diff --git a/packages/babel-cli/src/_babel-node.js b/packages/babel-cli/src/_babel-node.js
index 3a6f4e309d..8e89bf0e63 100644
--- a/packages/babel-cli/src/_babel-node.js
+++ b/packages/babel-cli/src/_babel-node.js
@@ -9,7 +9,7 @@ import * as babel from "babel-core";
 import vm from "vm";
 import _ from "lodash";
 
-var program = new commander.Command("babel-node");
+let program = new commander.Command("babel-node");
 
 program.option("-e, --eval [script]", "Evaluate script");
 program.option("-p, --print [code]", "Evaluate script and print result");
@@ -18,7 +18,7 @@ program.option("-x, --extensions [extensions]", "List of extensions to hook into
 program.option("-w, --plugins [string]", "TODO", util.list);
 program.option("-b, --presets [string]", "TODO", util.list);
 
-var pkg = require("../package.json");
+let pkg = require("../package.json");
 program.version(pkg.version);
 program.usage("[options] [ -e script | script.js ] [arguments]");
 program.parse(process.argv);
@@ -35,7 +35,7 @@ babel.register({
 
 //
 
-var replPlugin = new babel.Plugin("repl", {
+let replPlugin = new babel.Plugin("repl", {
   visitor: {
     ModuleDeclaration() {
       throw this.errorWithNode("Modules aren't supported in the REPL");
@@ -51,7 +51,7 @@ var replPlugin = new babel.Plugin("repl", {
 
 //
 
-var _eval = function (code, filename) {
+let _eval = function (code, filename) {
   code = code.trim();
   if (!code) return undefined;
 
@@ -70,13 +70,13 @@ var _eval = function (code, filename) {
 };
 
 if (program.eval || program.print) {
-  var code = program.eval;
+  let code = program.eval;
   if (!code || code === true) code = program.print;
 
   global.__filename = "[eval]";
   global.__dirname = process.cwd();
 
-  var module = new Module(global.__filename);
+  let module = new Module(global.__filename);
   module.filename = global.__filename;
   module.paths    = Module._nodeModulePaths(global.__dirname);
 
@@ -84,18 +84,18 @@ if (program.eval || program.print) {
   global.module  = module;
   global.require = module.require.bind(module);
 
-  var result = _eval(code, global.__filename);
+  let result = _eval(code, global.__filename);
   if (program.print) {
-    var output = _.isString(result) ? result : inspect(result);
+    let output = _.isString(result) ? result : inspect(result);
     process.stdout.write(output + "\n");
   }
 } else {
   if (program.args.length) {
     // slice all arguments up to the first filename since they're babel args that we handle
-    var args = process.argv.slice(2);
+    let args = process.argv.slice(2);
 
-    var i = 0;
-    var ignoreNext = false;
+    let i = 0;
+    let ignoreNext = false;
     _.each(args, function (arg, i2) {
       if (ignoreNext) {
         ignoreNext = false;
@@ -103,7 +103,7 @@ if (program.eval || program.print) {
       }
 
       if (arg[0] === "-") {
-        var parsedArg = program[arg.slice(2)];
+        let parsedArg = program[arg.slice(2)];
         if (parsedArg && parsedArg !== true) {
           ignoreNext = true;
         }
@@ -115,7 +115,7 @@ if (program.eval || program.print) {
     args = args.slice(i);
 
     // make the filename absolute
-    var filename = args[0];
+    let filename = args[0];
     if (!pathIsAbsolute(filename)) args[0] = path.join(process.cwd(), filename);
 
     // add back on node and concat the sliced args
@@ -139,8 +139,8 @@ function replStart() {
 }
 
 function replEval(code, context, filename, callback) {
-  var err;
-  var result;
+  let err;
+  let result;
 
   try {
     if (code[0] === "(" && code[code.length - 1] === ")") {
diff --git a/packages/babel-cli/src/babel-node.js b/packages/babel-cli/src/babel-node.js
index fadf11fcae..603835dc91 100755
--- a/packages/babel-cli/src/babel-node.js
+++ b/packages/babel-cli/src/babel-node.js
@@ -3,16 +3,16 @@
  * when found, before invoking the "real" _babel-node(1) executable.
  */
 
-var getV8Flags = require("v8flags");
-var path = require("path");
+let getV8Flags = require("v8flags");
+let path = require("path");
 
-var args = [path.join(__dirname, "_babel-node")];
+let args = [path.join(__dirname, "_babel-node")];
 
-var babelArgs = process.argv.slice(2);
-var userArgs;
+let babelArgs = process.argv.slice(2);
+let userArgs;
 
 // separate node arguments from script arguments
-var argSeparator = babelArgs.indexOf("--");
+let argSeparator = babelArgs.indexOf("--");
 if (argSeparator > -1) {
   userArgs  = babelArgs.slice(argSeparator); // including the  --
   babelArgs = babelArgs.slice(0, argSeparator);
@@ -20,7 +20,7 @@ if (argSeparator > -1) {
 
 getV8Flags(function (v8Flags) {
   babelArgs.forEach(function(arg){
-    var flag = arg.split("=")[0];
+    let flag = arg.split("=")[0];
 
     switch (flag) {
       case "-d":
@@ -54,13 +54,13 @@ getV8Flags(function (v8Flags) {
   }
 
   try {
-    var kexec = require("kexec");
+    let kexec = require("kexec");
     kexec(process.argv[0], args);
   } catch (err) {
     if (err.code !== "MODULE_NOT_FOUND") throw err;
 
-    var child_process = require("child_process");
-    var proc = child_process.spawn(process.argv[0], args, { stdio: "inherit" });
+    let child_process = require("child_process");
+    let proc = child_process.spawn(process.argv[0], args, { stdio: "inherit" });
     proc.on("exit", function (code, signal) {
       process.on("exit", function () {
         if (signal) {
diff --git a/packages/babel-cli/src/babel-plugin/index.js b/packages/babel-cli/src/babel-plugin/index.js
index 522d2e7f40..de8d326b02 100755
--- a/packages/babel-cli/src/babel-plugin/index.js
+++ b/packages/babel-cli/src/babel-plugin/index.js
@@ -7,7 +7,7 @@ import fs from "fs";
 function spawn(cmd, args, callback) {
   console.log(">", cmd, args);
 
-  var spawn = child.spawn(cmd, args, { stdio: "inherit" });
+  let spawn = child.spawn(cmd, args, { stdio: "inherit" });
 
   spawn.on("exit", function (code) {
     if (code === 0) {
@@ -21,7 +21,7 @@ function spawn(cmd, args, callback) {
 
 function spawnMultiple(cmds) {
   function next() {
-    var cmd = cmds.shift();
+    let cmd = cmds.shift();
     if (cmd) {
       spawn(cmd.command, cmd.args, next);
     } else {
@@ -33,7 +33,7 @@ function spawnMultiple(cmds) {
 }
 
 function template(name, data = {}) {
-  var source = fs.readFileSync(path.join(__dirname, "templates", name), "utf8");
+  let source = fs.readFileSync(path.join(__dirname, "templates", name), "utf8");
   source = source.replace(/[A-Z_]+/g, function (key) {
     return data[key] === undefined ? key : data[key];
   });
@@ -53,23 +53,23 @@ function execMaybe(cmd) {
   }
 }
 
-var rl = readline.createInterface({
+let rl = readline.createInterface({
   input: process.stdin,
   output: process.stdout
 });
 
-var BABEL_PLUGIN_PREFIX = "babel-plugin-";
+let BABEL_PLUGIN_PREFIX = "babel-plugin-";
 
-var cmds = {
+let cmds = {
   init: function () {
-    var name = path.basename(process.cwd());
+    let name = path.basename(process.cwd());
 
     if (name.indexOf(BABEL_PLUGIN_PREFIX) === 0) {
       name = name.slice(BABEL_PLUGIN_PREFIX.length);
     }
 
     rl.question("Description (optional): ", function (description) {
-      var remote = execMaybe("git config --get remote.origin.url").trim().match(/git@github.com:(.*?).git/);
+      let remote = execMaybe("git config --get remote.origin.url").trim().match(/git@github.com:(.*?).git/);
       if (remote) {
         build(description, remote[1]);
       } else {
@@ -82,7 +82,7 @@ var cmds = {
     function build(description, repo) {
       rl.close();
 
-      var templateData = {
+      let templateData = {
         DESCRIPTION: description,
         FULL_NAME: BABEL_PLUGIN_PREFIX + name,
         NAME: name
@@ -133,7 +133,7 @@ var cmds = {
   },
 
   publish: function () {
-    var pkg = require(process.cwd() + "/package.json");
+    let pkg = require(process.cwd() + "/package.json");
     console.log("Current version:", pkg.version);
 
     rl.question("New version (enter nothing for patch): ", function (newVersion) {
@@ -153,7 +153,7 @@ var cmds = {
   }
 };
 
-var cmd = cmds[process.argv[2]];
+let cmd = cmds[process.argv[2]];
 if (cmd) {
   cmd();
 } else {
diff --git a/packages/babel-cli/src/babel/dir.js b/packages/babel-cli/src/babel/dir.js
index f5010fa571..7507a78f47 100644
--- a/packages/babel-cli/src/babel/dir.js
+++ b/packages/babel-cli/src/babel/dir.js
@@ -1,27 +1,27 @@
-var outputFileSync = require("output-file-sync");
-var pathExists     = require("path-exists");
-var chokidar       = require("chokidar");
-var slash          = require("slash");
-var path           = require("path");
-var util           = require("./util");
-var fs             = require("fs");
-var _              = require("lodash");
+let outputFileSync = require("output-file-sync");
+let pathExists     = require("path-exists");
+let chokidar       = require("chokidar");
+let slash          = require("slash");
+let path           = require("path");
+let util           = require("./util");
+let fs             = require("fs");
+let _              = require("lodash");
 
 module.exports = function (commander, filenames) {
   function write(src, relative) {
     // remove extension and then append back on .js
     relative = relative.replace(/\.(\w*?)$/, "") + ".js";
 
-    var dest = path.join(commander.outDir, relative);
+    let dest = path.join(commander.outDir, relative);
 
-    var data = util.compile(src, {
+    let data = util.compile(src, {
       sourceFileName: slash(path.relative(dest + "/..", src)),
       sourceMapTarget: path.basename(relative)
     });
     if (!commander.copyFiles && data.ignored) return;
 
     if (data.map && commander.sourceMaps && commander.sourceMaps !== "inline") {
-      var mapLoc = dest + ".map";
+      let mapLoc = dest + ".map";
       data.code = util.addSourceMappingUrl(data.code, mapLoc);
       outputFileSync(mapLoc, JSON.stringify(data.map));
     }
@@ -44,13 +44,13 @@ module.exports = function (commander, filenames) {
   function handle(filename) {
     if (!pathExists.sync(filename)) return;
 
-    var stat = fs.statSync(filename);
+    let stat = fs.statSync(filename);
 
     if (stat.isDirectory(filename)) {
-      var dirname = filename;
+      let dirname = filename;
 
       _.each(util.readdir(dirname), function (filename) {
-        var src = path.join(dirname, filename);
+        let src = path.join(dirname, filename);
         handleFile(src, filename);
       });
     } else {
@@ -62,14 +62,14 @@ module.exports = function (commander, filenames) {
 
   if (commander.watch) {
     _.each(filenames, function (dirname) {
-      var watcher = chokidar.watch(dirname, {
+      let watcher = chokidar.watch(dirname, {
         persistent: true,
         ignoreInitial: true
       });
 
       _.each(["add", "change"], function (type) {
         watcher.on(type, function (filename) {
-          var relative = path.relative(dirname, filename) || filename;
+          let relative = path.relative(dirname, filename) || filename;
           try {
             handleFile(filename, relative);
           } catch (err) {
diff --git a/packages/babel-cli/src/babel/file.js b/packages/babel-cli/src/babel/file.js
index 9b2f9db4a5..e7120f5af5 100644
--- a/packages/babel-cli/src/babel/file.js
+++ b/packages/babel-cli/src/babel/file.js
@@ -1,37 +1,37 @@
-var convertSourceMap = require("convert-source-map");
-var pathExists       = require("path-exists");
-var sourceMap        = require("source-map");
-var chokidar         = require("chokidar");
-var slash            = require("slash");
-var path             = require("path");
-var util             = require("./util");
-var fs               = require("fs");
-var _                = require("lodash");
+let convertSourceMap = require("convert-source-map");
+let pathExists       = require("path-exists");
+let sourceMap        = require("source-map");
+let chokidar         = require("chokidar");
+let slash            = require("slash");
+let path             = require("path");
+let util             = require("./util");
+let fs               = require("fs");
+let _                = require("lodash");
 
 module.exports = function (commander, filenames, opts) {
   if (commander.sourceMaps === "inline") {
     opts.sourceMaps = true;
   }
 
-  var results = [];
+  let results = [];
 
-  var buildResult = function () {
-    var map = new sourceMap.SourceMapGenerator({
+  let buildResult = function () {
+    let map = new sourceMap.SourceMapGenerator({
       file: path.basename(commander.outFile) || "stdout",
       sourceRoot: opts.sourceRoot
     });
 
-    var code = "";
-    var offset = 0;
+    let code = "";
+    let offset = 0;
 
     _.each(results, function (result) {
-      var filename = result.filename;
+      let filename = result.filename;
       code += result.code + "\n";
 
       if (result.map) {
-        var consumer = new sourceMap.SourceMapConsumer(result.map);
+        let consumer = new sourceMap.SourceMapConsumer(result.map);
 
-        var sourceFilename = filename;
+        let sourceFilename = filename;
         if (commander.outFile) {
           sourceFilename = path.relative(path.dirname(commander.outFile), sourceFilename);
         }
@@ -64,12 +64,12 @@ module.exports = function (commander, filenames, opts) {
     };
   };
 
-  var output = function () {
-    var result = buildResult();
+  let output = function () {
+    let result = buildResult();
 
     if (commander.outFile) {
       if (commander.sourceMaps && commander.sourceMaps !== "inline") {
-        var mapLoc = commander.outFile + ".map";
+        let mapLoc = commander.outFile + ".map";
         result.code = util.addSourceMappingUrl(result.code, mapLoc);
         fs.writeFileSync(mapLoc, JSON.stringify(result.map));
       }
@@ -80,13 +80,13 @@ module.exports = function (commander, filenames, opts) {
     }
   };
 
-  var stdin = function () {
-    var code = "";
+  let stdin = function () {
+    let code = "";
 
     process.stdin.setEncoding("utf8");
 
     process.stdin.on("readable", function () {
-      var chunk = process.stdin.read();
+      let chunk = process.stdin.read();
       if (chunk !== null) code += chunk;
     });
 
@@ -96,16 +96,16 @@ module.exports = function (commander, filenames, opts) {
     });
   };
 
-  var walk = function () {
-    var _filenames = [];
+  let walk = function () {
+    let _filenames = [];
     results = [];
 
     _.each(filenames, function (filename) {
       if (!pathExists.sync(filename)) return;
 
-      var stat = fs.statSync(filename);
+      let stat = fs.statSync(filename);
       if (stat.isDirectory()) {
-        var dirname = filename;
+        let dirname = filename;
 
         _.each(util.readdirFilter(filename), function (filename) {
           _filenames.push(path.join(dirname, filename));
@@ -118,7 +118,7 @@ module.exports = function (commander, filenames, opts) {
     _.each(_filenames, function (filename) {
       if (util.shouldIgnore(filename)) return;
 
-      var data = util.compile(filename);
+      let data = util.compile(filename);
       if (data.ignored) return;
       results.push(data);
     });
@@ -126,7 +126,7 @@ module.exports = function (commander, filenames, opts) {
     output();
   };
 
-  var files = function () {
+  let files = function () {
     walk();
 
     if (commander.watch) {
diff --git a/packages/babel-cli/src/babel/index.js b/packages/babel-cli/src/babel/index.js
index 46fc228c15..ac5c856164 100755
--- a/packages/babel-cli/src/babel/index.js
+++ b/packages/babel-cli/src/babel/index.js
@@ -2,19 +2,19 @@
 
 require("babel-core");
 
-var pathExists = require("path-exists");
-var commander  = require("commander");
-var kebabCase  = require("lodash/string/kebabCase");
-var options    = require("babel-core").options;
-var util       = require("babel-core").util;
-var uniq       = require("lodash/array/uniq");
-var each       = require("lodash/collection/each");
-var glob       = require("glob");
+let pathExists = require("path-exists");
+let commander  = require("commander");
+let kebabCase  = require("lodash/string/kebabCase");
+let options    = require("babel-core").options;
+let util       = require("babel-core").util;
+let uniq       = require("lodash/array/uniq");
+let each       = require("lodash/collection/each");
+let glob       = require("glob");
 
 each(options, function (option, key) {
   if (option.hidden) return;
 
-  var arg = kebabCase(key);
+  let arg = kebabCase(key);
 
   if (option.type !== "boolean") {
     arg += " [" + (option.type || "string") + "]";
@@ -30,7 +30,7 @@ each(options, function (option, key) {
     arg = "-" + option.shorthand + ", " + arg;
   }
 
-  var desc = [];
+  let desc = [];
   if (option.deprecated) desc.push("[DEPRECATED] " + option.deprecated);
   if (option.description) desc.push(option.description);
 
@@ -44,7 +44,7 @@ commander.option("-d, --out-dir [out]", "Compile an input directory of modules i
 commander.option("-D, --copy-files", "When compiling a directory copy over non-compilable files");
 commander.option("-q, --quiet", "Don't log anything");
 
-var pkg = require("../../package.json");
+let pkg = require("../../package.json");
 commander.version(pkg.version + " (babel-core " + require("babel-core").version + ")");
 commander.usage("[options] <files ...>");
 commander.parse(process.argv);
@@ -57,10 +57,10 @@ if (commander.extensions) {
 
 //
 
-var errors = [];
+let errors = [];
 
-var filenames = commander.args.reduce(function (globbed, input) {
-  var files = glob.sync(input);
+let filenames = commander.args.reduce(function (globbed, input) {
+  let files = glob.sync(input);
   if (!files.length) files = [input];
   return globbed.concat(files);
 }, []);
@@ -98,7 +98,7 @@ if (errors.length) {
 
 //
 
-var opts = exports.opts = {};
+let opts = exports.opts = {};
 
 each(options, function (opt, key) {
   if (commander[key] !== undefined) {
@@ -112,7 +112,7 @@ if (opts.only) {
   opts.only = util.arrayify(opts.only, util.regexify);
 }
 
-var fn;
+let fn;
 
 if (commander.outDir) {
   fn = require("./dir");
diff --git a/packages/babel-cli/src/babel/util.js b/packages/babel-cli/src/babel/util.js
index 52c0801109..e215b38b17 100644
--- a/packages/babel-cli/src/babel/util.js
+++ b/packages/babel-cli/src/babel/util.js
@@ -1,11 +1,11 @@
-var commander = require("commander");
-var readdir   = require("fs-readdir-recursive");
-var index     = require("./index");
-var babel     = require("babel-core");
-var util      = require("babel-core").util;
-var path      = require("path");
-var fs        = require("fs");
-var _         = require("lodash");
+let commander = require("commander");
+let readdir   = require("fs-readdir-recursive");
+let index     = require("./index");
+let babel     = require("babel-core");
+let util      = require("babel-core").util;
+let path      = require("path");
+let fs        = require("fs");
+let _         = require("lodash");
 
 exports.readdirFilter = function (filename) {
   return readdir(filename).filter(function (filename) {
@@ -35,7 +35,7 @@ exports.transform = function (filename, code, opts) {
   opts.ignore = null;
   opts.only = null;
 
-  var result = babel.transform(code, opts);
+  let result = babel.transform(code, opts);
   result.filename = filename;
   result.actual = code;
   return result;
@@ -43,7 +43,7 @@ exports.transform = function (filename, code, opts) {
 
 exports.compile = function (filename, opts) {
   try {
-    var code = fs.readFileSync(filename, "utf8");
+    let code = fs.readFileSync(filename, "utf8");
     return exports.transform(filename, code, opts);
   } catch (err) {
     if (commander.watch) {
diff --git a/packages/babel-core/src/api/browser.js b/packages/babel-core/src/api/browser.js
index 68f3eaccb7..17d8883762 100644
--- a/packages/babel-core/src/api/browser.js
+++ b/packages/babel-core/src/api/browser.js
@@ -10,16 +10,16 @@ export function run(code, opts = {}) {
 export function load(url, callback, opts = {}, hold) {
   opts.filename = opts.filename || url;
 
-  var xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
+  let xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
   xhr.open("GET", url, true);
   if ("overrideMimeType" in xhr) xhr.overrideMimeType("text/plain");
 
   xhr.onreadystatechange = function () {
     if (xhr.readyState !== 4) return;
 
-    var status = xhr.status;
+    let status = xhr.status;
     if (status === 0 || status === 200) {
-      var param = [xhr.responseText, opts];
+      let param = [xhr.responseText, opts];
       if (!hold) run(param);
       if (callback) callback(param);
     } else {
@@ -31,16 +31,16 @@ export function load(url, callback, opts = {}, hold) {
 }
 
 function runScripts() {
-  var scripts = [];
-  var types   = ["text/ecmascript-6", "text/6to5", "text/babel", "module"];
-  var index   = 0;
+  let scripts = [];
+  let types   = ["text/ecmascript-6", "text/6to5", "text/babel", "module"];
+  let index   = 0;
 
   /**
    * Transform and execute script. Ensures correct load order.
    */
 
   function exec() {
-    var param = scripts[index];
+    let param = scripts[index];
     if (param instanceof Array) {
       run(param);
       index++;
@@ -53,7 +53,7 @@ function runScripts() {
    */
 
   function run(script, i) {
-    var opts = {};
+    let opts = {};
 
     if (script.src) {
       load(script.src, function (param) {
@@ -68,10 +68,10 @@ function runScripts() {
 
   // Collect scripts with Babel `types`.
 
-  var _scripts = global.document.getElementsByTagName("script");
+  let _scripts = global.document.getElementsByTagName("script");
 
-  for (var i = 0; i < _scripts.length; ++i) {
-    var _script = _scripts[i];
+  for (let i = 0; i < _scripts.length; ++i) {
+    let _script = _scripts[i];
     if (types.indexOf(_script.type) >= 0) scripts.push(_script);
   }
 
diff --git a/packages/babel-core/src/api/node.js b/packages/babel-core/src/api/node.js
index 6f6f6b1c11..a4630f6257 100644
--- a/packages/babel-core/src/api/node.js
+++ b/packages/babel-core/src/api/node.js
@@ -20,7 +20,7 @@ import * as t from "babel-types";
 export { t as types };
 
 export function register(opts?: Object) {
-  var callback = require("./register/node-polyfill");
+  let callback = require("./register/node-polyfill");
   if (opts != null) callback(opts);
   return callback;
 }
@@ -36,7 +36,7 @@ export function transformFile(filename: string, opts?: Object, callback: Functio
   fs.readFile(filename, function (err, code) {
     if (err) return callback(err);
 
-    var result;
+    let result;
 
     try {
       result = transform(code, opts);
@@ -63,7 +63,7 @@ export function parse(code, opts = {}) {
   };
   opts.features = {};
 
-  var ast = babylon.parse(code, opts);
+  let ast = babylon.parse(code, opts);
 
   if (opts.onToken) {
     opts.onToken.push(...ast.tokens);
diff --git a/packages/babel-core/src/api/register/cache.js b/packages/babel-core/src/api/register/cache.js
index 73ab94b175..b128ea4e47 100644
--- a/packages/babel-core/src/api/register/cache.js
+++ b/packages/babel-core/src/api/register/cache.js
@@ -4,7 +4,7 @@ import homeOrTmp from "home-or-tmp";
 import pathExists from "path-exists";
 
 const FILENAME = process.env.BABEL_CACHE_PATH || path.join(homeOrTmp, ".babel.json");
-var data = {};
+let data = {};
 
 /**
  * Write stringified cache to disk.
diff --git a/packages/babel-core/src/api/register/node.js b/packages/babel-core/src/api/register/node.js
index 723fcb391a..f130596a3b 100644
--- a/packages/babel-core/src/api/register/node.js
+++ b/packages/babel-core/src/api/register/node.js
@@ -15,7 +15,7 @@ import path from "path";
 sourceMapSupport.install({
   handleUncaughtExceptions: false,
   retrieveSourceMap(source) {
-    var map = maps && maps[source];
+    let map = maps && maps[source];
     if (map) {
       return {
         url: null,
@@ -32,27 +32,27 @@ sourceMapSupport.install({
  */
 
 registerCache.load();
-var cache = registerCache.get();
+let cache = registerCache.get();
 
 /**
  * Store options.
  */
 
-var transformOpts = {};
+let transformOpts = {};
 
-var ignore;
-var only;
+let ignore;
+let only;
 
-var oldHandlers   = {};
-var maps          = {};
+let oldHandlers   = {};
+let maps          = {};
 
-var cwd = process.cwd();
+let cwd = process.cwd();
 
 /**
  * Get path from `filename` relative to the current working directory.
  */
 
-var getRelativePath = function (filename){
+let getRelativePath = function (filename){
   return path.relative(cwd, filename);
 };
 
@@ -60,7 +60,7 @@ var getRelativePath = function (filename){
  * Get last modified time for a `filename`.
  */
 
-var mtime = function (filename) {
+let mtime = function (filename) {
   return +fs.statSync(filename).mtime;
 };
 
@@ -68,22 +68,22 @@ var mtime = function (filename) {
  * Compile a `filename` with optional `opts`.
  */
 
-var compile = function (filename, opts = {}) {
-  var result;
+let compile = function (filename, opts = {}) {
+  let result;
 
   opts.filename = filename;
 
-  var optsManager = new OptionManager;
+  let optsManager = new OptionManager;
   optsManager.mergeOptions(transformOpts);
   opts = optsManager.init(opts);
 
-  var cacheKey = `${JSON.stringify(opts)}:${babel.version}`;
+  let cacheKey = `${JSON.stringify(opts)}:${babel.version}`;
 
-  var env = process.env.BABEL_ENV || process.env.NODE_ENV;
+  let env = process.env.BABEL_ENV || process.env.NODE_ENV;
   if (env) cacheKey += `:${env}`;
 
   if (cache) {
-    var cached = cache[cacheKey];
+    let cached = cache[cacheKey];
     if (cached && cached.mtime === mtime(filename)) {
       result = cached;
     }
@@ -110,7 +110,7 @@ var compile = function (filename, opts = {}) {
  * Test if a `filename` should be ignored by Babel.
  */
 
-var shouldIgnore = function (filename) {
+let shouldIgnore = function (filename) {
   if (!ignore && !only) {
     return getRelativePath(filename).split(path.sep).indexOf("node_modules") >= 0;
   } else {
@@ -122,17 +122,17 @@ var shouldIgnore = function (filename) {
  * Monkey patch istanbul if it is running so that it works properly.
  */
 
-var istanbulMonkey = {};
+let istanbulMonkey = {};
 
 if (process.env.running_under_istanbul) {
   // we need to monkey patch fs.readFileSync so we can hook into
   // what istanbul gets, it's extremely dirty but it's the only way
-  var _readFileSync = fs.readFileSync;
+  let _readFileSync = fs.readFileSync;
 
   fs.readFileSync = function (filename) {
     if (istanbulMonkey[filename]) {
       delete istanbulMonkey[filename];
-      var code = compile(filename, {
+      let code = compile(filename, {
         auxiliaryCommentBefore: "istanbul ignore next"
       });
       istanbulMonkey[filename] = true;
@@ -147,7 +147,7 @@ if (process.env.running_under_istanbul) {
  * Replacement for the loader for istanbul.
  */
 
-var istanbulLoader = function (m, filename, old) {
+let istanbulLoader = function (m, filename, old) {
   istanbulMonkey[filename] = true;
   old(m, filename);
 };
@@ -156,7 +156,7 @@ var istanbulLoader = function (m, filename, old) {
  * Default loader.
  */
 
-var normalLoader = function (m, filename) {
+let normalLoader = function (m, filename) {
   m._compile(compile(filename), filename);
 };
 
@@ -164,10 +164,10 @@ var normalLoader = function (m, filename) {
  * Register a loader for an extension.
  */
 
-var registerExtension = function (ext) {
-  var old = oldHandlers[ext] || oldHandlers[".js"] || require.extensions[".js"];
+let registerExtension = function (ext) {
+  let old = oldHandlers[ext] || oldHandlers[".js"] || require.extensions[".js"];
 
-  var loader = normalLoader;
+  let loader = normalLoader;
   if (process.env.running_under_istanbul) loader = istanbulLoader;
 
   require.extensions[ext] = function (m, filename) {
@@ -183,7 +183,7 @@ var registerExtension = function (ext) {
  * Register loader for given extensions.
  */
 
-var hookExtensions = function (_exts) {
+let hookExtensions = function (_exts) {
   each(oldHandlers, function (old, ext) {
     if (old === undefined) {
       delete require.extensions[ext];
diff --git a/packages/babel-core/src/helpers/merge.js b/packages/babel-core/src/helpers/merge.js
index 33f433dbd9..dbbc07d35a 100644
--- a/packages/babel-core/src/helpers/merge.js
+++ b/packages/babel-core/src/helpers/merge.js
@@ -9,8 +9,8 @@ export default function (dest, src) {
 
   return merge(dest, src, function (a, b) {
     if (b && Array.isArray(a)) {
-      var c = a.slice(0);
-      for (var v of b) {
+      let c = a.slice(0);
+      for (let v of b) {
         if (a.indexOf(v) < 0) {
           c.push(v);
         }
diff --git a/packages/babel-core/src/helpers/parse.js b/packages/babel-core/src/helpers/parse.js
index cb6eb42ba9..0e4595b551 100644
--- a/packages/babel-core/src/helpers/parse.js
+++ b/packages/babel-core/src/helpers/parse.js
@@ -5,7 +5,7 @@ import * as babylon from "babylon";
  */
 
 export default function (code, opts = {}) {
-  var parseOpts = {
+  let parseOpts = {
     allowImportExportEverywhere: opts.looseModules,
     allowReturnOutsideFunction:  opts.looseModules,
     strictMode:    opts.strictMode,
diff --git a/packages/babel-core/src/store.js b/packages/babel-core/src/store.js
index e276346bb9..d83cdd71c2 100644
--- a/packages/babel-core/src/store.js
+++ b/packages/babel-core/src/store.js
@@ -11,11 +11,11 @@ export default class Store {
   }
 
   get(key: string): any {
-    var data = this.data[key];
+    let data = this.data[key];
     if (data) {
       return data;
     } else {
-      var dynamic = this.dynamicData[key];
+      let dynamic = this.dynamicData[key];
       if (dynamic) {
         return this.set(key, dynamic());
       }
diff --git a/packages/babel-core/src/tools/build-external-helpers.js b/packages/babel-core/src/tools/build-external-helpers.js
index 474da46340..d19971ac2e 100644
--- a/packages/babel-core/src/tools/build-external-helpers.js
+++ b/packages/babel-core/src/tools/build-external-helpers.js
@@ -6,9 +6,9 @@ import each from "lodash/collection/each";
 import * as t from "babel-types";
 
 function buildGlobal(namespace, builder) {
-  var body      = [];
-  var container = t.functionExpression(null, [t.identifier("global")], t.blockStatement(body));
-  var tree      = t.program([t.expressionStatement(t.callExpression(container, [util.template("helper-self-global")]))]);
+  let body      = [];
+  let container = t.functionExpression(null, [t.identifier("global")], t.blockStatement(body));
+  let tree      = t.program([t.expressionStatement(t.callExpression(container, [util.template("helper-self-global")]))]);
 
   body.push(t.variableDeclaration("var", [
     t.variableDeclarator(
@@ -23,14 +23,14 @@ function buildGlobal(namespace, builder) {
 }
 
 function buildUmd(namespace, builder) {
-  var body = [];
+  let body = [];
   body.push(t.variableDeclaration("var", [
     t.variableDeclarator(namespace, t.identifier("global"))
   ]));
 
   builder(body);
 
-  var container = util.template("umd-commonjs-strict", {
+  let container = util.template("umd-commonjs-strict", {
     FACTORY_PARAMETERS: t.identifier("global"),
     BROWSER_ARGUMENTS:  t.assignmentExpression("=", t.memberExpression(t.identifier("root"), namespace), t.objectExpression({})),
     COMMON_ARGUMENTS:   t.identifier("exports"),
@@ -42,7 +42,7 @@ function buildUmd(namespace, builder) {
 }
 
 function buildVar(namespace, builder) {
-  var body = [];
+  let body = [];
   body.push(t.variableDeclaration("var", [
     t.variableDeclarator(namespace, t.objectExpression({}))
   ]));
@@ -54,7 +54,7 @@ function buildHelpers(body, namespace, whitelist) {
   each(File.helpers, function (name) {
     if (whitelist && whitelist.indexOf(name) === -1) return;
 
-    var key = t.identifier(t.toIdentifier(name));
+    let key = t.identifier(t.toIdentifier(name));
     body.push(t.expressionStatement(
       t.assignmentExpression("=", t.memberExpression(namespace, key), util.template("helper-" + name))
     ));
@@ -62,15 +62,15 @@ function buildHelpers(body, namespace, whitelist) {
 }
 
 export default function (whitelist, outputType = "global") {
-  var namespace = t.identifier("babelHelpers");
+  let namespace = t.identifier("babelHelpers");
 
-  var builder = function (body) {
+  let builder = function (body) {
     return buildHelpers(body, namespace, whitelist);
   };
 
-  var tree;
+  let tree;
 
-  var build = {
+  let build = {
     global: buildGlobal,
     umd:    buildUmd,
     var:    buildVar
diff --git a/packages/babel-core/src/transformation/file/index.js b/packages/babel-core/src/transformation/file/index.js
index 79f38de0e0..e6c69ca1cd 100644
--- a/packages/babel-core/src/transformation/file/index.js
+++ b/packages/babel-core/src/transformation/file/index.js
@@ -21,9 +21,9 @@ import * as util from  "../../util";
 import path from "path";
 import * as t from "babel-types";
 
-var errorVisitor = {
+let errorVisitor = {
   enter(path, state) {
-    var loc = path.node.loc;
+    let loc = path.node.loc;
     if (loc) {
       state.loc = loc;
       path.stop();
@@ -155,16 +155,16 @@ export default class File extends Store {
   }
 
   buildTransformers() {
-    var file = this;
+    let file = this;
 
-    var transformers = this.transformers = {};
+    let transformers = this.transformers = {};
 
-    var secondaryStack = [];
-    var stack = [];
+    let secondaryStack = [];
+    let stack = [];
 
     // build internal transformers
-    for (var key in this.pipeline.transformers) {
-      var transformer = this.pipeline.transformers[key];
+    for (let key in this.pipeline.transformers) {
+      let transformer = this.pipeline.transformers[key];
       let pass = transformers[key] = transformer.buildPass(file);
 
       if (pass.canTransform()) {
@@ -181,15 +181,15 @@ export default class File extends Store {
     }
 
     // init plugins!
-    var beforePlugins = [];
-    var afterPlugins = [];
-    var pluginManager = new PluginManager({
+    let beforePlugins = [];
+    let afterPlugins = [];
+    let pluginManager = new PluginManager({
       file: this,
       transformers: this.transformers,
       before: beforePlugins,
       after: afterPlugins
     });
-    for (var i = 0; i < file.opts.plugins.length; i++) {
+    for (let i = 0; i < file.opts.plugins.length; i++) {
       pluginManager.add(file.opts.plugins[i]);
     }
     stack = beforePlugins.concat(stack, afterPlugins);
@@ -199,7 +199,7 @@ export default class File extends Store {
 
     // build dependency graph
     for (let pass of (stack: Array)) {
-      for (var dep of (pass.plugin.dependencies: Array)) {
+      for (let dep of (pass.plugin.dependencies: Array)) {
         this.transformerDependencies[dep] = pass.key;
       }
     }
@@ -209,14 +209,14 @@ export default class File extends Store {
   }
 
   collapseStack(_stack) {
-    var stack  = [];
-    var ignore = [];
+    let stack  = [];
+    let ignore = [];
 
     for (let pass of (_stack: Array)) {
       // been merged
       if (ignore.indexOf(pass) >= 0) continue;
 
-      var group = pass.plugin.metadata.group;
+      let group = pass.plugin.metadata.group;
 
       // can't merge
       if (!pass.canTransform() || !group) {
@@ -224,7 +224,7 @@ export default class File extends Store {
         continue;
       }
 
-      var mergeStack = [];
+      let mergeStack = [];
       for (let pass of (_stack: Array)) {
         if (pass.plugin.metadata.group === group) {
           mergeStack.push(pass);
@@ -236,12 +236,12 @@ export default class File extends Store {
       shuffle;
       //mergeStack = shuffle(mergeStack);
 
-      var visitors = [];
+      let visitors = [];
       for (let pass of (mergeStack: Array)) {
         visitors.push(pass.plugin.visitor);
       }
-      var visitor = traverse.visitors.merge(visitors);
-      var mergePlugin = new Plugin(group, { visitor });
+      let visitor = traverse.visitors.merge(visitors);
+      let mergePlugin = new Plugin(group, { visitor });
       stack.push(mergePlugin.buildPass(this));
     }
 
@@ -257,11 +257,11 @@ export default class File extends Store {
   }
 
   get(key: string): any {
-    var data = this.data[key];
+    let data = this.data[key];
     if (data) {
       return data;
     } else {
-      var dynamic = this.dynamicData[key];
+      let dynamic = this.dynamicData[key];
       if (dynamic) {
         return this.set(key, dynamic());
       }
@@ -269,25 +269,25 @@ export default class File extends Store {
   }
 
   resolveModuleSource(source: string): string {
-    var resolveModuleSource = this.opts.resolveModuleSource;
+    let resolveModuleSource = this.opts.resolveModuleSource;
     if (resolveModuleSource) source = resolveModuleSource(source, this.opts.filename);
     return source;
   }
 
   addImport(source: string, name?: string, type?: string): Object {
     name = name || source;
-    var id = this.dynamicImportIds[name];
+    let id = this.dynamicImportIds[name];
 
     if (!id) {
       source = this.resolveModuleSource(source);
       id = this.dynamicImportIds[name] = this.scope.generateUidIdentifier(name);
 
-      var specifiers = [t.importDefaultSpecifier(id)];
-      var declar = t.importDeclaration(specifiers, t.stringLiteral(source));
+      let specifiers = [t.importDefaultSpecifier(id)];
+      let declar = t.importDeclaration(specifiers, t.stringLiteral(source));
       declar._blockHoist = 3;
 
       if (type) {
-        var modules = this.dynamicImportTypes[type] = this.dynamicImportTypes[type] || [];
+        let modules = this.dynamicImportTypes[type] = this.dynamicImportTypes[type] || [];
         modules.push(declar);
       }
 
@@ -303,7 +303,7 @@ export default class File extends Store {
   }
 
   attachAuxiliaryComment(node: Object): Object {
-    var beforeComment = this.opts.auxiliaryCommentBefore;
+    let beforeComment = this.opts.auxiliaryCommentBefore;
     if (beforeComment) {
       node.leadingComments = node.leadingComments || [];
       node.leadingComments.push({
@@ -312,7 +312,7 @@ export default class File extends Store {
       });
     }
 
-    var afterComment = this.opts.auxiliaryCommentAfter;
+    let afterComment = this.opts.auxiliaryCommentAfter;
     if (afterComment) {
       node.trailingComments = node.trailingComments || [];
       node.trailingComments.push({
@@ -325,31 +325,31 @@ export default class File extends Store {
   }
 
   addHelper(name: string): Object {
-    var isSolo = includes(File.soloHelpers, name);
+    let isSolo = includes(File.soloHelpers, name);
 
     if (!isSolo && !includes(File.helpers, name)) {
       throw new ReferenceError(`Unknown helper ${name}`);
     }
 
-    var declar = this.declarations[name];
+    let declar = this.declarations[name];
     if (declar) return declar;
 
     this.usedHelpers[name] = true;
 
     if (!isSolo) {
-      var generator = this.get("helperGenerator");
-      var runtime   = this.get("helpersNamespace");
+      let generator = this.get("helperGenerator");
+      let runtime   = this.get("helpersNamespace");
       if (generator) {
         return generator(name);
       } else if (runtime) {
-        var id = t.identifier(t.toIdentifier(name));
+        let id = t.identifier(t.toIdentifier(name));
         return t.memberExpression(runtime, id);
       }
     }
 
-    var ref = util.template("helper-" + name);
+    let ref = util.template("helper-" + name);
 
-    var uid = this.declarations[name] = this.scope.generateUidIdentifier(name);
+    let uid = this.declarations[name] = this.scope.generateUidIdentifier(name);
 
     if (t.isFunctionExpression(ref) && !ref.id) {
       ref.body._compact = true;
@@ -373,18 +373,18 @@ export default class File extends Store {
   addTemplateObject(helperName: string, strings: Array, raw: Array): Object {
     // Generate a unique name based on the string literals so we dedupe
     // identical strings used in the program.
-    var stringIds = raw.elements.map(function(string) {
+    let stringIds = raw.elements.map(function(string) {
       return string.value;
     });
-    var name = `${helperName}_${raw.elements.length}_${stringIds.join(",")}`;
+    let name = `${helperName}_${raw.elements.length}_${stringIds.join(",")}`;
 
-    var declar = this.declarations[name];
+    let declar = this.declarations[name];
     if (declar) return declar;
 
-    var uid = this.declarations[name] = this.scope.generateUidIdentifier("templateObject");
+    let uid = this.declarations[name] = this.scope.generateUidIdentifier("templateObject");
 
-    var helperId = this.addHelper(helperName);
-    var init = t.callExpression(helperId, [strings, raw]);
+    let helperId = this.addHelper(helperName);
+    let init = t.callExpression(helperId, [strings, raw]);
     init._compact = true;
     this.scope.push({
       id: uid,
@@ -395,9 +395,9 @@ export default class File extends Store {
   }
 
   buildCodeFrameError(node, msg, Error = SyntaxError) {
-    var loc = node && (node.loc || node._loc);
+    let loc = node && (node.loc || node._loc);
 
-    var err = new Error(msg);
+    let err = new Error(msg);
 
     if (loc) {
       err.loc = loc.start;
@@ -417,19 +417,19 @@ export default class File extends Store {
   }
 
   mergeSourceMap(map: Object) {
-    var opts = this.opts;
+    let opts = this.opts;
 
-    var inputMap = opts.inputSourceMap;
+    let inputMap = opts.inputSourceMap;
 
     if (inputMap) {
       map.sources[0] = inputMap.file;
 
-      var inputMapConsumer   = new sourceMap.SourceMapConsumer(inputMap);
-      var outputMapConsumer  = new sourceMap.SourceMapConsumer(map);
-      var outputMapGenerator = sourceMap.SourceMapGenerator.fromSourceMap(outputMapConsumer);
+      let inputMapConsumer   = new sourceMap.SourceMapConsumer(inputMap);
+      let outputMapConsumer  = new sourceMap.SourceMapConsumer(map);
+      let outputMapGenerator = sourceMap.SourceMapGenerator.fromSourceMap(outputMapConsumer);
       outputMapGenerator.applySourceMap(inputMapConsumer);
 
-      var mergedMap = outputMapGenerator.toJSON();
+      let mergedMap = outputMapGenerator.toJSON();
       mergedMap.sources = inputMap.sources;
       mergedMap.file    = inputMap.file;
       return mergedMap;
@@ -443,10 +443,10 @@ export default class File extends Store {
       this.log.deprecate("Custom module formatters are deprecated and will be removed in the next major. Please use Babel plugins instead.");
     }
 
-    var ModuleFormatter = isFunction(type) ? type : moduleFormatters[type];
+    let ModuleFormatter = isFunction(type) ? type : moduleFormatters[type];
 
     if (!ModuleFormatter) {
-      var loc = resolve.relative(type);
+      let loc = resolve.relative(type);
       if (loc) ModuleFormatter = require(loc);
     }
 
@@ -458,11 +458,11 @@ export default class File extends Store {
   }
 
   parse(code: string) {
-    var opts = this.opts;
+    let opts = this.opts;
 
     //
 
-    var parseOpts = {
+    let parseOpts = {
       highlightCode: opts.highlightCode,
       nonStandard:   opts.nonStandard,
       sourceType:    opts.sourceType,
@@ -470,9 +470,9 @@ export default class File extends Store {
       plugins:       {}
     };
 
-    var features = parseOpts.features = {};
-    for (var key in this.transformers) {
-      var transformer = this.transformers[key];
+    let features = parseOpts.features = {};
+    for (let key in this.transformers) {
+      let transformer = this.transformers[key];
       features[key] = transformer.canRun();
     }
 
@@ -480,7 +480,7 @@ export default class File extends Store {
     parseOpts.strictMode = features.strict;
 
     this.log.debug("Parse start");
-    var ast = parse(code, parseOpts);
+    let ast = parse(code, parseOpts);
     this.log.debug("Parse stop");
     return ast;
   }
@@ -503,7 +503,7 @@ export default class File extends Store {
     this.log.debug("End set AST");
 
     this.log.debug("Start module formatter init");
-    var modFormatter = this.moduleFormatter = this.getModuleFormatter(this.opts.modules);
+    let modFormatter = this.moduleFormatter = this.getModuleFormatter(this.opts.modules);
     if (modFormatter.init && this.transformers["es6.modules"].canTransform()) {
       modFormatter.init();
     }
@@ -512,7 +512,7 @@ export default class File extends Store {
 
   transform() {
     this.call("pre");
-    for (var pass of (this.transformerStack: Array)) {
+    for (let pass of (this.transformerStack: Array)) {
       pass.transform();
     }
     this.call("post");
@@ -536,9 +536,9 @@ export default class File extends Store {
         err._babel = true;
       }
 
-      var message = err.message = `${this.opts.filename}: ${err.message}`;
+      let message = err.message = `${this.opts.filename}: ${err.message}`;
 
-      var loc = err.loc;
+      let loc = err.loc;
       if (loc) {
         err.codeFrame = codeFrame(code, loc.line, loc.column + 1, this.opts);
         message += "\n" + err.codeFrame;
@@ -551,7 +551,7 @@ export default class File extends Store {
       }
 
       if (err.stack) {
-        var newStack = err.stack.replace(err.message, message);
+        let newStack = err.stack.replace(err.message, message);
         try {
           err.stack = newStack;
         } catch (e) {
@@ -571,27 +571,27 @@ export default class File extends Store {
 
   parseCode() {
     this.parseShebang();
-    var ast = this.parse(this.code);
+    let ast = this.parse(this.code);
     this.addAst(ast);
   }
 
   shouldIgnore() {
-    var opts = this.opts;
+    let opts = this.opts;
     return util.shouldIgnore(opts.filename, opts.ignore, opts.only);
   }
 
   call(key: string) {
-    for (var pass of (this.uncollapsedTransformerStack: Array)) {
-      var fn = pass.plugin[key];
+    for (let pass of (this.uncollapsedTransformerStack: Array)) {
+      let fn = pass.plugin[key];
       if (fn) fn.call(pass, this);
     }
   }
 
   parseInputSourceMap(code: string) {
-    var opts = this.opts;
+    let opts = this.opts;
 
     if (opts.inputSourceMap !== false) {
-      var inputMap = convertSourceMap.fromSource(code);
+      let inputMap = convertSourceMap.fromSource(code);
       if (inputMap) {
         opts.inputSourceMap = inputMap.toObject();
         code = convertSourceMap.removeComments(code);
@@ -602,7 +602,7 @@ export default class File extends Store {
   }
 
   parseShebang() {
-    var shebangMatch = shebangRegex.exec(this.code);
+    let shebangMatch = shebangRegex.exec(this.code);
     if (shebangMatch) {
       this.shebang = shebangMatch[0];
       this.code = this.code.replace(shebangRegex, "");
@@ -610,7 +610,7 @@ export default class File extends Store {
   }
 
   makeResult({ code, map = null, ast, ignored }) {
-    var result = {
+    let result = {
       metadata: null,
       ignored:  !!ignored,
       code:     null,
@@ -635,15 +635,15 @@ export default class File extends Store {
   }
 
   generate() {
-    var opts = this.opts;
-    var ast  = this.ast;
+    let opts = this.opts;
+    let ast  = this.ast;
 
-    var result = { ast };
+    let result = { ast };
     if (!opts.code) return this.makeResult(result);
 
     this.log.debug("Generation start");
 
-    var _result = generate(ast, opts, this.code);
+    let _result = generate(ast, opts, this.code);
     result.code = _result.code;
     result.map  = _result.map;
 
diff --git a/packages/babel-core/src/transformation/file/logger.js b/packages/babel-core/src/transformation/file/logger.js
index 35db09efc5..39a17ddf12 100644
--- a/packages/babel-core/src/transformation/file/logger.js
+++ b/packages/babel-core/src/transformation/file/logger.js
@@ -1,10 +1,10 @@
 import type File from "./index";
 import buildDebug from "debug/node";
 
-var verboseDebug = buildDebug("babel:verbose");
-var generalDebug = buildDebug("babel");
+let verboseDebug = buildDebug("babel:verbose");
+let generalDebug = buildDebug("babel");
 
-var seenDeprecatedMessages = [];
+let seenDeprecatedMessages = [];
 
 export default class Logger {
   constructor(file: File, filename: string) {
@@ -13,7 +13,7 @@ export default class Logger {
   }
 
   _buildMessage(msg: string): string {
-    var parts = `[BABEL] ${this.filename}`;
+    let parts = `[BABEL] ${this.filename}`;
     if (msg) parts += `: ${msg}`;
     return parts;
   }
diff --git a/packages/babel-core/src/transformation/file/options/index.js b/packages/babel-core/src/transformation/file/options/index.js
index 513670a972..2aeb85c41c 100644
--- a/packages/babel-core/src/transformation/file/options/index.js
+++ b/packages/babel-core/src/transformation/file/options/index.js
@@ -8,8 +8,8 @@ export { config };
  */
 
 export function validateOption(key, val, pipeline) {
-  var opt = config[key];
-  var parser = opt && parsers[opt.type];
+  let opt = config[key];
+  let parser = opt && parsers[opt.type];
   if (parser && parser.validate) {
     return parser.validate(key, val, pipeline);
   } else {
@@ -22,14 +22,14 @@ export function validateOption(key, val, pipeline) {
  */
 
 export function normaliseOptions(options = {}) {
-  for (var key in options) {
-    var val = options[key];
+  for (let key in options) {
+    let val = options[key];
     if (val == null) continue;
 
-    var opt = config[key];
+    let opt = config[key];
     if (!opt) continue;
 
-    var parser = parsers[opt.type];
+    let parser = parsers[opt.type];
     if (parser) val = parser(val);
 
     options[key] = val;
diff --git a/packages/babel-core/src/transformation/file/options/option-manager.js b/packages/babel-core/src/transformation/file/options/option-manager.js
index 67eb61b44b..fd5cadc234 100644
--- a/packages/babel-core/src/transformation/file/options/option-manager.js
+++ b/packages/babel-core/src/transformation/file/options/option-manager.js
@@ -12,15 +12,15 @@ import config from "./config";
 import path from "path";
 import fs from "fs";
 
-var existsCache = {};
-var jsonCache   = {};
+let existsCache = {};
+let jsonCache   = {};
 
 const BABELIGNORE_FILENAME = ".babelignore";
 const BABELRC_FILENAME     = ".babelrc";
 const PACKAGE_FILENAME     = "package.json";
 
 function exists(filename) {
-  var cached = existsCache[filename];
+  let cached = existsCache[filename];
   if (cached != null) {
     return cached;
   } else {
@@ -39,14 +39,14 @@ export default class OptionManager {
   static memoisedPlugins = [];
 
   static memoisePluginContainer(fn, loc, i) {
-    for (var cache of (OptionManager.memoisedPlugins: Array)) {
+    for (let cache of (OptionManager.memoisedPlugins: Array)) {
       if (cache.container === fn) return cache.plugin;
     }
 
-    var obj = fn(context);
+    let obj = fn(context);
 
     if (typeof obj === "object") {
-      var plugin = new Plugin(obj);
+      let plugin = new Plugin(obj);
       OptionManager.memoisedPlugins.push({
         container: fn,
         plugin: plugin
@@ -58,10 +58,10 @@ export default class OptionManager {
   }
 
   static createBareOptions() {
-    var opts = {};
+    let opts = {};
 
-    for (var key in config) {
-      var opt = config[key];
+    for (let key in config) {
+      let opt = config[key];
       opts[key] = clone(opt.default);
     }
 
@@ -70,7 +70,7 @@ export default class OptionManager {
 
   static normalisePlugins(loc, dirname, plugins) {
     return plugins.map(function (val, i) {
-      var plugin, options;
+      let plugin, options;
 
       // destructure plugins
       if (Array.isArray(val)) {
@@ -81,7 +81,7 @@ export default class OptionManager {
 
       // allow plugins to be specified as strings
       if (typeof plugin === "string") {
-        var pluginLoc = resolve(`babel-plugin-${plugin}`, dirname) || resolve(plugin, dirname);
+        let pluginLoc = resolve(`babel-plugin-${plugin}`, dirname) || resolve(plugin, dirname);
         if (pluginLoc) {
           plugin = require(pluginLoc);
         } else {
@@ -106,8 +106,8 @@ export default class OptionManager {
   addConfig(loc, key?, json=json5) {
     if (this.resolvedConfigs.indexOf(loc) >= 0) return;
 
-    var content = fs.readFileSync(loc, "utf8");
-    var opts;
+    let content = fs.readFileSync(loc, "utf8");
+    let opts;
 
     try {
       opts = jsonCache[content] = jsonCache[content] || json.parse(content);
@@ -163,8 +163,8 @@ export default class OptionManager {
       delete opts.presets;
     }
 
-    var envOpts;
-    var envKey = process.env.BABEL_ENV || process.env.NODE_ENV || "development";
+    let envOpts;
+    let envKey = process.env.BABEL_ENV || process.env.NODE_ENV || "development";
     if (opts.env) {
       envOpts = opts.env[envKey];
       delete opts.env;
@@ -178,11 +178,11 @@ export default class OptionManager {
   }
 
   mergePresets(presets: Array, dirname) {
-    for (var val of presets) {
+    for (let val of presets) {
       if (typeof val === "string") {
-        var presetLoc = resolve(`babel-preset-${val}`, dirname) || resolve(val, dirname);
+        let presetLoc = resolve(`babel-preset-${val}`, dirname) || resolve(val, dirname);
         if (presetLoc) {
-          var presetOpts = require(presetLoc);
+          let presetOpts = require(presetLoc);
           this.mergeOptions(presetOpts, presetLoc, presetLoc, path.dirname(presetLoc));
         } else {
           throw new Error("todo");
@@ -196,8 +196,8 @@ export default class OptionManager {
   }
 
   addIgnoreConfig(loc) {
-    var file  = fs.readFileSync(loc, "utf8");
-    var lines = file.split("\n");
+    let file  = fs.readFileSync(loc, "utf8");
+    let lines = file.split("\n");
 
     lines = lines.map(function (line) {
       return line.replace(/#(.*?)$/, "").trim();
@@ -213,18 +213,18 @@ export default class OptionManager {
       loc = path.join(process.cwd(), loc);
     }
 
-    var foundConfig = false;
-    var foundIgnore = false;
+    let foundConfig = false;
+    let foundIgnore = false;
 
     while (loc !== (loc = path.dirname(loc))) {
       if (!foundConfig) {
-        var configLoc = path.join(loc, BABELRC_FILENAME);
+        let configLoc = path.join(loc, BABELRC_FILENAME);
         if (exists(configLoc)) {
           this.addConfig(configLoc);
           foundConfig = true;
         }
 
-        var pkgLoc = path.join(loc, PACKAGE_FILENAME);
+        let pkgLoc = path.join(loc, PACKAGE_FILENAME);
         if (exists(pkgLoc)) {
           this.addConfig(pkgLoc, "babel", JSON);
           foundConfig = true;
@@ -232,7 +232,7 @@ export default class OptionManager {
       }
 
       if (!foundIgnore) {
-        var ignoreLoc = path.join(loc, BABELIGNORE_FILENAME);
+        let ignoreLoc = path.join(loc, BABELIGNORE_FILENAME);
         if (exists(ignoreLoc)) {
           this.addIgnoreConfig(ignoreLoc);
           foundIgnore = true;
@@ -244,11 +244,11 @@ export default class OptionManager {
   }
 
   normaliseOptions() {
-    var opts = this.options;
+    let opts = this.options;
 
     for (let key in config) {
-      var option = config[key];
-      var val    = opts[key];
+      let option = config[key];
+      let val    = opts[key];
 
       // optional
       if (!val && option.optional) continue;
diff --git a/packages/babel-core/src/transformation/file/options/parsers.js b/packages/babel-core/src/transformation/file/options/parsers.js
index c6c6fd26f7..60d2752782 100644
--- a/packages/babel-core/src/transformation/file/options/parsers.js
+++ b/packages/babel-core/src/transformation/file/options/parsers.js
@@ -5,7 +5,7 @@ export function number(val) {
   return +val;
 }
 
-export var filename = slash;
+export let filename = slash;
 
 export function boolean(val) {
   return !!val;
diff --git a/packages/babel-core/src/transformation/helpers/build-binary-assignment-operator-transformer.js b/packages/babel-core/src/transformation/helpers/build-binary-assignment-operator-transformer.js
index 4e60a46e3d..4772176e69 100644
--- a/packages/babel-core/src/transformation/helpers/build-binary-assignment-operator-transformer.js
+++ b/packages/babel-core/src/transformation/helpers/build-binary-assignment-operator-transformer.js
@@ -2,13 +2,13 @@ import explode from "./explode-assignable-expression";
 import * as t from "babel-types";
 
 export default function (opts) {
-  var exports = {};
+  let exports = {};
 
-  var isAssignment = function (node) {
+  let isAssignment = function (node) {
     return node.operator === opts.operator + "=";
   };
 
-  var buildAssignment = function (left, right) {
+  let buildAssignment = function (left, right) {
     return t.assignmentExpression("=", left, right);
   };
 
@@ -16,11 +16,11 @@ export default function (opts) {
     // hit the `AssignmentExpression` one below
     if (path.isCompletionRecord()) return;
 
-    var expr = path.node.expression;
+    let expr = path.node.expression;
     if (!isAssignment(expr)) return;
 
-    var nodes    = [];
-    var exploded = explode(expr.left, nodes, file, path.scope, true);
+    let nodes    = [];
+    let exploded = explode(expr.left, nodes, file, path.scope, true);
 
     nodes.push(t.expressionStatement(
       buildAssignment(exploded.ref, opts.build(exploded.uid, expr.right))
@@ -32,8 +32,8 @@ export default function (opts) {
   exports.AssignmentExpression = function ({ node, scope }, file) {
     if (!isAssignment(node)) return;
 
-    var nodes    = [];
-    var exploded = explode(node.left, nodes, file, scope);
+    let nodes    = [];
+    let exploded = explode(node.left, nodes, file, scope);
     nodes.push(buildAssignment(exploded.ref, opts.build(exploded.uid, node.right)));
     return nodes;
   };
diff --git a/packages/babel-core/src/transformation/helpers/build-comprehension.js b/packages/babel-core/src/transformation/helpers/build-comprehension.js
index 3c491cc15c..2903f5fec4 100644
--- a/packages/babel-core/src/transformation/helpers/build-comprehension.js
+++ b/packages/babel-core/src/transformation/helpers/build-comprehension.js
@@ -1,10 +1,10 @@
 import * as t from "babel-types";
 
 export default function build(node, buildBody) {
-  var self = node.blocks.shift();
+  let self = node.blocks.shift();
   if (!self) return;
 
-  var child = build(node, buildBody);
+  let child = build(node, buildBody);
   if (!child) {
     // last item
     child = buildBody();
diff --git a/packages/babel-core/src/transformation/helpers/build-conditional-assignment-operator-transformer.js b/packages/babel-core/src/transformation/helpers/build-conditional-assignment-operator-transformer.js
index 94ae08d7ca..70275fa570 100644
--- a/packages/babel-core/src/transformation/helpers/build-conditional-assignment-operator-transformer.js
+++ b/packages/babel-core/src/transformation/helpers/build-conditional-assignment-operator-transformer.js
@@ -2,7 +2,7 @@ import explode from "./explode-assignable-expression";
 import * as t from "babel-types";
 
 export default function (exports, opts) {
-  var buildAssignment = function (left, right) {
+  let buildAssignment = function (left, right) {
     return t.assignmentExpression("=", left, right);
   };
 
@@ -10,12 +10,12 @@ export default function (exports, opts) {
     // hit the `AssignmentExpression` one below
     if (path.isCompletionRecord()) return;
 
-    var expr = path.node.expression;
+    let expr = path.node.expression;
     if (!opts.is(expr, file)) return;
 
-    var nodes = [];
+    let nodes = [];
 
-    var exploded = explode(expr.left, nodes, file, path.scope);
+    let exploded = explode(expr.left, nodes, file, path.scope);
 
     nodes.push(t.ifStatement(
       opts.build(exploded.uid, file),
@@ -26,11 +26,11 @@ export default function (exports, opts) {
   };
 
   exports.AssignmentExpression = function (path, file) {
-    var node = path.node;
+    let node = path.node;
     if (!opts.is(node, file)) return;
 
-    var nodes    = [];
-    var exploded = explode(node.left, nodes, file, path.scope);
+    let nodes    = [];
+    let exploded = explode(node.left, nodes, file, path.scope);
 
     nodes.push(t.logicalExpression(
       "&&",
diff --git a/packages/babel-core/src/transformation/helpers/build-react-transformer.js b/packages/babel-core/src/transformation/helpers/build-react-transformer.js
index dc77ac2b2a..6a2c904250 100644
--- a/packages/babel-core/src/transformation/helpers/build-react-transformer.js
+++ b/packages/babel-core/src/transformation/helpers/build-react-transformer.js
@@ -1,4 +1,4 @@
-// Based upon the excellent jsx-transpiler by Ingvar Stepanyan (RReverser)
+// Based upon the excellent jsx-transpiler by Inglet Stepanyan (RReverser)
 // https://github.com/RReverser/jsx-transpiler
 
 // jsx
@@ -10,7 +10,7 @@ import { react } from "babel-types";
 import * as t from "babel-types";
 
 export default function (opts) {
-  var visitor = {};
+  let visitor = {};
 
   visitor.JSXIdentifier = function (node) {
     if (node.name === "this" && this.isReferenced()) {
@@ -39,14 +39,14 @@ export default function (opts) {
 
   visitor.JSXAttribute = {
     enter(node) {
-      var value = node.value;
+      let value = node.value;
       if (t.isLiteral(value) && isString(value.value)) {
         value.value = value.value.replace(/\n\s+/g, " ");
       }
     },
 
     exit(node) {
-      var value = node.value || t.booleanLiteral(true);
+      let value = node.value || t.booleanLiteral(true);
       return t.inherits(t.property("init", node.name, value), node);
     }
   };
@@ -55,17 +55,17 @@ export default function (opts) {
     exit(node, parent, scope, file) {
       parent.children = react.buildChildren(parent);
 
-      var tagExpr = node.name;
-      var args = [];
+      let tagExpr = node.name;
+      let args = [];
 
-      var tagName;
+      let tagName;
       if (t.isIdentifier(tagExpr)) {
         tagName = tagExpr.name;
       } else if (t.isLiteral(tagExpr)) {
         tagName = tagExpr.value;
       }
 
-      var state = {
+      let state = {
         tagExpr: tagExpr,
         tagName: tagName,
         args:    args
@@ -75,7 +75,7 @@ export default function (opts) {
         opts.pre(state, file);
       }
 
-      var attribs = node.attributes;
+      let attribs = node.attributes;
       if (attribs.length) {
         attribs = buildJSXOpeningElementAttributes(attribs, file);
       } else {
@@ -99,11 +99,11 @@ export default function (opts) {
    * all prior attributes to an array for later processing.
    */
 
-  var buildJSXOpeningElementAttributes = function (attribs, file) {
-    var _props = [];
-    var objs = [];
+  let buildJSXOpeningElementAttributes = function (attribs, file) {
+    let _props = [];
+    let objs = [];
 
-    var pushProps = function () {
+    let pushProps = function () {
       if (!_props.length) return;
 
       objs.push(t.objectExpression(_props));
@@ -111,7 +111,7 @@ export default function (opts) {
     };
 
     while (attribs.length) {
-      var prop = attribs.shift();
+      let prop = attribs.shift();
       if (t.isJSXSpreadAttribute(prop)) {
         pushProps();
         objs.push(prop.argument);
@@ -143,7 +143,7 @@ export default function (opts) {
 
   visitor.JSXElement = {
     exit(node) {
-      var callExpr = node.openingElement;
+      let callExpr = node.openingElement;
 
       callExpr.arguments = callExpr.arguments.concat(node.children);
 
diff --git a/packages/babel-core/src/transformation/helpers/call-delegate.js b/packages/babel-core/src/transformation/helpers/call-delegate.js
index d98a4907a6..6fc6009251 100644
--- a/packages/babel-core/src/transformation/helpers/call-delegate.js
+++ b/packages/babel-core/src/transformation/helpers/call-delegate.js
@@ -1,6 +1,6 @@
 import * as t from "babel-types";
 
-var visitor = {
+let visitor = {
   enter(path, state) {
     if (path.isThisExpression() || path.isReferencedIdentifier({ name: "arguments" })) {
       state.found = true;
@@ -14,19 +14,19 @@ var visitor = {
 };
 
 export default function (node, scope) {
-  var container = t.functionExpression(null, [], node.body, node.generator, node.async);
+  let container = t.functionExpression(null, [], node.body, node.generator, node.async);
 
-  var callee = container;
-  var args   = [];
+  let callee = container;
+  let args   = [];
 
-  var state = { found: false };
+  let state = { found: false };
   scope.traverse(node, visitor, state);
   if (state.found) {
     callee = t.memberExpression(container, t.identifier("apply"));
     args = [t.thisExpression(), t.identifier("arguments")];
   }
 
-  var call = t.callExpression(callee, args);
+  let call = t.callExpression(callee, args);
   if (node.generator) call = t.yieldExpression(call, true);
 
   return t.returnStatement(call);
diff --git a/packages/babel-core/src/transformation/helpers/define-map.js b/packages/babel-core/src/transformation/helpers/define-map.js
index 7f8ae70c93..4fc596a5d4 100644
--- a/packages/babel-core/src/transformation/helpers/define-map.js
+++ b/packages/babel-core/src/transformation/helpers/define-map.js
@@ -3,11 +3,11 @@ import has from "lodash/object/has";
 import * as t from "babel-types";
 
 export function push(mutatorMap, node, kind, file) {
-  var alias = t.toKeyAlias(node);
+  let alias = t.toKeyAlias(node);
 
   //
 
-  var map = {};
+  let map = {};
   if (has(mutatorMap, alias)) map = mutatorMap[alias];
   mutatorMap[alias] = map;
 
@@ -23,7 +23,7 @@ export function push(mutatorMap, node, kind, file) {
   }
 
   if (node.decorators) {
-    var decorators = map.decorators = map.decorators || t.arrayExpression([]);
+    let decorators = map.decorators = map.decorators || t.arrayExpression([]);
     decorators.elements = decorators.elements.concat(node.decorators.map(dec => dec.expression).reverse());
   }
 
@@ -44,7 +44,7 @@ export function push(mutatorMap, node, kind, file) {
 }
 
 export function hasComputed(mutatorMap) {
-  for (var key in mutatorMap) {
+  for (let key in mutatorMap) {
     if (mutatorMap[key]._computed) {
       return true;
     }
@@ -53,11 +53,11 @@ export function hasComputed(mutatorMap) {
 }
 
 export function toComputedObjectFromClass(obj) {
-  var objExpr = t.arrayExpression([]);
+  let objExpr = t.arrayExpression([]);
 
-  for (var i = 0; i < obj.properties.length; i++) {
-    var prop = obj.properties[i];
-    var val = prop.value;
+  for (let i = 0; i < obj.properties.length; i++) {
+    let prop = obj.properties[i];
+    let val = prop.value;
     val.properties.unshift(t.property("init", t.identifier("key"), t.toComputedKey(prop)));
     objExpr.elements.push(val);
   }
@@ -66,20 +66,20 @@ export function toComputedObjectFromClass(obj) {
 }
 
 export function toClassObject(mutatorMap) {
-  var objExpr = t.objectExpression([]);
+  let objExpr = t.objectExpression([]);
 
   each(mutatorMap, function (map) {
-    var mapNode = t.objectExpression([]);
+    let mapNode = t.objectExpression([]);
 
-    var propNode = t.property("init", map._key, mapNode, map._computed);
+    let propNode = t.property("init", map._key, mapNode, map._computed);
 
     each(map, function (node, key) {
       if (key[0] === "_") return;
 
-      var inheritNode = node;
+      let inheritNode = node;
       if (t.isMethodDefinition(node) || t.isClassProperty(node)) node = node.value;
 
-      var prop = t.property("init", t.identifier(key), node);
+      let prop = t.property("init", t.identifier(key), node);
       t.inheritsComments(prop, inheritNode);
       t.removeComments(inheritNode);
 
diff --git a/packages/babel-core/src/transformation/helpers/explode-assignable-expression.js b/packages/babel-core/src/transformation/helpers/explode-assignable-expression.js
index c89388616c..eddcc5d4c8 100644
--- a/packages/babel-core/src/transformation/helpers/explode-assignable-expression.js
+++ b/packages/babel-core/src/transformation/helpers/explode-assignable-expression.js
@@ -1,7 +1,7 @@
 import * as t from "babel-types";
 
-var getObjRef = function (node, nodes, file, scope) {
-  var ref;
+let getObjRef = function (node, nodes, file, scope) {
+  let ref;
   if (t.isIdentifier(node)) {
     if (scope.hasBinding(node.name)) {
       // this variable is declared in scope so we can be 100% sure
@@ -26,19 +26,19 @@ var getObjRef = function (node, nodes, file, scope) {
     throw new Error(`We can't explode this node type ${node.type}`);
   }
 
-  var temp = scope.generateUidIdentifierBasedOnNode(ref);
+  let temp = scope.generateUidIdentifierBasedOnNode(ref);
   nodes.push(t.variableDeclaration("var", [
     t.variableDeclarator(temp, ref)
   ]));
   return temp;
 };
 
-var getPropRef = function (node, nodes, file, scope) {
-  var prop = node.property;
-  var key = t.toComputedKey(node, prop);
+let getPropRef = function (node, nodes, file, scope) {
+  let prop = node.property;
+  let key = t.toComputedKey(node, prop);
   if (t.isLiteral(key)) return key;
 
-  var temp = scope.generateUidIdentifierBasedOnNode(prop);
+  let temp = scope.generateUidIdentifierBasedOnNode(prop);
   nodes.push(t.variableDeclaration("var", [
     t.variableDeclarator(temp, prop)
   ]));
@@ -46,21 +46,21 @@ var getPropRef = function (node, nodes, file, scope) {
 };
 
 export default function (node, nodes, file, scope, allowedSingleIdent) {
-  var obj;
+  let obj;
   if (t.isIdentifier(node) && allowedSingleIdent) {
     obj = node;
   } else {
     obj = getObjRef(node, nodes, file, scope);
   }
 
-  var ref, uid;
+  let ref, uid;
 
   if (t.isIdentifier(node)) {
     ref = node;
     uid = obj;
   } else {
-    var prop = getPropRef(node, nodes, file, scope);
-    var computed = node.computed || t.isLiteral(prop);
+    let prop = getPropRef(node, nodes, file, scope);
+    let computed = node.computed || t.isLiteral(prop);
     uid = ref = t.memberExpression(obj, prop, computed);
   }
 
diff --git a/packages/babel-core/src/transformation/helpers/get-function-arity.js b/packages/babel-core/src/transformation/helpers/get-function-arity.js
index ce66f57207..756a102d9a 100644
--- a/packages/babel-core/src/transformation/helpers/get-function-arity.js
+++ b/packages/babel-core/src/transformation/helpers/get-function-arity.js
@@ -1,8 +1,8 @@
 import * as t from "babel-types";
 
 export default function (node) {
-  for (var i = 0; i < node.params.length; i++) {
-    var param = node.params[i];
+  for (let i = 0; i < node.params.length; i++) {
+    let param = node.params[i];
     if (t.isAssignmentPattern(param) || t.isRestElement(param)) {
       return i;
     }
diff --git a/packages/babel-core/src/transformation/helpers/memoise-decorators.js b/packages/babel-core/src/transformation/helpers/memoise-decorators.js
index 1764781df9..d05d4c4e33 100644
--- a/packages/babel-core/src/transformation/helpers/memoise-decorators.js
+++ b/packages/babel-core/src/transformation/helpers/memoise-decorators.js
@@ -1,15 +1,15 @@
 import * as t from "babel-types";
 
 export default function (decorators, scope) {
-  for (var i = 0; i < decorators.length; i++) {
-    var decorator = decorators[i];
-    var expression = decorator.expression;
+  for (let i = 0; i < decorators.length; i++) {
+    let decorator = decorators[i];
+    let expression = decorator.expression;
     if (!t.isMemberExpression(expression)) continue;
 
-    var temp = scope.maybeGenerateMemoised(expression.object);
-    var ref;
+    let temp = scope.maybeGenerateMemoised(expression.object);
+    let ref;
 
-    var nodes = [];
+    let nodes = [];
 
     if (temp) {
       ref = temp;
diff --git a/packages/babel-core/src/transformation/helpers/name-method.js b/packages/babel-core/src/transformation/helpers/name-method.js
index 9b3fc20ebb..df1a25e88c 100644
--- a/packages/babel-core/src/transformation/helpers/name-method.js
+++ b/packages/babel-core/src/transformation/helpers/name-method.js
@@ -2,14 +2,14 @@ import getFunctionArity from "./get-function-arity";
 import * as util from  "../../util";
 import * as t from "babel-types";
 
-var visitor = {
+let visitor = {
   "ReferencedIdentifier|BindingIdentifier"(path, state) {
     // check if this node matches our function id
     if (path.node.name !== state.name) return;
 
     // check that we don't have a local variable declared as that removes the need
     // for the wrapper
-    var localDeclar = path.scope.getBindingIdentifier(state.name);
+    let localDeclar = path.scope.getBindingIdentifier(state.name);
     if (localDeclar !== state.outerDeclar) return;
 
     state.selfReference = true;
@@ -24,9 +24,9 @@ function wrap(state, method, id, scope) {
       scope.rename(id.name);
     } else {
       // need to add a wrapper since we can't change the references
-      var templateName = "property-method-assignment-wrapper";
+      let templateName = "property-method-assignment-wrapper";
       if (method.generator) templateName += "-generator";
-      var template = util.template(templateName, {
+      let template = util.template(templateName, {
         FUNCTION: method,
         FUNCTION_ID: id,
         FUNCTION_KEY: scope.generateUidIdentifier(id.name)
@@ -35,8 +35,8 @@ function wrap(state, method, id, scope) {
 
       // shim in dummy params to retain function arity, if you try to read the
       // source then you'll get the original since it's proxied so it's all good
-      var params = template.callee.body.body[0].params;
-      for (var i = 0, len = getFunctionArity(method); i < len; i++) {
+      let params = template.callee.body.body[0].params;
+      for (let i = 0, len = getFunctionArity(method); i < len; i++) {
         params.push(scope.generateUidIdentifier("x"));
       }
 
@@ -49,7 +49,7 @@ function wrap(state, method, id, scope) {
 }
 
 function visit(node, name, scope) {
-  var state = {
+  let state = {
     selfAssignment: false,
     selfReference:  false,
     outerDeclar:    scope.getBindingIdentifier(name),
@@ -60,13 +60,13 @@ function visit(node, name, scope) {
   // check to see if we have a local binding of the id we're setting inside of
   // the function, this is important as there are caveats associated
 
-  var binding = scope.getOwnBinding(name);
+  let binding = scope.getOwnBinding(name);
 
   if (binding) {
     if (binding.kind === "param") {
       // safari will blow up in strict mode with code like:
       //
-      //   var t = function t(t) {};
+      //   let t = function t(t) {};
       //
       // with the error:
       //
@@ -80,8 +80,8 @@ function visit(node, name, scope) {
     } else {
       // otherwise it's defined somewhere in scope like:
       //
-      //   var t = function () {
-      //     var t = 2;
+      //   let t = function () {
+      //     let t = 2;
       //   };
       //
       // so we can safely just set the id and move along as it shadows the
@@ -95,19 +95,19 @@ function visit(node, name, scope) {
 }
 
 export function custom(node, id, scope) {
-  var state = visit(node, id.name, scope);
+  let state = visit(node, id.name, scope);
   return wrap(state, node, id, scope);
 }
 
 export function property(node, file, scope) {
-  var key = t.toComputedKey(node, node.key);
+  let key = t.toComputedKey(node, node.key);
   if (!t.isLiteral(key)) return; // we can't set a function id with this
 
-  var name = t.toBindingIdentifierName(key.value);
-  var id = t.identifier(name);
+  let name = t.toBindingIdentifierName(key.value);
+  let id = t.identifier(name);
 
-  var method = node.value;
-  var state  = visit(method, name, scope);
+  let method = node.value;
+  let state  = visit(method, name, scope);
   node.value = wrap(state, method, id, scope) || method;
 }
 
@@ -115,16 +115,16 @@ export function bare(node, parent, scope) {
   // has an `id` so we don't need to infer one
   if (node.id) return;
 
-  var id;
+  let id;
   if (t.isProperty(parent) && parent.kind === "init" && (!parent.computed || t.isLiteral(parent.key))) {
     // { foo() {} };
     id = parent.key;
   } else if (t.isVariableDeclarator(parent)) {
-    // var foo = function () {};
+    // let foo = function () {};
     id = parent.id;
 
     if (t.isIdentifier(id)) {
-      var binding = scope.parent.getBinding(id.name);
+      let binding = scope.parent.getBinding(id.name);
       if (binding && binding.constant && scope.getBinding(id.name) === binding) {
         // always going to reference this method
         node.id = id;
@@ -135,7 +135,7 @@ export function bare(node, parent, scope) {
     return;
   }
 
-  var name;
+  let name;
   if (t.isLiteral(id)) {
     name = id.value;
   } else if (t.isIdentifier(id)) {
@@ -147,6 +147,6 @@ export function bare(node, parent, scope) {
   name = t.toBindingIdentifierName(name);
   id = t.identifier(name);
 
-  var state = visit(node, name, scope);
+  let state = visit(node, name, scope);
   return wrap(state, node, id, scope);
 }
diff --git a/packages/babel-core/src/transformation/helpers/regex.js b/packages/babel-core/src/transformation/helpers/regex.js
index 012775591e..c99a183760 100644
--- a/packages/babel-core/src/transformation/helpers/regex.js
+++ b/packages/babel-core/src/transformation/helpers/regex.js
@@ -6,7 +6,7 @@ export function is(node, flag) {
 }
 
 export function pullFlag(node, flag) {
-  var flags = node.flags.split("");
+  let flags = node.flags.split("");
   if (node.flags.indexOf(flag) < 0) return;
   pull(flags, flag);
   node.flags = flags.join("");
diff --git a/packages/babel-core/src/transformation/helpers/remap-async-to-generator.js b/packages/babel-core/src/transformation/helpers/remap-async-to-generator.js
index 49351fdca4..561418e44f 100644
--- a/packages/babel-core/src/transformation/helpers/remap-async-to-generator.js
+++ b/packages/babel-core/src/transformation/helpers/remap-async-to-generator.js
@@ -1,6 +1,6 @@
 import * as t from "babel-types";
 
-var awaitVisitor = {
+let awaitVisitor = {
   Function(path) {
     path.skip();
   },
@@ -16,9 +16,9 @@ var awaitVisitor = {
   }
 };
 
-var referenceVisitor = {
+let referenceVisitor = {
   ReferencedIdentifier({ node, scope }, state) {
-    var name = state.id.name;
+    let name = state.id.name;
     if (node.name === name && scope.bindingIdentifierEquals(name, state.id)) {
       return state.ref = state.ref || scope.generateUidIdentifier(name);
     }
@@ -26,20 +26,20 @@ var referenceVisitor = {
 };
 
 export default function (path, callId) {
-  var node = path.node;
+  let node = path.node;
 
   node.async = false;
   node.generator = true;
 
   path.traverse(awaitVisitor, state);
 
-  var call = t.callExpression(callId, [node]);
+  let call = t.callExpression(callId, [node]);
 
-  var id = node.id;
+  let id = node.id;
   node.id = null;
 
   if (t.isFunctionDeclaration(node)) {
-    var declar = t.variableDeclaration("let", [
+    let declar = t.variableDeclaration("let", [
       t.variableDeclarator(id, call)
     ]);
     declar._blockHoist = true;
@@ -48,7 +48,7 @@ export default function (path, callId) {
     node.type = "FunctionExpression";
 
     if (id) {
-      var state = { id };
+      let state = { id };
       path.traverse(referenceVisitor, state);
 
       if (state.ref) {
diff --git a/packages/babel-core/src/transformation/helpers/replace-supers.js b/packages/babel-core/src/transformation/helpers/replace-supers.js
index 5eb11077fc..8239e9d481 100644
--- a/packages/babel-core/src/transformation/helpers/replace-supers.js
+++ b/packages/babel-core/src/transformation/helpers/replace-supers.js
@@ -13,10 +13,10 @@ function isMemberExpressionSuper(node) {
   return t.isMemberExpression(node) && t.isSuper(node.object);
 }
 
-var visitor = {
+let visitor = {
   enter(path, state) {
-    var topLevel = state.topLevel;
-    var self = state.self;
+    let topLevel = state.topLevel;
+    let self = state.self;
 
     if (path.isFunction() && !path.isArrowFunctionExpression()) {
       // we need to call traverseLevel again so we're context aware
@@ -29,15 +29,15 @@ var visitor = {
       return path.skip();
     }
 
-    var getThisReference = topLevel ?
+    let getThisReference = topLevel ?
       // top level so `this` is the instance
       t.thisExpression :
       // not in the top level so we need to create a reference
       self.getThisReference.bind(self);
 
-    var callback = self.specHandle;
+    let callback = self.specHandle;
     if (self.isLoose) callback = self.looseHandle;
-    var result = callback.call(self, path, getThisReference);
+    let result = callback.call(self, path, getThisReference);
     if (result) path.hasSuper = true;
     if (result === true) return;
     return result;
@@ -119,7 +119,7 @@ export default class ReplaceSupers {
   }
 
   traverseLevel(path: NodePath, topLevel: boolean) {
-    var state = { self: this, topLevel: topLevel };
+    let state = { self: this, topLevel: topLevel };
     path.traverse(visitor, state);
   }
 
@@ -127,7 +127,7 @@ export default class ReplaceSupers {
     if (this.topLevelThisReference) {
       return this.topLevelThisReference;
     } else {
-      var ref = this.topLevelThisReference = this.scope.generateUidIdentifier("this");
+      let ref = this.topLevelThisReference = this.scope.generateUidIdentifier("this");
       this.methodNode.value.body.body.unshift(t.variableDeclaration("var", [
         t.variableDeclarator(this.topLevelThisReference, t.thisExpression())
       ]));
@@ -136,9 +136,9 @@ export default class ReplaceSupers {
   }
 
   getLooseSuperProperty(id: Object, parent: Object) {
-    var methodNode = this.methodNode;
-    var methodName = methodNode.key;
-    var superRef   = this.superRef || t.identifier("Function");
+    let methodNode = this.methodNode;
+    let methodName = methodNode.key;
+    let superRef   = this.superRef || t.identifier("Function");
 
     if (parent.property === id) {
       return;
@@ -175,11 +175,11 @@ export default class ReplaceSupers {
   }
 
   looseHandle(path: NodePath, getThisReference: Function) {
-    var node = path.node;
+    let node = path.node;
     if (path.isSuper()) {
       return this.getLooseSuperProperty(node, path.parent);
     } else if (path.isCallExpression()) {
-      var callee = node.callee;
+      let callee = node.callee;
       if (!t.isMemberExpression(callee)) return;
       if (!t.isSuper(callee.object)) return;
 
@@ -195,7 +195,7 @@ export default class ReplaceSupers {
       // super.name = "val"; -> _set(Object.getPrototypeOf(objectRef.prototype), "name", this);
       return this.setSuperProperty(node.left.property, node.right, node.left.computed, getThisReference());
     } else {
-      // super.age += 2; -> var _ref = super.age; super.age = _ref + 2;
+      // super.age += 2; -> let _ref = super.age; super.age = _ref + 2;
       ref = ref || path.scope.generateUidIdentifier("ref");
       return [
         t.variableDeclaration("var", [
@@ -209,21 +209,21 @@ export default class ReplaceSupers {
   }
 
   specHandle(path: NodePath, getThisReference: Function) {
-    var methodNode = this.methodNode;
-    var property;
-    var computed;
-    var args;
-    var thisReference;
+    let methodNode = this.methodNode;
+    let property;
+    let computed;
+    let args;
+    let thisReference;
 
-    var parent = path.parent;
-    var node = path.node;
+    let parent = path.parent;
+    let node = path.node;
 
     if (isIllegalBareSuper(node, parent)) {
       throw path.buildCodeFrameError(messages.get("classesIllegalBareSuper"));
     }
 
     if (t.isCallExpression(node)) {
-      var callee = node.callee;
+      let callee = node.callee;
       if (t.isSuper(callee)) {
         // super(); -> _get(Object.getPrototypeOf(objectRef), "MethodName", this).call(this);
         property = methodNode.key;
@@ -234,7 +234,7 @@ export default class ReplaceSupers {
         //  - https://esdiscuss.org/topic/super-call-in-methods
         //  - https://twitter.com/wycats/status/544553184396836864
         if (methodNode.key.name !== "constructor" || !this.inClass) {
-          var methodName = methodNode.key.name || "METHOD_NAME";
+          let methodName = methodNode.key.name || "METHOD_NAME";
           throw this.file.buildCodeFrameError(node, messages.get("classesIllegalSuperCall", methodName));
         }
       } else if (isMemberExpressionSuper(callee)) {
@@ -248,13 +248,13 @@ export default class ReplaceSupers {
       property = node.property;
       computed = node.computed;
     } else if (t.isUpdateExpression(node) && isMemberExpressionSuper(node.argument)) {
-      var binary = t.binaryExpression(node.operator[0], node.argument, t.numberLiteral(1));
+      let binary = t.binaryExpression(node.operator[0], node.argument, t.numberLiteral(1));
       if (node.prefix) {
         // ++super.foo; -> super.foo += 1;
         return this.specHandleAssignmentExpression(null, path, binary, getThisReference);
       } else {
-        // super.foo++; -> var _ref = super.foo; super.foo = _ref + 1;
-        var ref = path.scope.generateUidIdentifier("ref");
+        // super.foo++; -> let _ref = super.foo; super.foo = _ref + 1;
+        let ref = path.scope.generateUidIdentifier("ref");
         return this.specHandleAssignmentExpression(ref, path, binary, getThisReference).concat(t.expressionStatement(ref));
       }
     } else if (t.isAssignmentExpression(node) && isMemberExpressionSuper(node.left)) {
@@ -264,7 +264,7 @@ export default class ReplaceSupers {
     if (!property) return;
 
     thisReference = getThisReference();
-    var superProperty = this.getSuperProperty(property, computed, thisReference);
+    let superProperty = this.getSuperProperty(property, computed, thisReference);
     if (args) {
       if (args.length === 1 && t.isSpreadElement(args[0])) {
         // super(...arguments);
diff --git a/packages/babel-core/src/transformation/index.js b/packages/babel-core/src/transformation/index.js
index da4c0dc0f4..3719de4bd0 100644
--- a/packages/babel-core/src/transformation/index.js
+++ b/packages/babel-core/src/transformation/index.js
@@ -1,7 +1,7 @@
 import Pipeline from "./pipeline";
 
-var pipeline = new Pipeline;
-var transform = pipeline.transform.bind(pipeline);
+let pipeline = new Pipeline;
+let transform = pipeline.transform.bind(pipeline);
 transform.fromAst = pipeline.transformFromAst.bind(pipeline);
 transform.lint = pipeline.lint.bind(pipeline);
 transform.pipeline = pipeline;
diff --git a/packages/babel-core/src/transformation/internal-plugins/block-hoist.js b/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
index e033bc39f0..014b9e7711 100644
--- a/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
+++ b/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
@@ -16,9 +16,9 @@ export default new Plugin({
   visitor: {
     Block: {
       exit({ node }) {
-        var hasChange = false;
-        for (var i = 0; i < node.body.length; i++) {
-          var bodyNode = node.body[i];
+        let hasChange = false;
+        for (let i = 0; i < node.body.length; i++) {
+          let bodyNode = node.body[i];
           if (bodyNode && bodyNode._blockHoist != null) {
             hasChange = true;
             break;
@@ -27,7 +27,7 @@ export default new Plugin({
         if (!hasChange) return;
 
         node.body = sortBy(node.body, function(bodyNode){
-          var priority = bodyNode && bodyNode._blockHoist;
+          let priority = bodyNode && bodyNode._blockHoist;
           if (priority == null) priority = 1;
           if (priority === true) priority = 2;
 
diff --git a/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js b/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
index b386a44202..397b48b67a 100644
--- a/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
+++ b/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
@@ -25,13 +25,13 @@ function shouldShadow(path, shadowPath) {
 
 function remap(path, key, create) {
   // ensure that we're shadowed
-  var shadowPath = path.inShadow(key);
+  let shadowPath = path.inShadow(key);
   if (!shouldShadow(path, shadowPath)) return;
 
-  var shadowFunction = path.node._shadowedFunctionLiteral;
-  var currentFunction;
+  let shadowFunction = path.node._shadowedFunctionLiteral;
+  let currentFunction;
 
-  var fnPath = path.findParent(function (path) {
+  let fnPath = path.findParent(function (path) {
     if (path.isProgram() || path.isFunction()) {
       // catch current function in case this is the shadowed one and we can ignore it
       currentFunction = currentFunction || path;
@@ -53,11 +53,11 @@ function remap(path, key, create) {
   // no point in realiasing if we're in this function
   if (fnPath === currentFunction) return;
 
-  var cached = fnPath.getData(key);
+  let cached = fnPath.getData(key);
   if (cached) return cached;
 
-  var init = create();
-  var id   = path.scope.generateUidIdentifier(key);
+  let init = create();
+  let id   = path.scope.generateUidIdentifier(key);
 
   fnPath.setData(key, id);
   fnPath.scope.push({ id, init });
diff --git a/packages/babel-core/src/transformation/modules/_default.js b/packages/babel-core/src/transformation/modules/_default.js
index 73535a5d0e..48dd52bbd0 100644
--- a/packages/babel-core/src/transformation/modules/_default.js
+++ b/packages/babel-core/src/transformation/modules/_default.js
@@ -32,10 +32,10 @@ export default class DefaultFormatter {
   }
 
   addScope(path) {
-    var source = path.node.source && path.node.source.value;
+    let source = path.node.source && path.node.source.value;
     if (!source) return;
 
-    var existingScope = this.sourceScopes[source];
+    let existingScope = this.sourceScopes[source];
     if (existingScope && existingScope !== path.scope) {
       throw path.buildCodeFrameError(messages.get("modulesDuplicateDeclarations"));
     }
@@ -44,7 +44,7 @@ export default class DefaultFormatter {
   }
 
   isModuleType(node, type) {
-    var modules = this.file.dynamicImportTypes[type];
+    let modules = this.file.dynamicImportTypes[type];
     return modules && modules.indexOf(node) >= 0;
   }
 
@@ -57,8 +57,8 @@ export default class DefaultFormatter {
   }
 
   getMetadata() {
-    var has = false;
-    for (var node of (this.file.ast.program.body: Array)) {
+    let has = false;
+    for (let node of (this.file.ast.program.body: Array)) {
       if (t.isModuleDeclaration(node)) {
         has = true;
         break;
@@ -76,9 +76,9 @@ export default class DefaultFormatter {
   }
 
   remapExportAssignment(node, exported) {
-    var assign = node;
+    let assign = node;
 
-    for (var i = 0; i < exported.length; i++) {
+    for (let i = 0; i < exported.length; i++) {
       assign = t.assignmentExpression(
         "=",
         t.memberExpression(t.identifier("exports"), exported[i]),
@@ -90,7 +90,7 @@ export default class DefaultFormatter {
   }
 
   _addExport(name, exported) {
-    var info = this.localExports[name] = this.localExports[name] || {
+    let info = this.localExports[name] = this.localExports[name] || {
       binding: this.scope.getBindingIdentifier(name),
       exported: []
     };
@@ -100,21 +100,21 @@ export default class DefaultFormatter {
   getExport(node, scope) {
     if (!t.isIdentifier(node)) return;
 
-    var local = this.localExports[node.name];
+    let local = this.localExports[node.name];
     if (local && local.binding === scope.getBindingIdentifier(node.name)) {
       return local.exported;
     }
   }
 
   getModuleName() {
-    var opts = this.file.opts;
+    let opts = this.file.opts;
     // moduleId is n/a if a `getModuleId()` is provided
     if (opts.moduleId != null && !opts.getModuleId) {
       return opts.moduleId;
     }
 
-    var filenameRelative = opts.filenameRelative;
-    var moduleName = "";
+    let filenameRelative = opts.filenameRelative;
+    let moduleName = "";
 
     if (opts.moduleRoot != null) {
       moduleName = opts.moduleRoot + "/";
@@ -126,7 +126,7 @@ export default class DefaultFormatter {
 
     if (opts.sourceRoot != null) {
       // remove sourceRoot from filename
-      var sourceRootRegEx = new RegExp("^" + opts.sourceRoot + "\/?");
+      let sourceRootRegEx = new RegExp("^" + opts.sourceRoot + "\/?");
       filenameRelative = filenameRelative.replace(sourceRootRegEx, "");
     }
 
@@ -168,8 +168,8 @@ export default class DefaultFormatter {
   }
 
   getExternalReference(node, nodes) {
-    var ids = this.ids;
-    var id = node.source.value;
+    let ids = this.ids;
+    let id = node.source.value;
 
     if (ids[id]) {
       return ids[id];
@@ -185,7 +185,7 @@ export default class DefaultFormatter {
   }
 
   exportAllDeclaration(node, nodes) {
-    var ref = this.getExternalReference(node, nodes);
+    let ref = this.getExternalReference(node, nodes);
     nodes.push(this.buildExportsWildcard(ref, node));
   }
 
@@ -195,7 +195,7 @@ export default class DefaultFormatter {
 
   exportSpecifier(specifier, node, nodes) {
     if (node.source) {
-      var ref = this.getExternalReference(node, nodes);
+      let ref = this.getExternalReference(node, nodes);
 
       if (specifier.local.name === "default" && !this.noInteropRequireExport) {
         // importing a default so we need to normalize it
@@ -244,28 +244,28 @@ export default class DefaultFormatter {
   }
 
   exportDeclaration(node, nodes) {
-    var declar = node.declaration;
+    let declar = node.declaration;
 
-    var id = declar.id;
+    let id = declar.id;
 
     if (t.isExportDefaultDeclaration(node)) {
       id = t.identifier("default");
     }
 
-    var assign;
+    let assign;
 
     if (t.isVariableDeclaration(declar)) {
-      for (var i = 0; i < declar.declarations.length; i++) {
-        var decl = declar.declarations[i];
+      for (let i = 0; i < declar.declarations.length; i++) {
+        let decl = declar.declarations[i];
 
         decl.init = this.buildExportsAssignment(decl.id, decl.init, node).expression;
 
-        var newDeclar = t.variableDeclaration(declar.kind, [decl]);
+        let newDeclar = t.variableDeclaration(declar.kind, [decl]);
         if (i === 0) t.inherits(newDeclar, declar);
         nodes.push(newDeclar);
       }
     } else {
-      var ref = declar;
+      let ref = declar;
 
       if (t.isFunctionDeclaration(declar) || t.isClassDeclaration(declar)) {
         ref = declar.id;
diff --git a/packages/babel-core/src/transformation/modules/_strict.js b/packages/babel-core/src/transformation/modules/_strict.js
index 02c8aba004..2a615dfc4a 100644
--- a/packages/babel-core/src/transformation/modules/_strict.js
+++ b/packages/babel-core/src/transformation/modules/_strict.js
@@ -1,7 +1,7 @@
 import * as util from "../../util";
 
 export default function (Parent) {
-  var Constructor = function () {
+  let Constructor = function () {
     this.noInteropRequireImport = true;
     this.noInteropRequireExport = true;
     Parent.apply(this, arguments);
diff --git a/packages/babel-core/src/transformation/modules/amd.js b/packages/babel-core/src/transformation/modules/amd.js
index 3a5f5c4341..6f2a1319a4 100644
--- a/packages/babel-core/src/transformation/modules/amd.js
+++ b/packages/babel-core/src/transformation/modules/amd.js
@@ -11,8 +11,8 @@ export default class AMDFormatter extends DefaultFormatter {
   }
 
   buildDependencyLiterals() {
-    var names = [];
-    for (var name in this.ids) {
+    let names = [];
+    for (let name in this.ids) {
       names.push(t.stringLiteral(name));
     }
     return names;
@@ -25,28 +25,28 @@ export default class AMDFormatter extends DefaultFormatter {
   transform(program) {
     CommonFormatter.prototype.transform.apply(this, arguments);
 
-    var body = program.body;
+    let body = program.body;
 
     // build an array of module names
 
-    var names = [t.stringLiteral("exports")];
+    let names = [t.stringLiteral("exports")];
     if (this.passModuleArg) names.push(t.stringLiteral("module"));
     names = names.concat(this.buildDependencyLiterals());
     names = t.arrayExpression(names);
 
     // build up define container
 
-    var params = values(this.ids);
+    let params = values(this.ids);
     if (this.passModuleArg) params.unshift(t.identifier("module"));
     params.unshift(t.identifier("exports"));
 
-    var container = t.functionExpression(null, params, t.blockStatement(body));
+    let container = t.functionExpression(null, params, t.blockStatement(body));
 
-    var defineArgs = [names, container];
-    var moduleName = this.getModuleName();
+    let defineArgs = [names, container];
+    let moduleName = this.getModuleName();
     if (moduleName) defineArgs.unshift(t.stringLiteral(moduleName));
 
-    var call = t.callExpression(t.identifier("define"), defineArgs);
+    let call = t.callExpression(t.identifier("define"), defineArgs);
 
     program.body = [t.expressionStatement(call)];
   }
@@ -73,8 +73,8 @@ export default class AMDFormatter extends DefaultFormatter {
   }
 
   importSpecifier(specifier, node, nodes, scope) {
-    var key = node.source.value;
-    var ref = this.getExternalReference(node);
+    let key = node.source.value;
+    let ref = this.getExternalReference(node);
 
     if (t.isImportNamespaceSpecifier(specifier) || t.isImportDefaultSpecifier(specifier)) {
       this.defaultIds[key] = specifier.local;
@@ -90,14 +90,14 @@ export default class AMDFormatter extends DefaultFormatter {
       // import * as bar from "foo";
     } else if (!includes(this.file.dynamicImported, node) && t.isSpecifierDefault(specifier) && !this.noInteropRequireImport) {
       // import foo from "foo";
-      var uid = scope.generateUidIdentifier(specifier.local.name);
+      let uid = scope.generateUidIdentifier(specifier.local.name);
       nodes.push(t.variableDeclaration("var", [
         t.variableDeclarator(uid, t.callExpression(this.file.addHelper("interop-require-default"), [ref]))
       ]));
       ref = t.memberExpression(uid, t.identifier("default"));
     } else {
       // import { foo } from "foo";
-      var imported = specifier.imported;
+      let imported = specifier.imported;
       if (t.isSpecifierDefault(specifier)) imported = t.identifier("default");
       ref = t.memberExpression(ref, imported);
     }
@@ -124,8 +124,8 @@ export default class AMDFormatter extends DefaultFormatter {
     if (this.doDefaultExportInterop(node)) {
       this.passModuleArg = true;
 
-      var declar = node.declaration;
-      var assign = util.template("exports-default-assign", {
+      let declar = node.declaration;
+      let assign = util.template("exports-default-assign", {
         VALUE: this._pushStatement(declar, nodes)
       }, true);
 
diff --git a/packages/babel-core/src/transformation/modules/common.js b/packages/babel-core/src/transformation/modules/common.js
index b6b18acf0c..1f81d20b68 100644
--- a/packages/babel-core/src/transformation/modules/common.js
+++ b/packages/babel-core/src/transformation/modules/common.js
@@ -8,16 +8,16 @@ export default class CommonJSFormatter extends DefaultFormatter {
   }
 
   _setup(conditional) {
-    var file  = this.file;
-    var scope = file.scope;
+    let file  = this.file;
+    let scope = file.scope;
 
     scope.rename("module");
     scope.rename("exports");
 
     if (!this.noInteropRequireImport && conditional) {
-      var templateName = "exports-module-declaration";
+      let templateName = "exports-module-declaration";
       if (this.file.isLoose("es6.modules")) templateName += "-loose";
-      var declar = util.template(templateName, true);
+      let declar = util.template(templateName, true);
       declar._blockHoist = 3;
       file.path.unshiftContainer("body", [declar]);
     }
@@ -38,9 +38,9 @@ export default class CommonJSFormatter extends DefaultFormatter {
   }
 
   importSpecifier(specifier, node, nodes, scope) {
-    var variableName = specifier.local;
+    let variableName = specifier.local;
 
-    var ref = this.getExternalReference(node, nodes);
+    let ref = this.getExternalReference(node, nodes);
 
     // import foo from "foo";
     if (t.isSpecifierDefault(specifier)) {
@@ -51,7 +51,7 @@ export default class CommonJSFormatter extends DefaultFormatter {
       } else if (this.noInteropRequireImport) {
         this.remaps.add(scope, variableName.name, t.memberExpression(ref, t.identifier("default")));
       } else {
-        var uid = this.scope.generateUidIdentifierBasedOnNode(node, "import");
+        let uid = this.scope.generateUidIdentifierBasedOnNode(node, "import");
 
         nodes.push(t.variableDeclaration("var", [
           t.variableDeclarator(uid, t.callExpression(this.file.addHelper("interop-require-default"), [ref]))
@@ -101,8 +101,8 @@ export default class CommonJSFormatter extends DefaultFormatter {
   }
 
   _getExternalReference(node, nodes) {
-    var call = t.callExpression(t.identifier("require"), [node.source]);
-    var uid;
+    let call = t.callExpression(t.identifier("require"), [node.source]);
+    let uid;
 
     if (this.isModuleType(node, "absolute")) {
       // absolute module reference
@@ -114,7 +114,7 @@ export default class CommonJSFormatter extends DefaultFormatter {
 
     uid = uid || node.specifiers[0].local;
 
-    var declar = t.variableDeclaration("var", [
+    let declar = t.variableDeclaration("var", [
       t.variableDeclarator(uid, call)
     ]);
     nodes.push(declar);
diff --git a/packages/babel-core/src/transformation/modules/ignore.js b/packages/babel-core/src/transformation/modules/ignore.js
index aaef9dfb27..98056791a7 100644
--- a/packages/babel-core/src/transformation/modules/ignore.js
+++ b/packages/babel-core/src/transformation/modules/ignore.js
@@ -3,7 +3,7 @@ import * as t from "babel-types";
 
 export default class IgnoreFormatter extends DefaultFormatter {
   exportDeclaration(node, nodes) {
-    var declar = t.toStatement(node.declaration, true);
+    let declar = t.toStatement(node.declaration, true);
     if (declar) nodes.push(t.inherits(declar, node));
   }
 
diff --git a/packages/babel-core/src/transformation/modules/lib/metadata.js b/packages/babel-core/src/transformation/modules/lib/metadata.js
index e22f9aa0a0..4d248ada07 100644
--- a/packages/babel-core/src/transformation/modules/lib/metadata.js
+++ b/packages/babel-core/src/transformation/modules/lib/metadata.js
@@ -1,7 +1,7 @@
 import extend from "lodash/object/extend";
 import * as t from "babel-types";
 
-export var ModuleDeclaration = {
+export let ModuleDeclaration = {
   enter(node, parent, scope, formatter) {
     if (node.source) {
       node.source.value = formatter.file.resolveModuleSource(node.source.value);
@@ -10,23 +10,23 @@ export var ModuleDeclaration = {
   }
 };
 
-export var ImportDeclaration = {
+export let ImportDeclaration = {
   exit(node, parent, scope, formatter) {
     formatter.hasLocalImports = true;
 
-    var specifiers = [];
-    var imported = [];
+    let specifiers = [];
+    let imported = [];
     formatter.metadata.imports.push({
       source: node.source.value,
       imported,
       specifiers
     });
 
-    for (var specifier of (this.get("specifiers"): Array)) {
-      var ids = specifier.getBindingIdentifiers();
+    for (let specifier of (this.get("specifiers"): Array)) {
+      let ids = specifier.getBindingIdentifiers();
       extend(formatter.localImports, ids);
 
-      var local = specifier.node.local.name;
+      let local = specifier.node.local.name;
 
       if (specifier.isImportDefaultSpecifier()) {
         imported.push("default");
@@ -38,7 +38,7 @@ export var ImportDeclaration = {
       }
 
       if (specifier.isImportSpecifier()) {
-        var importedName = specifier.node.imported.name;
+        let importedName = specifier.node.imported.name;
         imported.push(importedName);
         specifiers.push({
           kind: "named",
@@ -61,17 +61,17 @@ export var ImportDeclaration = {
 export function ExportDeclaration(node, parent, scope, formatter) {
   formatter.hasLocalExports = true;
 
-  var source = node.source ? node.source.value : null;
-  var exports = formatter.metadata.exports;
+  let source = node.source ? node.source.value : null;
+  let exports = formatter.metadata.exports;
 
   // export function foo() {}
-  // export var foo = "bar";
-  var declar = this.get("declaration");
+  // export let foo = "bar";
+  let declar = this.get("declaration");
   if (declar.isStatement()) {
-    var bindings = declar.getBindingIdentifiers();
+    let bindings = declar.getBindingIdentifiers();
 
-    for (var name in bindings) {
-      var binding = bindings[name];
+    for (let name in bindings) {
+      let binding = bindings[name];
       formatter._addExport(name, binding);
 
       exports.exported.push(name);
@@ -84,8 +84,8 @@ export function ExportDeclaration(node, parent, scope, formatter) {
   }
 
   if (this.isExportNamedDeclaration() && node.specifiers) {
-    for (var specifier of (node.specifiers: Array)) {
-      var exported = specifier.exported.name;
+    for (let specifier of (node.specifiers: Array)) {
+      let exported = specifier.exported.name;
       exports.exported.push(exported);
 
       // export foo from "bar";
@@ -107,7 +107,7 @@ export function ExportDeclaration(node, parent, scope, formatter) {
         });
       }
 
-      var local = specifier.local;
+      let local = specifier.local;
       if (!local) continue;
 
       formatter._addExport(local.name, specifier.exported);
@@ -144,7 +144,7 @@ export function ExportDeclaration(node, parent, scope, formatter) {
   }
 
   if (!t.isExportDefaultDeclaration(node) && !declar.isTypeAlias()) {
-    var onlyDefault = node.specifiers && node.specifiers.length === 1 && t.isSpecifierDefault(node.specifiers[0]);
+    let onlyDefault = node.specifiers && node.specifiers.length === 1 && t.isSpecifierDefault(node.specifiers[0]);
     if (!onlyDefault) {
       formatter.hasNonDefaultExports = true;
     }
diff --git a/packages/babel-core/src/transformation/modules/lib/remaps.js b/packages/babel-core/src/transformation/modules/lib/remaps.js
index dcd4737124..9e8ee531a3 100644
--- a/packages/babel-core/src/transformation/modules/lib/remaps.js
+++ b/packages/babel-core/src/transformation/modules/lib/remaps.js
@@ -1,6 +1,6 @@
 import * as t from "babel-types";
 
-var remapVisitor = {
+let remapVisitor = {
   enter(node) {
     if (node._skipModulesRemap) {
       return this.skip();
@@ -8,9 +8,9 @@ var remapVisitor = {
   },
 
   ReferencedIdentifier(node, parent, scope, remaps) {
-    var { formatter } = remaps;
+    let { formatter } = remaps;
 
-    var remap = remaps.get(scope, node.name);
+    let remap = remaps.get(scope, node.name);
     if (!remap || node === remap) return;
 
     if (!scope.hasBinding(node.name) ||
@@ -26,7 +26,7 @@ var remapVisitor = {
   AssignmentExpression: {
     exit(node, parent, scope, { formatter }) {
       if (!node._ignoreModulesRemap) {
-        var exported = formatter.getExport(node.left, scope);
+        let exported = formatter.getExport(node.left, scope);
         if (exported) {
           return formatter.remapExportAssignment(node, exported);
         }
@@ -35,26 +35,26 @@ var remapVisitor = {
   },
 
   UpdateExpression(node, parent, scope, { formatter }) {
-    var exported = formatter.getExport(node.argument, scope);
+    let exported = formatter.getExport(node.argument, scope);
     if (!exported) return;
 
     this.skip();
 
     // expand to long file assignment expression
-    var assign = t.assignmentExpression(node.operator[0] + "=", node.argument, t.numberLiteral(1));
+    let assign = t.assignmentExpression(node.operator[0] + "=", node.argument, t.numberLiteral(1));
 
     // remap this assignment expression
-    var remapped = formatter.remapExportAssignment(assign, exported);
+    let remapped = formatter.remapExportAssignment(assign, exported);
 
     // we don't need to change the result
     if (t.isExpressionStatement(parent) || node.prefix) {
       return remapped;
     }
 
-    var nodes = [];
+    let nodes = [];
     nodes.push(remapped);
 
-    var operator;
+    let operator;
     if (node.operator === "--") {
       operator = "+";
     } else { // "++"
@@ -112,7 +112,7 @@ export default class Remaps {
 
   clearAll() {
     if (this.all) {
-      for (var remap of (this.all: Array)) {
+      for (let remap of (this.all: Array)) {
         remap.scope.removeData(this._getKey(remap.name));
       }
     }
diff --git a/packages/babel-core/src/transformation/modules/system.js b/packages/babel-core/src/transformation/modules/system.js
index a5b051efe3..18817b5600 100644
--- a/packages/babel-core/src/transformation/modules/system.js
+++ b/packages/babel-core/src/transformation/modules/system.js
@@ -5,7 +5,7 @@ import last from "lodash/array/last";
 import map from "lodash/collection/map";
 import * as t from "babel-types";
 
-var hoistVariablesVisitor = {
+let hoistVariablesVisitor = {
   Function(path) {
     // nothing inside is accessible
     path.skip();
@@ -20,19 +20,19 @@ var hoistVariablesVisitor = {
     // ignore block hoisted nodes as these can be left in
     if (state.formatter._canHoist(node)) return;
 
-    var nodes = [];
+    let nodes = [];
 
-    for (var i = 0; i < node.declarations.length; i++) {
-      var declar = node.declarations[i];
+    for (let i = 0; i < node.declarations.length; i++) {
+      let declar = node.declarations[i];
       state.hoistDeclarators.push(t.variableDeclarator(declar.id));
       if (declar.init) {
         // no initializer so we can just hoist it as-is
-        var assign = t.expressionStatement(t.assignmentExpression("=", declar.id, declar.init));
+        let assign = t.expressionStatement(t.assignmentExpression("=", declar.id, declar.init));
         nodes.push(assign);
       }
     }
 
-    // for (var i in test)
+    // for (let i in test)
     if (t.isFor(parent) && parent.left === node) {
       return node.declarations[0].id;
     }
@@ -41,7 +41,7 @@ var hoistVariablesVisitor = {
   }
 };
 
-var hoistFunctionsVisitor = {
+let hoistFunctionsVisitor = {
   Function(path) {
     path.skip();
   },
@@ -54,13 +54,13 @@ var hoistFunctionsVisitor = {
   }
 };
 
-var runnerSettersVisitor = {
+let runnerSettersVisitor = {
   enter(path, state) {
-    var { node } = path;
+    let { node } = path;
 
     if (node._importSource === state.source) {
       if (t.isVariableDeclaration(node)) {
-        for (var declar of (node.declarations: Array)) {
+        for (let declar of (node.declarations: Array)) {
           state.hoistDeclarators.push(t.variableDeclarator(declar.id));
           state.nodes.push(t.expressionStatement(
             t.assignmentExpression("=", declar.id, declar.init)
@@ -93,16 +93,16 @@ export default class SystemFormatter extends AMDFormatter {
   }
 
   buildExportsWildcard(objectIdentifier, node) {
-    var leftIdentifier = this.scope.generateUidIdentifier("key");
-    var valIdentifier  = t.memberExpression(objectIdentifier, leftIdentifier, true);
+    let leftIdentifier = this.scope.generateUidIdentifier("key");
+    let valIdentifier  = t.memberExpression(objectIdentifier, leftIdentifier, true);
 
-    var left = t.variableDeclaration("var", [
+    let left = t.variableDeclaration("var", [
       t.variableDeclarator(leftIdentifier)
     ]);
 
-    var right = objectIdentifier;
+    let right = objectIdentifier;
 
-    var block = t.blockStatement([
+    let block = t.blockStatement([
       t.ifStatement(
         t.binaryExpression("!==", leftIdentifier, t.stringLiteral("default")),
         t.expressionStatement(this._buildExportCall(leftIdentifier, valIdentifier))
@@ -113,7 +113,7 @@ export default class SystemFormatter extends AMDFormatter {
   }
 
   buildExportsAssignment(id, init, node) {
-    var call = this._buildExportCall(t.stringLiteral(id.name), init, true);
+    let call = this._buildExportCall(t.stringLiteral(id.name), init, true);
     return this._addImportSource(call, node);
   }
 
@@ -122,9 +122,9 @@ export default class SystemFormatter extends AMDFormatter {
   }
 
   remapExportAssignment(node, exported) {
-    var assign = node;
+    let assign = node;
 
-    for (var i = 0; i < exported.length; i++) {
+    for (let i = 0; i < exported.length; i++) {
       assign = this._buildExportCall(t.stringLiteral(exported[i].name), assign);
     }
 
@@ -132,7 +132,7 @@ export default class SystemFormatter extends AMDFormatter {
   }
 
   _buildExportCall(id, init, isStatement) {
-    var call = t.callExpression(this.exportIdentifier, [id, init]);
+    let call = t.callExpression(this.exportIdentifier, [id, init]);
     if (isStatement) {
       return t.expressionStatement(call);
     } else {
@@ -143,7 +143,7 @@ export default class SystemFormatter extends AMDFormatter {
   importSpecifier(specifier, node, nodes) {
     AMDFormatter.prototype.importSpecifier.apply(this, arguments);
 
-    for (var remap of (this.remaps.getAll(): Array)) {
+    for (let remap of (this.remaps.getAll(): Array)) {
       nodes.push(t.variableDeclaration("var", [
         t.variableDeclarator(t.identifier(remap.name), remap.node)
       ]));
@@ -155,10 +155,10 @@ export default class SystemFormatter extends AMDFormatter {
   }
 
   _buildRunnerSetters(block, hoistDeclarators) {
-    var scope = this.file.scope;
+    let scope = this.file.scope;
 
     return t.arrayExpression(map(this.ids, function (uid, source) {
-      var state = {
+      let state = {
         hoistDeclarators: hoistDeclarators,
         source:           source,
         nodes:            []
@@ -177,16 +177,16 @@ export default class SystemFormatter extends AMDFormatter {
   transform(program) {
     DefaultFormatter.prototype.transform.apply(this, arguments);
 
-    var hoistDeclarators = [];
-    var moduleName = this.getModuleName();
-    var moduleNameLiteral = t.stringLiteral(moduleName);
+    let hoistDeclarators = [];
+    let moduleName = this.getModuleName();
+    let moduleNameLiteral = t.stringLiteral(moduleName);
 
-    var block = t.blockStatement(program.body);
+    let block = t.blockStatement(program.body);
 
-    var setterListNode = this._buildRunnerSetters(block, hoistDeclarators);
+    let setterListNode = this._buildRunnerSetters(block, hoistDeclarators);
     this._setters = setterListNode;
 
-    var runner = util.template("system", {
+    let runner = util.template("system", {
       MODULE_DEPENDENCIES: t.arrayExpression(this.buildDependencyLiterals()),
       EXPORT_IDENTIFIER:   this.exportIdentifier,
       MODULE_NAME:         moduleNameLiteral,
@@ -194,10 +194,10 @@ export default class SystemFormatter extends AMDFormatter {
       EXECUTE:             t.functionExpression(null, [], block)
     }, true);
 
-    var handlerBody = runner.expression.arguments[2].body.body;
+    let handlerBody = runner.expression.arguments[2].body.body;
     if (!moduleName) runner.expression.arguments.shift();
 
-    var returnStatement = handlerBody.pop();
+    let returnStatement = handlerBody.pop();
 
     // hoist up all variable declarations
     this.file.scope.traverse(block, hoistVariablesVisitor, {
@@ -206,7 +206,7 @@ export default class SystemFormatter extends AMDFormatter {
     });
 
     if (hoistDeclarators.length) {
-      var hoistDeclar = t.variableDeclaration("var", hoistDeclarators);
+      let hoistDeclar = t.variableDeclaration("var", hoistDeclarators);
       hoistDeclar._blockHoist = true;
       handlerBody.unshift(hoistDeclar);
     }
diff --git a/packages/babel-core/src/transformation/modules/umd.js b/packages/babel-core/src/transformation/modules/umd.js
index a6f83c1911..60244a8fb2 100644
--- a/packages/babel-core/src/transformation/modules/umd.js
+++ b/packages/babel-core/src/transformation/modules/umd.js
@@ -9,38 +9,38 @@ export default class UMDFormatter extends AMDFormatter {
   transform(program) {
     DefaultFormatter.prototype.transform.apply(this, arguments);
 
-    var body = program.body;
+    let body = program.body;
 
     // build an array of module names
 
-    var names = [];
+    let names = [];
     for (let name in this.ids) {
       names.push(t.stringLiteral(name));
     }
 
     // factory
 
-    var ids  = values(this.ids);
-    var args = [t.identifier("exports")];
+    let ids  = values(this.ids);
+    let args = [t.identifier("exports")];
     if (this.passModuleArg) args.push(t.identifier("module"));
     args = args.concat(ids);
 
-    var factory = t.functionExpression(null, args, t.blockStatement(body));
+    let factory = t.functionExpression(null, args, t.blockStatement(body));
 
     // amd
 
-    var defineArgs = [t.stringLiteral("exports")];
+    let defineArgs = [t.stringLiteral("exports")];
     if (this.passModuleArg) defineArgs.push(t.stringLiteral("module"));
     defineArgs = defineArgs.concat(names);
     defineArgs = [t.arrayExpression(defineArgs)];
 
     // common
 
-    var testExports = util.template("test-exports");
-    var testModule  = util.template("test-module");
-    var commonTests = this.passModuleArg ? t.logicalExpression("&&", testExports, testModule) : testExports;
+    let testExports = util.template("test-exports");
+    let testModule  = util.template("test-module");
+    let commonTests = this.passModuleArg ? t.logicalExpression("&&", testExports, testModule) : testExports;
 
-    var commonArgs = [t.identifier("exports")];
+    let commonArgs = [t.identifier("exports")];
     if (this.passModuleArg) commonArgs.push(t.identifier("module"));
     commonArgs = commonArgs.concat(names.map(function (name) {
       return t.callExpression(t.identifier("require"), [name]);
@@ -48,25 +48,25 @@ export default class UMDFormatter extends AMDFormatter {
 
     // globals
 
-    var browserArgs = [];
+    let browserArgs = [];
     if (this.passModuleArg) browserArgs.push(t.identifier("mod"));
 
     for (let name in this.ids) {
-      var id = this.defaultIds[name] || t.identifier(t.toIdentifier(path.basename(name, path.extname(name))));
+      let id = this.defaultIds[name] || t.identifier(t.toIdentifier(path.basename(name, path.extname(name))));
       browserArgs.push(t.memberExpression(t.identifier("global"), id));
     }
 
     //
 
-    var moduleName = this.getModuleName();
+    let moduleName = this.getModuleName();
     if (moduleName) defineArgs.unshift(t.stringLiteral(moduleName));
 
     //
-    var globalArg = this.file.opts.basename;
+    let globalArg = this.file.opts.basename;
     if (moduleName) globalArg = moduleName;
     globalArg = t.identifier(t.toIdentifier(globalArg));
 
-    var runner = util.template("umd-runner-body", {
+    let runner = util.template("umd-runner-body", {
       AMD_ARGUMENTS: defineArgs,
       COMMON_TEST: commonTests,
       COMMON_ARGUMENTS: commonArgs,
diff --git a/packages/babel-core/src/transformation/pipeline.js b/packages/babel-core/src/transformation/pipeline.js
index 460861ef74..9b7abaf207 100644
--- a/packages/babel-core/src/transformation/pipeline.js
+++ b/packages/babel-core/src/transformation/pipeline.js
@@ -9,7 +9,7 @@ export default class Pipeline {
   }
 
   pretransform(code: string, opts?: Object) {
-    var file = new File(opts, this);
+    let file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
       file.parseCode(code);
@@ -18,7 +18,7 @@ export default class Pipeline {
   }
 
   transform(code: string, opts?: Object) {
-    var file = new File(opts, this);
+    let file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
       file.parseCode(code);
@@ -29,7 +29,7 @@ export default class Pipeline {
   transformFromAst(ast, code, opts) {
     ast = normalizeAst(ast);
 
-    var file = new File(opts, this);
+    let file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
       file.addAst(ast);
diff --git a/packages/babel-core/src/transformation/plugin-pass.js b/packages/babel-core/src/transformation/plugin-pass.js
index c1b560041c..5ef78e0a69 100644
--- a/packages/babel-core/src/transformation/plugin-pass.js
+++ b/packages/babel-core/src/transformation/plugin-pass.js
@@ -10,7 +10,7 @@ export default class PluginPass {
   }
 
   transform() {
-    var file = this.file;
+    let file = this.file;
     file.log.debug(`Start transformer ${this.key}`);
     traverse(file.ast, this.plugin.visitor, file.scope, file);
     file.log.debug(`Finish transformer ${this.key}`);
diff --git a/packages/babel-core/src/transformation/plugin.js b/packages/babel-core/src/transformation/plugin.js
index c960ffbda1..1ba7c3b135 100644
--- a/packages/babel-core/src/transformation/plugin.js
+++ b/packages/babel-core/src/transformation/plugin.js
@@ -8,7 +8,7 @@ export default class Plugin {
     plugin = this.raw = assign({}, plugin);
 
     function take(key) {
-      var val = plugin[key];
+      let val = plugin[key];
       delete plugin[key];
       return val;
     }
diff --git a/packages/babel-core/src/transformation/transformers/es5/properties.mutators.js b/packages/babel-core/src/transformation/transformers/es5/properties.mutators.js
index d326bdf0b8..344e1d081b 100644
--- a/packages/babel-core/src/transformation/transformers/es5/properties.mutators.js
+++ b/packages/babel-core/src/transformation/transformers/es5/properties.mutators.js
@@ -8,7 +8,7 @@ import * as t from "babel-types";
  * **In**
  *
  * ```javascript
- * var foo = {
+ * let foo = {
  *   get bar() {
  *     return "bar";
  *   }
@@ -18,7 +18,7 @@ import * as t from "babel-types";
  * **Out**
  *
  * ```javascript
- * var foo = Object.defineProperties({}, {
+ * let foo = Object.defineProperties({}, {
  *   bar: {
  *     get: function () {
  *       return "bar";
@@ -30,7 +30,7 @@ import * as t from "babel-types";
  * ```
  */
 
-export var visitor = {
+export let visitor = {
   /**
    * Look for getters and setters on an object.
    * Filter them out and wrap the object with an `Object.defineProperties` that
@@ -38,8 +38,8 @@ export var visitor = {
    */
 
   ObjectExpression(node, parent, scope, file) {
-    var hasAny = false;
-    for (var prop of (node.properties: Array)) {
+    let hasAny = false;
+    for (let prop of (node.properties: Array)) {
       if (prop.kind === "get" || prop.kind === "set") {
         hasAny = true;
         break;
@@ -47,7 +47,7 @@ export var visitor = {
     }
     if (!hasAny) return;
 
-    var mutatorMap = {};
+    let mutatorMap = {};
 
     node.properties = node.properties.filter(function (prop) {
       if (prop.kind === "get" || prop.kind === "set") {
diff --git a/packages/babel-core/src/transformation/transformers/es6/block-scoping/index.js b/packages/babel-core/src/transformation/transformers/es6/block-scoping/index.js
index 435406b0b9..71691c584f 100644
--- a/packages/babel-core/src/transformation/transformers/es6/block-scoping/index.js
+++ b/packages/babel-core/src/transformation/transformers/es6/block-scoping/index.js
@@ -15,8 +15,8 @@ function isLet(node, parent) {
 
   // https://github.com/babel/babel/issues/255
   if (isLetInitable(node, parent)) {
-    for (var i = 0; i < node.declarations.length; i++) {
-      var declar = node.declarations[i];
+    for (let i = 0; i < node.declarations.length; i++) {
+      let declar = node.declarations[i];
       declar.init = declar.init || scope.buildUndefinedNode();
     }
   }
@@ -35,26 +35,26 @@ function isVar(node, parent, scope) {
 }
 
 function standardizeLets(declars) {
-  for (var declar of (declars: Array)) {
+  for (let declar of (declars: Array)) {
     delete declar._let;
   }
 }
 
-export var metadata = {
+export let metadata = {
   group: "builtin-advanced"
 };
 
-export var visitor = {
+export let visitor = {
   VariableDeclaration(node, parent, scope, file) {
     if (!isLet(node, parent, scope)) return;
 
     if (isLetInitable(node) && node._tdzThis) {
-      var nodes = [node];
+      let nodes = [node];
 
-      for (var i = 0; i < node.declarations.length; i++) {
-        var decl = node.declarations[i];
+      for (let i = 0; i < node.declarations.length; i++) {
+        let decl = node.declarations[i];
         if (decl.init) {
-          var assign = t.assignmentExpression("=", decl.id, decl.init);
+          let assign = t.assignmentExpression("=", decl.id, decl.init);
           assign._ignoreBlockScopingTDZ = true;
           nodes.push(t.expressionStatement(assign));
         }
@@ -74,29 +74,29 @@ export var visitor = {
   },
 
   Loop(node, parent, scope, file) {
-    var init = node.left || node.init;
+    let init = node.left || node.init;
     if (isLet(init, node, scope)) {
       t.ensureBlock(node);
       node.body._letDeclarators = [init];
     }
 
-    var blockScoping = new BlockScoping(this, this.get("body"), parent, scope, file);
+    let blockScoping = new BlockScoping(this, this.get("body"), parent, scope, file);
     return blockScoping.run();
   },
 
   "BlockStatement|Program"(block, parent, scope, file) {
     if (!t.isLoop(parent)) {
-      var blockScoping = new BlockScoping(null, this, parent, scope, file);
+      let blockScoping = new BlockScoping(null, this, parent, scope, file);
       blockScoping.run();
     }
   }
 };
 
 function replace(node, parent, scope, remaps) {
-  var remap = remaps[node.name];
+  let remap = remaps[node.name];
   if (!remap) return;
 
-  var ownBinding = scope.getBindingIdentifier(node.name);
+  let ownBinding = scope.getBindingIdentifier(node.name);
   if (ownBinding === remap.binding) {
     node.name = remap.uid;
   } else {
@@ -106,12 +106,12 @@ function replace(node, parent, scope, remaps) {
   }
 }
 
-var replaceVisitor = {
+let replaceVisitor = {
   ReferencedIdentifier: replace,
 
   AssignmentExpression(node, parent, scope, remaps) {
-    var ids = this.getBindingIdentifiers();
-    for (var name in ids) {
+    let ids = this.getBindingIdentifiers();
+    for (let name in ids) {
       replace(ids[name], node, scope, remaps);
     }
   },
@@ -123,8 +123,8 @@ function traverseReplace(node, parent, scope, remaps) {
   }
 
   if (t.isAssignmentExpression(node)) {
-    var ids = t.getBindingIdentifiers(node);
-    for (var name in ids) {
+    let ids = t.getBindingIdentifiers(node);
+    for (let name in ids) {
       replace(ids[name], parent, scope, remaps);
     }
   }
@@ -132,34 +132,34 @@ function traverseReplace(node, parent, scope, remaps) {
   scope.traverse(node, replaceVisitor, remaps);
 }
 
-var letReferenceBlockVisitor = traverse.visitors.merge([{
+let letReferenceBlockVisitor = traverse.visitors.merge([{
   Function(node, parent, scope, state) {
     this.traverse(letReferenceFunctionVisitor, state);
     return this.skip();
   }
 }, tdzVisitor]);
 
-var letReferenceFunctionVisitor = traverse.visitors.merge([{
+let letReferenceFunctionVisitor = traverse.visitors.merge([{
   ReferencedIdentifier(node, parent, scope, state) {
-    var ref = state.letReferences[node.name];
+    let ref = state.letReferences[node.name];
 
     // not a part of our scope
     if (!ref) return;
 
     // this scope has a variable with the same name so it couldn't belong
     // to our let scope
-    var localBinding = scope.getBindingIdentifier(node.name);
+    let localBinding = scope.getBindingIdentifier(node.name);
     if (localBinding && localBinding !== ref) return;
 
     state.closurify = true;
   }
 }, tdzVisitor]);
 
-var hoistVarDeclarationsVisitor = {
+let hoistVarDeclarationsVisitor = {
   enter(node, parent, scope, self) {
     if (this.isForStatement()) {
       if (isVar(node.init, node, scope)) {
-        var nodes = self.pushDeclar(node.init);
+        let nodes = self.pushDeclar(node.init);
         if (nodes.length === 1) {
           node.init = nodes[0];
         } else {
@@ -179,17 +179,17 @@ var hoistVarDeclarationsVisitor = {
   }
 };
 
-var loopLabelVisitor = {
+let loopLabelVisitor = {
   LabeledStatement(node, parent, scope, state) {
     state.innerLabels.push(node.label.name);
   }
 };
 
-var continuationVisitor = {
+let continuationVisitor = {
   enter(node, parent, scope, state) {
     if (this.isAssignmentExpression() || this.isUpdateExpression()) {
-      var bindings = this.getBindingIdentifiers();
-      for (var name in bindings) {
+      let bindings = this.getBindingIdentifiers();
+      for (let name in bindings) {
         if (state.outsideReferences[name] !== scope.getBindingIdentifier(name)) continue;
         state.reassignments[name] = true;
       }
@@ -205,9 +205,9 @@ function loopNodeTo(node) {
   }
 }
 
-var loopVisitor = {
+let loopVisitor = {
   Loop(node, parent, scope, state) {
-    var oldIgnoreLabeless = state.ignoreLabeless;
+    let oldIgnoreLabeless = state.ignoreLabeless;
     state.ignoreLabeless = true;
     this.traverse(loopVisitor, state);
     state.ignoreLabeless = oldIgnoreLabeless;
@@ -219,7 +219,7 @@ var loopVisitor = {
   },
 
   SwitchCase(node, parent, scope, state) {
-    var oldInSwitchCase = state.inSwitchCase;
+    let oldInSwitchCase = state.inSwitchCase;
     state.inSwitchCase = true;
     this.traverse(loopVisitor, state);
     state.inSwitchCase = oldInSwitchCase;
@@ -227,8 +227,8 @@ var loopVisitor = {
   },
 
   "BreakStatement|ContinueStatement|ReturnStatement"(node, parent, scope, state) {
-    var replace;
-    var loopText = loopNodeTo(node);
+    let replace;
+    let loopText = loopNodeTo(node);
 
     if (loopText) {
       if (node.label) {
@@ -297,11 +297,11 @@ class BlockScoping {
    */
 
   run() {
-    var block = this.block;
+    let block = this.block;
     if (block._letDone) return;
     block._letDone = true;
 
-    var needsClosure = this.getLetReferences();
+    let needsClosure = this.getLetReferences();
 
     // this is a block within a `Function/Program` so we can safely leave it be
     if (t.isFunction(this.parent) || t.isProgram(this.block)) return;
@@ -321,24 +321,24 @@ class BlockScoping {
   }
 
   remap() {
-    var hasRemaps = false;
-    var letRefs   = this.letReferences;
-    var scope     = this.scope;
+    let hasRemaps = false;
+    let letRefs   = this.letReferences;
+    let scope     = this.scope;
 
     // alright, so since we aren't wrapping this block in a closure
     // we have to check if any of our let variables collide with
     // those in upper scopes and then if they do, generate a uid
     // for them and replace all references with it
-    var remaps = Object.create(null);
+    let remaps = Object.create(null);
 
-    for (var key in letRefs) {
+    for (let key in letRefs) {
       // just an Identifier node we collected in `getLetReferences`
       // this is the defining identifier of a declaration
-      var ref = letRefs[key];
+      let ref = letRefs[key];
 
       // todo: could skip this if the colliding binding is in another function
       if (scope.parentHasBinding(key) || scope.hasGlobal(key)) {
-        var uid = scope.generateUidIdentifier(ref.name).name;
+        let uid = scope.generateUidIdentifier(ref.name).name;
         ref.name = uid;
 
         hasRemaps = true;
@@ -353,7 +353,7 @@ class BlockScoping {
 
     //
 
-    var loop = this.loop;
+    let loop = this.loop;
     if (loop) {
       traverseReplace(loop.right, loop, scope, remaps);
       traverseReplace(loop.test, loop, scope, remaps);
@@ -364,14 +364,14 @@ class BlockScoping {
   }
 
   wrapClosure() {
-    var block = this.block;
+    let block = this.block;
 
-    var outsideRefs = this.outsideLetReferences;
+    let outsideRefs = this.outsideLetReferences;
 
     // remap loop heads with colliding variables
     if (this.loop) {
-      for (var name in outsideRefs) {
-        var id = outsideRefs[name];
+      for (let name in outsideRefs) {
+        let id = outsideRefs[name];
 
         if (this.scope.hasGlobal(id.name) || this.scope.parentHasBinding(id.name)) {
           delete outsideRefs[id.name];
@@ -389,15 +389,15 @@ class BlockScoping {
     // `break`s, `continue`s, `return`s etc
     this.has = this.checkLoop();
 
-    // hoist var references to retain scope
+    // hoist let references to retain scope
     this.hoistVarDeclarations();
 
     // turn outsideLetReferences into an array
-    var params = values(outsideRefs);
-    var args   = values(outsideRefs);
+    let params = values(outsideRefs);
+    let args   = values(outsideRefs);
 
     // build the closure that we're going to wrap the block with
-    var fn = t.functionExpression(null, params, t.blockStatement(block.body));
+    let fn = t.functionExpression(null, params, t.blockStatement(block.body));
     fn.shadow = true;
 
     // continuation
@@ -406,7 +406,7 @@ class BlockScoping {
     // replace the current block body with the one we're going to build
     block.body = this.body;
 
-    var ref = fn;
+    let ref = fn;
 
     if (this.loop) {
       ref = this.scope.generateUidIdentifier("loop");
@@ -416,18 +416,18 @@ class BlockScoping {
     }
 
     // build a call and a unique id that we can assign the return value to
-    var call = t.callExpression(ref, args);
-    var ret  = this.scope.generateUidIdentifier("ret");
+    let call = t.callExpression(ref, args);
+    let ret  = this.scope.generateUidIdentifier("ret");
 
     // handle generators
-    var hasYield = traverse.hasType(fn.body, this.scope, "YieldExpression", t.FUNCTION_TYPES);
+    let hasYield = traverse.hasType(fn.body, this.scope, "YieldExpression", t.FUNCTION_TYPES);
     if (hasYield) {
       fn.generator = true;
       call = t.yieldExpression(call, true);
     }
 
     // handlers async functions
-    var hasAsync = traverse.hasType(fn.body, this.scope, "AwaitExpression", t.FUNCTION_TYPES);
+    let hasAsync = traverse.hasType(fn.body, this.scope, "AwaitExpression", t.FUNCTION_TYPES);
     if (hasAsync) {
       fn.async = true;
       call = t.awaitExpression(call);
@@ -441,7 +441,7 @@ class BlockScoping {
    */
 
   buildClosure(ret: { type: "Identifier" }, call: { type: "CallExpression" }) {
-    var has = this.has;
+    let has = this.has;
     if (has.hasReturn || has.hasBreakContinue) {
       this.buildHas(ret, call);
     } else {
@@ -458,18 +458,18 @@ class BlockScoping {
    */
 
   addContinuations(fn) {
-    var state = {
+    let state = {
       reassignments: {},
       outsideReferences: this.outsideLetReferences
     };
 
     this.scope.traverse(fn, continuationVisitor, state);
 
-    for (var i = 0; i < fn.params.length; i++) {
-      var param = fn.params[i];
+    for (let i = 0; i < fn.params.length; i++) {
+      let param = fn.params[i];
       if (!state.reassignments[param.name]) continue;
 
-      var newParam = this.scope.generateUidIdentifier(param.name);
+      let newParam = this.scope.generateUidIdentifier(param.name);
       fn.params[i] = newParam;
 
       this.scope.rename(param.name, newParam.name, fn);
@@ -480,9 +480,9 @@ class BlockScoping {
   }
 
   getLetReferences() {
-    var block = this.block;
+    let block = this.block;
 
-    var declarators = block._letDeclarators || [];
+    let declarators = block._letDeclarators || [];
 
     //
     for (let i = 0; i < declarators.length; i++) {
@@ -503,7 +503,7 @@ class BlockScoping {
     //
     for (let i = 0; i < declarators.length; i++) {
       let declar = declarators[i];
-      var keys = t.getBindingIdentifiers(declar);
+      let keys = t.getBindingIdentifiers(declar);
       extend(this.letReferences, keys);
       this.hasLetReferences = true;
     }
@@ -511,10 +511,10 @@ class BlockScoping {
     // no let references so we can just quit
     if (!this.hasLetReferences) return;
 
-    // set let references to plain var references
+    // set let references to plain let references
     standardizeLets(declarators);
 
-    var state = {
+    let state = {
       letReferences: this.letReferences,
       closurify:     false,
       file:          this.file
@@ -535,7 +535,7 @@ class BlockScoping {
    */
 
   checkLoop(): Object {
-    var state = {
+    let state = {
       hasBreakContinue: false,
       ignoreLabeless:   false,
       inSwitchCase:     false,
@@ -552,7 +552,7 @@ class BlockScoping {
   }
 
   /**
-   * Hoist all var declarations in this block to before it so they retain scope
+   * Hoist all let declarations in this block to before it so they retain scope
    * once we wrap everything in a closure.
    */
 
@@ -566,21 +566,21 @@ class BlockScoping {
    */
 
   pushDeclar(node: { type: "VariableDeclaration" }): Array<Object> {
-    var declars = [];
-    var names = t.getBindingIdentifiers(node);
-    for (var name in names) {
+    let declars = [];
+    let names = t.getBindingIdentifiers(node);
+    for (let name in names) {
       declars.push(t.variableDeclarator(names[name]));
     }
 
     this.body.push(t.variableDeclaration(node.kind, declars));
 
-    var replace = [];
+    let replace = [];
 
-    for (var i = 0; i < node.declarations.length; i++) {
-      var declar = node.declarations[i];
+    for (let i = 0; i < node.declarations.length; i++) {
+      let declar = node.declarations[i];
       if (!declar.init) continue;
 
-      var expr = t.assignmentExpression("=", declar.id, declar.init);
+      let expr = t.assignmentExpression("=", declar.id, declar.init);
       replace.push(t.inherits(expr, declar));
     }
 
@@ -588,15 +588,15 @@ class BlockScoping {
   }
 
   buildHas(ret: { type: "Identifier" }, call: { type: "CallExpression" }) {
-    var body = this.body;
+    let body = this.body;
 
     body.push(t.variableDeclaration("var", [
       t.variableDeclarator(ret, call)
     ]));
 
-    var retCheck;
-    var has = this.has;
-    var cases = [];
+    let retCheck;
+    let has = this.has;
+    let cases = [];
 
     if (has.hasReturn) {
       // typeof ret === "object"
@@ -606,7 +606,7 @@ class BlockScoping {
     }
 
     if (has.hasBreakContinue) {
-      for (var key in has.map) {
+      for (let key in has.map) {
         cases.push(t.switchCase(t.stringLiteral(key), [has.map[key]]));
       }
 
@@ -615,15 +615,15 @@ class BlockScoping {
       }
 
       if (cases.length === 1) {
-        var single = cases[0];
+        let single = cases[0];
         body.push(this.file.attachAuxiliaryComment(t.ifStatement(
           t.binaryExpression("===", ret, single.test),
           single.consequent[0]
         )));
       } else {
         // https://github.com/babel/babel/issues/998
-        for (var i = 0; i < cases.length; i++) {
-          var caseConsequent = cases[i].consequent[0];
+        for (let i = 0; i < cases.length; i++) {
+          let caseConsequent = cases[i].consequent[0];
           if (t.isBreakStatement(caseConsequent) && !caseConsequent.label) {
             caseConsequent.label = this.loopLabel = this.loopLabel || this.file.scope.generateUidIdentifier("loop");
           }
diff --git a/packages/babel-core/src/transformation/transformers/es6/block-scoping/tdz.js b/packages/babel-core/src/transformation/transformers/es6/block-scoping/tdz.js
index cdd254fc62..718976b6f1 100644
--- a/packages/babel-core/src/transformation/transformers/es6/block-scoping/tdz.js
+++ b/packages/babel-core/src/transformation/transformers/es6/block-scoping/tdz.js
@@ -1,7 +1,7 @@
 import * as t from "babel-types";
 
 function getTDZStatus(refPath, bindingPath) {
-  var executionStatus = bindingPath._guessExecutionStatusRelativeTo(refPath);
+  let executionStatus = bindingPath._guessExecutionStatusRelativeTo(refPath);
 
   if (executionStatus === "before") {
     return "inside";
@@ -20,25 +20,25 @@ function buildTDZAssert(node, file) {
 }
 
 function isReference(node, scope, state) {
-  var declared = state.letReferences[node.name];
+  let declared = state.letReferences[node.name];
   if (!declared) return false;
 
   // declared node is different in this scope
   return scope.getBindingIdentifier(node.name) === declared;
 }
 
-export var visitor = {
+export let visitor = {
   ReferencedIdentifier(node, parent, scope, state) {
     if (t.isFor(parent, { left: node })) return;
     if (!isReference(node, scope, state)) return;
 
-    var bindingPath = scope.getBinding(node.name).path;
+    let bindingPath = scope.getBinding(node.name).path;
 
-    var status = getTDZStatus(this, bindingPath);
+    let status = getTDZStatus(this, bindingPath);
     if (status === "inside") return;
 
     if (status === "maybe") {
-      var assert = buildTDZAssert(node, state.file);
+      let assert = buildTDZAssert(node, state.file);
 
       // add tdzThis to parent variable declarator so it's exploded
       bindingPath.parent._tdzThis = true;
@@ -65,11 +65,11 @@ export var visitor = {
     exit(node, parent, scope, state) {
       if (node._ignoreBlockScopingTDZ) return;
 
-      var nodes = [];
-      var ids = this.getBindingIdentifiers();
+      let nodes = [];
+      let ids = this.getBindingIdentifiers();
 
-      for (var name in ids) {
-        var id = ids[name];
+      for (let name in ids) {
+        let id = ids[name];
 
         if (isReference(id, scope, state)) {
           nodes.push(buildTDZAssert(id, state.file));
diff --git a/packages/babel-core/src/transformation/transformers/es6/classes/index.js b/packages/babel-core/src/transformation/transformers/es6/classes/index.js
index 9f35895124..16b80720f9 100644
--- a/packages/babel-core/src/transformation/transformers/es6/classes/index.js
+++ b/packages/babel-core/src/transformation/transformers/es6/classes/index.js
@@ -3,7 +3,7 @@ import VanillaTransformer from "./vanilla";
 import * as t from "babel-types";
 import { bare } from "../../../helpers/name-method";
 
-export var visitor = {
+export let visitor = {
   ClassDeclaration(node) {
     return t.variableDeclaration("var", [
       t.variableDeclarator(node.id, t.toExpression(node))
@@ -11,7 +11,7 @@ export var visitor = {
   },
 
   ClassExpression(node, parent, scope, file) {
-    var inferred = bare(node, parent, scope);
+    let inferred = bare(node, parent, scope);
     if (inferred) return inferred;
 
     if (file.isLoose("es6.classes")) {
diff --git a/packages/babel-core/src/transformation/transformers/es6/classes/loose.js b/packages/babel-core/src/transformation/transformers/es6/classes/loose.js
index 10f7b2e622..4af2320e6c 100644
--- a/packages/babel-core/src/transformation/transformers/es6/classes/loose.js
+++ b/packages/babel-core/src/transformation/transformers/es6/classes/loose.js
@@ -11,11 +11,11 @@ export default class LooseClassTransformer extends VanillaTransformer {
     if (!node.decorators) {
       // use assignments instead of define properties for loose classes
 
-      var classRef = this.classRef;
+      let classRef = this.classRef;
       if (!node.static) classRef = t.memberExpression(classRef, t.identifier("prototype"));
-      var methodName = t.memberExpression(classRef, node.key, node.computed || t.isLiteral(node.key));
+      let methodName = t.memberExpression(classRef, node.key, node.computed || t.isLiteral(node.key));
 
-      var expr = t.expressionStatement(t.assignmentExpression("=", methodName, node.value));
+      let expr = t.expressionStatement(t.assignmentExpression("=", methodName, node.value));
       t.inheritsComments(expr, node);
       this.body.push(expr);
       return true;
diff --git a/packages/babel-core/src/transformation/transformers/es6/classes/vanilla.js b/packages/babel-core/src/transformation/transformers/es6/classes/vanilla.js
index 6e0f4ab1b3..af8ca86023 100644
--- a/packages/babel-core/src/transformation/transformers/es6/classes/vanilla.js
+++ b/packages/babel-core/src/transformation/transformers/es6/classes/vanilla.js
@@ -10,7 +10,7 @@ import * as t from "babel-types";
 
 const PROPERTY_COLLISION_METHOD_NAME = "__initializeProperties";
 
-var collectPropertyReferencesVisitor = {
+let collectPropertyReferencesVisitor = {
   Identifier: {
     enter(node, parent, scope, state) {
       if (this.parentPath.isClassProperty({ key: node })) {
@@ -24,7 +24,7 @@ var collectPropertyReferencesVisitor = {
   }
 };
 
-var verifyConstructorVisitor = {
+let verifyConstructorVisitor = {
   MethodDefinition() {
     this.skip();
   },
@@ -54,7 +54,7 @@ var verifyConstructorVisitor = {
     if (state.isDerived && !state.hasBareSuper) {
       if (this.inShadow()) {
         // https://github.com/babel/babel/issues/1920
-        var thisAlias = state.constructorPath.getData("this");
+        let thisAlias = state.constructorPath.getData("this");
 
         if (!thisAlias) {
           thisAlias = state.constructorPath.setData(
@@ -111,19 +111,19 @@ export default class ClassTransformer {
   }
 
   run() {
-    var superName = this.superName;
-    var file      = this.file;
-    var body      = this.body;
+    let superName = this.superName;
+    let file      = this.file;
+    let body      = this.body;
 
     //
 
-    var constructorBody = this.constructorBody = t.blockStatement([]);
+    let constructorBody = this.constructorBody = t.blockStatement([]);
     this.constructor    = this.buildConstructor();
 
     //
 
-    var closureParams = [];
-    var closureArgs = [];
+    let closureParams = [];
+    let closureArgs = [];
 
     //
     if (this.isDerived) {
@@ -136,7 +136,7 @@ export default class ClassTransformer {
     }
 
     //
-    var decorators = this.node.decorators;
+    let decorators = this.node.decorators;
     if (decorators) {
       // this is so super calls and the decorators have access to the raw function
       this.directRef = this.scope.generateUidIdentifier(this.classRef);
@@ -166,19 +166,19 @@ export default class ClassTransformer {
     //
     body.push(t.returnStatement(this.classRef));
 
-    var container = t.functionExpression(null, closureParams, t.blockStatement(body));
+    let container = t.functionExpression(null, closureParams, t.blockStatement(body));
     container.shadow = true;
     return t.callExpression(container, closureArgs);
   }
 
   buildConstructor() {
-    var func = t.functionDeclaration(this.classRef, [], this.constructorBody);
+    let func = t.functionDeclaration(this.classRef, [], this.constructorBody);
     t.inherits(func, this.node);
     return func;
   }
 
   pushToMap(node, enumerable, kind = "value") {
-    var mutatorMap;
+    let mutatorMap;
     if (node.static) {
       this.hasStaticDescriptors = true;
       mutatorMap = this.staticMutatorMap;
@@ -187,7 +187,7 @@ export default class ClassTransformer {
       mutatorMap = this.instanceMutatorMap;
     }
 
-    var map = defineMap.push(mutatorMap, node, kind, this.file);
+    let map = defineMap.push(mutatorMap, node, kind, this.file);
 
     if (enumerable) {
       map.enumerable = t.booleanLiteral(true);
@@ -204,15 +204,15 @@ export default class ClassTransformer {
    */
 
   constructorMeMaybe() {
-    var hasConstructor = false;
-    var paths = this.path.get("body.body");
-    for (var path of (paths: Array)) {
+    let hasConstructor = false;
+    let paths = this.path.get("body.body");
+    for (let path of (paths: Array)) {
       hasConstructor = path.equals("kind", "constructor");
       if (hasConstructor) break;
     }
     if (hasConstructor) return;
 
-    var constructor;
+    let constructor;
     if (this.isDerived) {
       constructor = util.template("class-derived-default-constructor");
     } else {
@@ -232,7 +232,7 @@ export default class ClassTransformer {
     this.placePropertyInitializers();
 
     if (this.userConstructor) {
-      var constructorBody = this.constructorBody;
+      let constructorBody = this.constructorBody;
       constructorBody.body = constructorBody.body.concat(this.userConstructor.body.body);
       t.inherits(this.constructor, this.userConstructor);
       t.inherits(constructorBody, this.userConstructor.body);
@@ -242,20 +242,20 @@ export default class ClassTransformer {
   }
 
   pushBody() {
-    var classBodyPaths = this.path.get("body.body");
+    let classBodyPaths = this.path.get("body.body");
 
-    for (var path of (classBodyPaths: Array)) {
-      var node = path.node;
+    for (let path of (classBodyPaths: Array)) {
+      let node = path.node;
 
       if (node.decorators) {
         memoiseDecorators(node.decorators, this.scope);
       }
 
       if (t.isMethodDefinition(node)) {
-        var isConstructor = node.kind === "constructor";
+        let isConstructor = node.kind === "constructor";
         if (isConstructor) this.verifyConstructor(path);
 
-        var replaceSupers = new ReplaceSupers({
+        let replaceSupers = new ReplaceSupers({
           methodPath: path,
           methodNode: node,
           objectRef:  this.directRef,
@@ -290,11 +290,11 @@ export default class ClassTransformer {
   pushDescriptors() {
     this.pushInherits();
 
-    var body = this.body;
+    let body = this.body;
 
-    var instanceProps;
-    var staticProps;
-    var classHelper = "create-class";
+    let instanceProps;
+    let staticProps;
+    let classHelper = "create-class";
     if (this.hasDecorators) classHelper = "create-decorated-class";
 
     if (this.hasInstanceDescriptors) {
@@ -309,10 +309,10 @@ export default class ClassTransformer {
       if (instanceProps) instanceProps = defineMap.toComputedObjectFromClass(instanceProps);
       if (staticProps) staticProps = defineMap.toComputedObjectFromClass(staticProps);
 
-      var nullNode = t.nullLiteral();
+      let nullNode = t.nullLiteral();
 
       // (Constructor, instanceDescriptors, staticDescriptors, instanceInitializers, staticInitializers)
-      var args = [this.classRef, nullNode, nullNode, nullNode, nullNode];
+      let args = [this.classRef, nullNode, nullNode, nullNode, nullNode];
 
       if (instanceProps) args[1] = instanceProps;
       if (staticProps) args[2] = staticProps;
@@ -327,7 +327,7 @@ export default class ClassTransformer {
         body.unshift(this.buildObjectAssignment(this.staticInitializersId));
       }
 
-      var lastNonNullIndex = 0;
+      let lastNonNullIndex = 0;
       for (let i = 0; i < args.length; i++) {
         if (args[i] !== nullNode) lastNonNullIndex = i;
       }
@@ -349,11 +349,11 @@ export default class ClassTransformer {
   }
 
   placePropertyInitializers() {
-    var body = this.instancePropBody;
+    let body = this.instancePropBody;
     if (!body.length) return;
 
     if (this.hasPropertyCollision()) {
-      var call = t.expressionStatement(t.callExpression(
+      let call = t.expressionStatement(t.callExpression(
         t.memberExpression(t.thisExpression(), t.identifier(PROPERTY_COLLISION_METHOD_NAME)),
         []
       ));
@@ -379,7 +379,7 @@ export default class ClassTransformer {
 
    hasPropertyCollision(): boolean {
     if (this.userConstructorPath) {
-      for (var name in this.instancePropRefs) {
+      for (let name in this.instancePropRefs) {
         if (this.userConstructorPath.scope.hasOwnBinding(name)) {
           return true;
         }
@@ -390,7 +390,7 @@ export default class ClassTransformer {
   }
 
    verifyConstructor(path: NodePath) {
-    var state = {
+    let state = {
       constructorPath: path.get("value"),
       hasBareSuper:    false,
       bareSuper:       null,
@@ -400,7 +400,7 @@ export default class ClassTransformer {
 
     state.constructorPath.traverse(verifyConstructorVisitor, state);
 
-    var thisAlias = state.constructorPath.getData("this");
+    let thisAlias = state.constructorPath.getData("this");
     if (thisAlias && state.bareSuper) {
       state.bareSuper.insertAfter(t.variableDeclaration("var", [
         t.variableDeclarator(thisAlias, t.thisExpression())
@@ -442,7 +442,7 @@ export default class ClassTransformer {
     });
 
     if (node.decorators) {
-      var body = [];
+      let body = [];
       if (node.value) {
         body.push(t.returnStatement(node.value));
         node.value = t.functionExpression(null, [], t.blockStatement(body));
@@ -451,8 +451,8 @@ export default class ClassTransformer {
       }
       this.pushToMap(node, true, "initializer");
 
-      var initializers;
-      var target;
+      let initializers;
+      let target;
       if (node.static) {
         initializers = this.staticInitializersId = this.staticInitializersId || this.scope.generateUidIdentifier("staticInitializers");
         body = this.staticPropBody;
@@ -492,13 +492,13 @@ export default class ClassTransformer {
 
   pushConstructor(method: { type: "MethodDefinition" }, path: NodePath) {
     // https://github.com/babel/babel/issues/1077
-    var fnPath = path.get("value");
+    let fnPath = path.get("value");
     if (fnPath.scope.hasOwnBinding(this.classRef.name)) {
       fnPath.scope.rename(this.classRef.name);
     }
 
-    var construct = this.constructor;
-    var fn        = method.value;
+    let construct = this.constructor;
+    let fn        = method.value;
 
     this.userConstructorPath = fnPath;
     this.userConstructor     = fn;
@@ -550,7 +550,7 @@ export default class ClassTransformer {
    */
 
   pushDecorators() {
-    var decorators = this.node.decorators;
+    let decorators = this.node.decorators;
     if (!decorators) return;
 
     this.body.push(t.variableDeclaration("var", [
@@ -560,8 +560,8 @@ export default class ClassTransformer {
     // reverse the decorators so we execute them in the right order
     decorators = decorators.reverse();
 
-    for (var decorator of (decorators: Array)) {
-      var decoratorNode = util.template("class-decorator", {
+    for (let decorator of (decorators: Array)) {
+      let decoratorNode = util.template("class-decorator", {
         DECORATOR: decorator.expression,
         CLASS_REF: this.classRef
       }, true);
diff --git a/packages/babel-core/src/transformation/transformers/es6/for-of.js b/packages/babel-core/src/transformation/transformers/es6/for-of.js
index fea662b0ce..de224dc727 100644
--- a/packages/babel-core/src/transformation/transformers/es6/for-of.js
+++ b/packages/babel-core/src/transformation/transformers/es6/for-of.js
@@ -2,19 +2,19 @@ import * as messages from "babel-messages";
 import * as util from  "../../../util";
 import * as t from "babel-types";
 
-export var visitor = {
+export let visitor = {
   ForOfStatement(node, parent, scope, file) {
     if (this.get("right").isArrayExpression()) {
       return _ForOfStatementArray.call(this, node, scope, file);
     }
 
-    var callback = spec;
+    let callback = spec;
     if (file.isLoose("es6.forOf")) callback = loose;
 
-    var build  = callback(node, parent, scope, file);
-    var declar = build.declar;
-    var loop   = build.loop;
-    var block  = loop.body;
+    let build  = callback(node, parent, scope, file);
+    let declar = build.declar;
+    let loop   = build.loop;
+    let block  = loop.body;
 
     // ensure that it's a block so we can take all its statements
     this.ensureBlock();
@@ -40,20 +40,20 @@ export var visitor = {
 };
 
 export function _ForOfStatementArray(node, scope) {
-  var nodes = [];
-  var right = node.right;
+  let nodes = [];
+  let right = node.right;
 
   if (!t.isIdentifier(right) || !scope.hasBinding(right.name)) {
-    var uid = scope.generateUidIdentifier("arr");
+    let uid = scope.generateUidIdentifier("arr");
     nodes.push(t.variableDeclaration("var", [
       t.variableDeclarator(uid, right)
     ]));
     right = uid;
   }
 
-  var iterationKey = scope.generateUidIdentifier("i");
+  let iterationKey = scope.generateUidIdentifier("i");
 
-  var loop = util.template("for-of-array", {
+  let loop = util.template("for-of-array", {
     BODY: node.body,
     KEY:  iterationKey,
     ARR:  right
@@ -62,9 +62,9 @@ export function _ForOfStatementArray(node, scope) {
   t.inherits(loop, node);
   t.ensureBlock(loop);
 
-  var iterationValue = t.memberExpression(right, iterationKey, true);
+  let iterationValue = t.memberExpression(right, iterationKey, true);
 
-  var left = node.left;
+  let left = node.left;
   if (t.isVariableDeclaration(left)) {
     left.declarations[0].init = iterationValue;
     loop.body.body.unshift(left);
@@ -81,15 +81,15 @@ export function _ForOfStatementArray(node, scope) {
   return nodes;
 }
 
-var loose = function (node, parent, scope, file) {
-  var left = node.left;
-  var declar, id;
+let loose = function (node, parent, scope, file) {
+  let left = node.left;
+  let declar, id;
 
   if (t.isIdentifier(left) || t.isPattern(left) || t.isMemberExpression(left)) {
     // for (i of test), for ({ i } of test)
     id = left;
   } else if (t.isVariableDeclaration(left)) {
-    // for (var i of test)
+    // for (let i of test)
     id = scope.generateUidIdentifier("ref");
     declar = t.variableDeclaration(left.kind, [
       t.variableDeclarator(left.declarations[0].id, id)
@@ -98,10 +98,10 @@ var loose = function (node, parent, scope, file) {
     throw file.errorWithNode(left, messages.get("unknownForHead", left.type));
   }
 
-  var iteratorKey = scope.generateUidIdentifier("iterator");
-  var isArrayKey  = scope.generateUidIdentifier("isArray");
+  let iteratorKey = scope.generateUidIdentifier("iterator");
+  let isArrayKey  = scope.generateUidIdentifier("isArray");
 
-  var loop = util.template("for-of-loose", {
+  let loop = util.template("for-of-loose", {
     LOOP_OBJECT:  iteratorKey,
     IS_ARRAY:     isArrayKey,
     OBJECT:       node.right,
@@ -124,18 +124,18 @@ var loose = function (node, parent, scope, file) {
   };
 };
 
-var spec = function (node, parent, scope, file) {
-  var left = node.left;
-  var declar;
+let spec = function (node, parent, scope, file) {
+  let left = node.left;
+  let declar;
 
-  var stepKey   = scope.generateUidIdentifier("step");
-  var stepValue = t.memberExpression(stepKey, t.identifier("value"));
+  let stepKey   = scope.generateUidIdentifier("step");
+  let stepValue = t.memberExpression(stepKey, t.identifier("value"));
 
   if (t.isIdentifier(left) || t.isPattern(left) || t.isMemberExpression(left)) {
     // for (i of test), for ({ i } of test)
     declar = t.expressionStatement(t.assignmentExpression("=", left, stepValue));
   } else if (t.isVariableDeclaration(left)) {
-    // for (var i of test)
+    // for (let i of test)
     declar = t.variableDeclaration(left.kind, [
       t.variableDeclarator(left.declarations[0].id, stepValue)
     ]);
@@ -145,9 +145,9 @@ var spec = function (node, parent, scope, file) {
 
   //
 
-  var iteratorKey = scope.generateUidIdentifier("iterator");
+  let iteratorKey = scope.generateUidIdentifier("iterator");
 
-  var template = util.template("for-of", {
+  let template = util.template("for-of", {
     ITERATOR_HAD_ERROR_KEY: scope.generateUidIdentifier("didIteratorError"),
     ITERATOR_COMPLETION:    scope.generateUidIdentifier("iteratorNormalCompletion"),
     ITERATOR_ERROR_KEY:     scope.generateUidIdentifier("iteratorError"),
@@ -157,10 +157,10 @@ var spec = function (node, parent, scope, file) {
     BODY:                   null
   });
 
-  var isLabeledParent = t.isLabeledStatement(parent);
+  let isLabeledParent = t.isLabeledStatement(parent);
 
-  var tryBody = template[3].block.body;
-  var loop = tryBody[0];
+  let tryBody = template[3].block.body;
+  let loop = tryBody[0];
 
   if (isLabeledParent) {
     tryBody[0] = t.labeledStatement(parent.label, loop);
diff --git a/packages/babel-core/src/transformation/transformers/es6/modules.js b/packages/babel-core/src/transformation/transformers/es6/modules.js
index 16c5ea5b2c..afde8d1f6a 100644
--- a/packages/babel-core/src/transformation/transformers/es6/modules.js
+++ b/packages/babel-core/src/transformation/transformers/es6/modules.js
@@ -8,19 +8,19 @@ function keepBlockHoist(node, nodes) {
   }
 }
 
-export var metadata = {
+export let metadata = {
   group: "builtin-modules"
 };
 
-export var visitor = {
+export let visitor = {
   ImportDeclaration(node, parent, scope, file) {
     // flow type
     if (node.importKind === "type" || node.importKind === "typeof") return;
 
-    var nodes = [];
+    let nodes = [];
 
     if (node.specifiers.length) {
-      for (var specifier of (node.specifiers: Array)) {
+      for (let specifier of (node.specifiers: Array)) {
         file.moduleFormatter.importSpecifier(specifier, node, nodes, scope);
       }
     } else {
@@ -36,14 +36,14 @@ export var visitor = {
   },
 
   ExportAllDeclaration(node, parent, scope, file) {
-    var nodes = [];
+    let nodes = [];
     file.moduleFormatter.exportAllDeclaration(node, nodes, scope);
     keepBlockHoist(node, nodes);
     return nodes;
   },
 
   ExportDefaultDeclaration(node, parent, scope, file) {
-    var nodes = [];
+    let nodes = [];
     file.moduleFormatter.exportDeclaration(node, nodes, scope);
     keepBlockHoist(node, nodes);
     return nodes;
@@ -53,13 +53,13 @@ export var visitor = {
     // flow type
     if (this.get("declaration").isTypeAlias()) return;
 
-    var nodes = [];
+    let nodes = [];
 
     if (node.declaration) {
       // make sure variable exports have an initializer
       // this is done here to avoid duplicating it in the module formatters
       if (t.isVariableDeclaration(node.declaration)) {
-        var declar = node.declaration.declarations[0];
+        let declar = node.declaration.declarations[0];
         declar.init = declar.init || scope.buildUndefinedNode();
       }
 
diff --git a/packages/babel-core/src/transformation/transformers/es6/object-super.js b/packages/babel-core/src/transformation/transformers/es6/object-super.js
index f3afd17552..b4e6a9006c 100644
--- a/packages/babel-core/src/transformation/transformers/es6/object-super.js
+++ b/packages/babel-core/src/transformation/transformers/es6/object-super.js
@@ -5,7 +5,7 @@ function Property(path, node, scope, getObjectRef, file) {
   if (!node.method && node.kind === "init") return;
   if (!t.isFunction(node.value)) return;
 
-  var replaceSupers = new ReplaceSupers({
+  let replaceSupers = new ReplaceSupers({
     getObjectRef: getObjectRef,
     methodNode:   node,
     methodPath:   path,
@@ -17,13 +17,13 @@ function Property(path, node, scope, getObjectRef, file) {
   replaceSupers.replace();
 }
 
-export var visitor = {
+export let visitor = {
   ObjectExpression(node, parent, scope, file) {
-    var objectRef;
-    var getObjectRef = () => objectRef = objectRef || scope.generateUidIdentifier("obj");
+    let objectRef;
+    let getObjectRef = () => objectRef = objectRef || scope.generateUidIdentifier("obj");
 
-    var propPaths = this.get("properties");
-    for (var i = 0; i < node.properties.length; i++) {
+    let propPaths = this.get("properties");
+    for (let i = 0; i < node.properties.length; i++) {
       Property(propPaths[i], node.properties[i], scope, getObjectRef, file);
     }
 
diff --git a/packages/babel-core/src/transformation/transformers/es6/parameters/default.js b/packages/babel-core/src/transformation/transformers/es6/parameters/default.js
index cb8fabebae..4a49aee048 100644
--- a/packages/babel-core/src/transformation/transformers/es6/parameters/default.js
+++ b/packages/babel-core/src/transformation/transformers/es6/parameters/default.js
@@ -3,14 +3,14 @@ import getFunctionArity from "../../../helpers/get-function-arity";
 import * as util from  "../../../../util";
 import * as t from "babel-types";
 
-var hasDefaults = function (node) {
-  for (var i = 0; i < node.params.length; i++) {
+let hasDefaults = function (node) {
+  for (let i = 0; i < node.params.length; i++) {
     if (!t.isIdentifier(node.params[i])) return true;
   }
   return false;
 };
 
-var iifeVisitor = {
+let iifeVisitor = {
   ReferencedIdentifier(node, parent, scope, state) {
     if (node.name !== "eval") {
       if (!state.scope.hasOwnBinding(node.name)) return;
@@ -22,27 +22,27 @@ var iifeVisitor = {
   }
 };
 
-export var visitor = {
+export let visitor = {
   Function(node, parent, scope, file) {
     if (!hasDefaults(node)) return;
 
     // ensure it's a block, useful for arrow functions
     this.ensureBlock();
 
-    var state = {
+    let state = {
       iife: false,
       scope: scope
     };
 
-    var body = [];
+    let body = [];
 
     //
-    var argsIdentifier = t.identifier("arguments");
+    let argsIdentifier = t.identifier("arguments");
     argsIdentifier._shadowedFunctionLiteral = this;
 
     // push a default parameter definition
     function pushDefNode(left, right, i) {
-      var defNode;
+      let defNode;
       if (exceedsLastNonDefault(i) || t.isPattern(left) || file.transformers["es6.spec.blockScoping"].canTransform()) {
         defNode = util.template("default-parameter", {
           VARIABLE_NAME: left,
@@ -66,12 +66,12 @@ export var visitor = {
     }
 
     //
-    var lastNonDefaultParam = getFunctionArity(node);
+    let lastNonDefaultParam = getFunctionArity(node);
 
     //
-    var params = this.get("params");
-    for (var i = 0; i < params.length; i++) {
-      var param = params[i];
+    let params = this.get("params");
+    for (let i = 0; i < params.length; i++) {
+      let param = params[i];
 
       if (!param.isAssignmentPattern()) {
         if (!param.isIdentifier()) {
@@ -85,11 +85,11 @@ export var visitor = {
         continue;
       }
 
-      var left  = param.get("left");
-      var right = param.get("right");
+      let left  = param.get("left");
+      let right = param.get("right");
 
       if (exceedsLastNonDefault(i) || left.isPattern()) {
-        var placeholder = scope.generateUidIdentifier("x");
+        let placeholder = scope.generateUidIdentifier("x");
         placeholder._isDefaultPlaceholder = true;
         node.params[i] = placeholder;
       } else {
diff --git a/packages/babel-core/src/transformation/transformers/es6/parameters/destructuring.js b/packages/babel-core/src/transformation/transformers/es6/parameters/destructuring.js
new file mode 100644
index 0000000000..da8b0de40f
--- /dev/null
+++ b/packages/babel-core/src/transformation/transformers/es6/parameters/destructuring.js
@@ -0,0 +1,23 @@
+import * as t from "babel-types";
+
+export let visitor = {
+  Function(path) {
+    let params: Array = path.get("params");
+    let body = path.get("body");
+
+    for (let i = 0; i < params.length; i++) {
+      let param = params[i];
+      if (param.isArrayPattern() || param.isObjectPattern()) {
+        let uid = path.scope.generateUidIdentifierBasedOnNode(param.node);
+
+        let declar = t.variableDeclaration("let", [
+          t.variableDeclarator(param.node, uid)
+        ]);
+        declar._blockHoist = params.length - i;
+        body.unshiftContainer("body", declar);
+
+        param.replaceWith(uid);
+      }
+    }
+  }
+};
diff --git a/packages/babel-core/src/transformation/transformers/es6/parameters/index.js b/packages/babel-core/src/transformation/transformers/es6/parameters/index.js
index e1061a0b68..6e154173aa 100644
--- a/packages/babel-core/src/transformation/transformers/es6/parameters/index.js
+++ b/packages/babel-core/src/transformation/transformers/es6/parameters/index.js
@@ -3,8 +3,8 @@ import { visitors } from "babel-traverse";
 import * as def from "./default";
 import * as rest from "./rest";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-advanced"
 };
 
-export var visitor = visitors.merge([rest.visitor, def.visitor]);
+export let visitor = visitors.merge([rest.visitor, def.visitor]);
diff --git a/packages/babel-core/src/transformation/transformers/es6/parameters/rest.js b/packages/babel-core/src/transformation/transformers/es6/parameters/rest.js
index 8142737a54..6eb37b76d2 100644
--- a/packages/babel-core/src/transformation/transformers/es6/parameters/rest.js
+++ b/packages/babel-core/src/transformation/transformers/es6/parameters/rest.js
@@ -1,7 +1,7 @@
 import * as util from  "../../../../util";
 import * as t from "babel-types";
 
-var memberExpressionOptimisationVisitor = {
+let memberExpressionOptimisationVisitor = {
   Scope(node, parent, scope, state) {
     // check if this scope has a local binding that will shadow the rest parameter
     if (!scope.bindingIdentifierEquals(state.name, state.outerBinding)) {
@@ -17,7 +17,7 @@ var memberExpressionOptimisationVisitor = {
   Function(node, parent, scope, state) {
     // skip over functions as whatever `arguments` we reference inside will refer
     // to the wrong function
-    var oldNoOptimise = state.noOptimise;
+    let oldNoOptimise = state.noOptimise;
     state.noOptimise = true;
     this.traverse(memberExpressionOptimisationVisitor, state);
     state.noOptimise = oldNoOptimise;
@@ -39,7 +39,7 @@ var memberExpressionOptimisationVisitor = {
       if (this.parentPath.isMemberExpression({ computed: true, object: node })) {
         // if we know that this member expression is referencing a number then we can safely
         // optimise it
-        var prop = this.parentPath.get("property");
+        let prop = this.parentPath.get("property");
         if (prop.isBaseType("number")) {
           state.candidates.push(this);
           return;
@@ -48,7 +48,7 @@ var memberExpressionOptimisationVisitor = {
 
       // optimise single spread args in calls
       if (this.parentPath.isSpreadElement() && state.offset === 0) {
-        var call = this.parentPath.parentPath;
+        let call = this.parentPath.parentPath;
         if (call.isCallExpression() && call.node.arguments.length === 1) {
           state.candidates.push(this);
           return;
@@ -75,8 +75,8 @@ var memberExpressionOptimisationVisitor = {
 function optimiseMemberExpression(parent, offset) {
   if (offset === 0) return;
 
-  var newExpr;
-  var prop = parent.property;
+  let newExpr;
+  let prop = parent.property;
 
   if (t.isLiteral(prop)) {
     prop.value += offset;
@@ -91,32 +91,32 @@ function hasRest(node) {
   return t.isRestElement(node.params[node.params.length - 1]);
 }
 
-export var visitor = {
+export let visitor = {
   Function(node, parent, scope) {
     if (!hasRest(node)) return;
 
-    var restParam = node.params.pop();
-    var rest = restParam.argument;
+    let restParam = node.params.pop();
+    let rest = restParam.argument;
 
-    var argsId = t.identifier("arguments");
+    let argsId = t.identifier("arguments");
 
     // otherwise `arguments` will be remapped in arrow functions
     argsId._shadowedFunctionLiteral = this;
 
     // support patterns
     if (t.isPattern(rest)) {
-      var pattern = rest;
+      let pattern = rest;
       rest = scope.generateUidIdentifier("ref");
 
-      var declar = t.variableDeclaration("let", pattern.elements.map(function (elem, index) {
-        var accessExpr = t.memberExpression(rest, t.numberLiteral(index), true);
+      let declar = t.variableDeclaration("let", pattern.elements.map(function (elem, index) {
+        let accessExpr = t.memberExpression(rest, t.numberLiteral(index), true);
         return t.variableDeclarator(elem, accessExpr);
       }));
       node.body.body.unshift(declar);
     }
 
     // check and optimise for extremely common cases
-    var state = {
+    let state = {
       references: [],
       offset:     node.params.length,
 
@@ -138,7 +138,7 @@ export var visitor = {
     if (!state.deopted && !state.references.length) {
       // we only have shorthands and there are no other references
       if (state.candidates.length) {
-        for (var candidate of (state.candidates: Array)) {
+        for (let candidate of (state.candidates: Array)) {
           candidate.replaceWith(argsId);
           if (candidate.parentPath.isMemberExpression()) {
             optimiseMemberExpression(candidate.parent, state.offset);
@@ -155,12 +155,12 @@ export var visitor = {
 
     //
 
-    var start = t.numberLiteral(node.params.length);
-    var key = scope.generateUidIdentifier("key");
-    var len = scope.generateUidIdentifier("len");
+    let start = t.numberLiteral(node.params.length);
+    let key = scope.generateUidIdentifier("key");
+    let len = scope.generateUidIdentifier("len");
 
-    var arrKey = key;
-    var arrLen = len;
+    let arrKey = key;
+    let arrLen = len;
     if (node.params.length) {
       // this method has additional params, so we need to subtract
       // the index of the current argument position from the
@@ -180,7 +180,7 @@ export var visitor = {
       );
     }
 
-    var loop = util.template("rest", {
+    let loop = util.template("rest", {
       ARRAY_TYPE: restParam.typeAnnotation,
       ARGUMENTS:  argsId,
       ARRAY_KEY:  arrKey,
@@ -198,10 +198,10 @@ export var visitor = {
       // perform allocation at the lowest common ancestor of all references
       loop._blockHoist = 1;
 
-      var target = this.getEarliestCommonAncestorFrom(state.references).getStatementParent();
+      let target = this.getEarliestCommonAncestorFrom(state.references).getStatementParent();
 
       // don't perform the allocation inside a loop
-      var highestLoop;
+      let highestLoop;
       target.findParent(function (path) {
         if (path.isLoop()) {
           highestLoop = path;
diff --git a/packages/babel-core/src/transformation/transformers/es6/regex.sticky.js b/packages/babel-core/src/transformation/transformers/es6/regex.sticky.js
index 26efd93f0d..bffc2e6dff 100644
--- a/packages/babel-core/src/transformation/transformers/es6/regex.sticky.js
+++ b/packages/babel-core/src/transformation/transformers/es6/regex.sticky.js
@@ -1,7 +1,7 @@
 import * as regex from "../../helpers/regex";
 import * as t from "babel-types";
 
-export var visitor = {
+export let visitor = {
   RegexLiteral(node) {
     if (!regex.is(node, "y")) return;
     return t.newExpression(t.identifier("RegExp"), [
diff --git a/packages/babel-core/src/transformation/transformers/es6/regex.unicode.js b/packages/babel-core/src/transformation/transformers/es6/regex.unicode.js
index 8bb2763c0c..cb7266940f 100644
--- a/packages/babel-core/src/transformation/transformers/es6/regex.unicode.js
+++ b/packages/babel-core/src/transformation/transformers/es6/regex.unicode.js
@@ -1,7 +1,7 @@
 import rewritePattern from "regexpu/rewrite-pattern";
 import * as regex from "../../helpers/regex";
 
-export var visitor = {
+export let visitor = {
   RegexLiteral(node) {
     if (!regex.is(node, "u")) return;
     node.regex.pattern = rewritePattern(node.pattern, node.flags);
diff --git a/packages/babel-core/src/transformation/transformers/es6/spec.arrow-functions.js b/packages/babel-core/src/transformation/transformers/es6/spec.arrow-functions.js
index e8a70a1b6e..48acf05eca 100644
--- a/packages/babel-core/src/transformation/transformers/es6/spec.arrow-functions.js
+++ b/packages/babel-core/src/transformation/transformers/es6/spec.arrow-functions.js
@@ -1,16 +1,16 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-pre",
   optional: true
 };
 
-export var visitor = {
+export let visitor = {
   ArrowFunctionExpression(node, parent, scope, file) {
     if (node.shadow) return;
     node.shadow = { this: false };
 
-    var boundThis = t.thisExpression();
+    let boundThis = t.thisExpression();
     boundThis._forceShadow = this;
 
     // make sure that arrow function won't be instantiated
diff --git a/packages/babel-core/src/transformation/transformers/es6/spec.modules.js b/packages/babel-core/src/transformation/transformers/es6/spec.modules.js
index 9e3f22640f..a51e2ea9ea 100644
--- a/packages/babel-core/src/transformation/transformers/es6/spec.modules.js
+++ b/packages/babel-core/src/transformation/transformers/es6/spec.modules.js
@@ -1,13 +1,13 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-pre",
   optional: true
 };
 
-export var visitor = {
+export let visitor = {
   Program() {
-    var id = this.scope.generateUidIdentifier("null");
+    let id = this.scope.generateUidIdentifier("null");
     this.unshiftContainer("body", [
       t.variableDeclaration("var", [
         t.variableDeclarator(id, t.nullLiteral())
diff --git a/packages/babel-core/src/transformation/transformers/es6/spec.symbols.js b/packages/babel-core/src/transformation/transformers/es6/spec.symbols.js
index 0886017864..73826474b8 100644
--- a/packages/babel-core/src/transformation/transformers/es6/spec.symbols.js
+++ b/packages/babel-core/src/transformation/transformers/es6/spec.symbols.js
@@ -1,24 +1,24 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   optional: true
 };
 
-export var visitor = {
+export let visitor = {
   UnaryExpression(node, parent, scope, file) {
     if (node._ignoreSpecSymbols) return;
 
     if (this.parentPath.isBinaryExpression() && t.EQUALITY_BINARY_OPERATORS.indexOf(parent.operator) >= 0) {
       // optimise `typeof foo === "string"` since we can determine that they'll never need to handle symbols
-      var opposite = this.getOpposite();
+      let opposite = this.getOpposite();
       if (opposite.isLiteral() && opposite.node.value !== "symbol" && opposite.node.value !== "object") return;
     }
 
     if (node.operator === "typeof") {
-      var call = t.callExpression(file.addHelper("typeof"), [node.argument]);
+      let call = t.callExpression(file.addHelper("typeof"), [node.argument]);
       if (this.get("argument").isIdentifier()) {
-        var undefLiteral = t.stringLiteral("undefined");
-        var unary = t.unaryExpression("typeof", node.argument);
+        let undefLiteral = t.stringLiteral("undefined");
+        let unary = t.unaryExpression("typeof", node.argument);
         unary._ignoreSpecSymbols = true;
         return t.conditionalExpression(
           t.binaryExpression("===", unary, undefLiteral),
diff --git a/packages/babel-core/src/transformation/transformers/es6/spec.template-literals.js b/packages/babel-core/src/transformation/transformers/es6/spec.template-literals.js
index 7498b1820e..798718552b 100644
--- a/packages/babel-core/src/transformation/transformers/es6/spec.template-literals.js
+++ b/packages/babel-core/src/transformation/transformers/es6/spec.template-literals.js
@@ -1,15 +1,15 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   optional: true,
   group: "builtin-pre"
 };
 
-export var visitor = {
+export let visitor = {
   TemplateLiteral(node, parent) {
     if (t.isTaggedTemplateExpression(parent)) return;
 
-    for (var i = 0; i < node.expressions.length; i++) {
+    for (let i = 0; i < node.expressions.length; i++) {
       node.expressions[i] = t.callExpression(t.identifier("String"), [node.expressions[i]]);
     }
   }
diff --git a/packages/babel-core/src/transformation/transformers/es6/tail-call.js b/packages/babel-core/src/transformation/transformers/es6/tail-call.js
index efe7e7eeb7..8400873e22 100644
--- a/packages/babel-core/src/transformation/transformers/es6/tail-call.js
+++ b/packages/babel-core/src/transformation/transformers/es6/tail-call.js
@@ -5,14 +5,14 @@ import * as util from  "../../../util";
 import map from "lodash/collection/map";
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-trailing"
 };
 
-export var visitor = {
+export let visitor = {
   Function(node, parent, scope, file) {
     if (node.generator || node.async) return;
-    var tailCall = new TailCallTransformer(this, scope, file);
+    let tailCall = new TailCallTransformer(this, scope, file);
     tailCall.run();
   }
 };
@@ -21,7 +21,7 @@ function returnBlock(expr) {
   return t.blockStatement([t.returnStatement(expr)]);
 }
 
-var visitor = {
+let visitor = {
   enter(node, parent) {
     if (t.isTryStatement(parent)) {
       if (node === parent.block) {
@@ -101,14 +101,14 @@ class TailCallTransformer {
   }
 
   getParams() {
-    var params = this.params;
+    let params = this.params;
 
     if (!params) {
       params = this.node.params;
       this.paramDecls = [];
 
-      for (var i = 0; i < params.length; i++) {
-        var param = params[i];
+      for (let i = 0; i < params.length; i++) {
+        let param = params[i];
         if (!param._isDefaultPlaceholder) {
           this.paramDecls.push(t.variableDeclarator(
             param,
@@ -124,16 +124,16 @@ class TailCallTransformer {
   hasDeopt() {
     // check if the ownerId has been reassigned, if it has then it's not safe to
     // perform optimisations
-    var ownerIdInfo = this.scope.getBinding(this.ownerId.name);
+    let ownerIdInfo = this.scope.getBinding(this.ownerId.name);
     return ownerIdInfo && !ownerIdInfo.constant;
   }
 
   run() {
-    var node  = this.node;
+    let node  = this.node;
 
     // only tail recursion can be optimized as for now, so we can skip anonymous
     // functions entirely
-    var ownerId = this.ownerId;
+    let ownerId = this.ownerId;
     if (!ownerId) return;
 
     // traverse the function and look for tail recursion
@@ -150,10 +150,10 @@ class TailCallTransformer {
 
     //
 
-    var body = this.path.ensureBlock().body;
+    let body = this.path.ensureBlock().body;
 
-    for (var i = 0; i < body.length; i++) {
-      var bodyNode = body[i];
+    for (let i = 0; i < body.length; i++) {
+      let bodyNode = body[i];
       if (!t.isFunctionDeclaration(bodyNode)) continue;
 
       bodyNode = body[i] = t.variableDeclaration("var", [
@@ -163,22 +163,22 @@ class TailCallTransformer {
     }
 
     if (this.vars.length > 0) {
-      var declarations = flatten(map(this.vars, function (decl) {
+      let declarations = flatten(map(this.vars, function (decl) {
         return decl.declarations;
       }));
 
-      var assignment = reduceRight(declarations, function (expr, decl) {
+      let assignment = reduceRight(declarations, function (expr, decl) {
         return t.assignmentExpression("=", decl.id, expr);
       }, this.scope.buildUndefinedNode());
 
-      var statement = t.expressionStatement(assignment);
+      let statement = t.expressionStatement(assignment);
       statement._blockHoist = Infinity;
       body.unshift(statement);
     }
 
-    var paramDecls = this.paramDecls;
+    let paramDecls = this.paramDecls;
     if (paramDecls.length > 0) {
-      var paramDecl = t.variableDeclaration("var", paramDecls);
+      let paramDecl = t.variableDeclaration("var", paramDecls);
       paramDecl._blockHoist = Infinity;
       body.unshift(paramDecl);
     }
@@ -193,10 +193,10 @@ class TailCallTransformer {
       BLOCK:       node.body
     });
 
-    var topVars = [];
+    let topVars = [];
 
     if (this.needsThis) {
-      for (var path of (this.thisPaths: Array)) {
+      for (let path of (this.thisPaths: Array)) {
         path.replaceWith(this.getThisId());
       }
 
@@ -208,7 +208,7 @@ class TailCallTransformer {
         path.replaceWith(this.argumentsId);
       }
 
-      var decl = t.variableDeclarator(this.argumentsId);
+      let decl = t.variableDeclarator(this.argumentsId);
       if (this.argumentsId) {
         decl.init = t.identifier("arguments");
         decl.init._shadowedFunctionLiteral = this.path;
@@ -216,7 +216,7 @@ class TailCallTransformer {
       topVars.push(decl);
     }
 
-    var leftId = this.leftId;
+    let leftId = this.leftId;
     if (leftId) {
       topVars.push(t.variableDeclarator(leftId));
     }
@@ -229,13 +229,13 @@ class TailCallTransformer {
   subTransform(node) {
     if (!node) return;
 
-    var handler = this[`subTransform${node.type}`];
+    let handler = this[`subTransform${node.type}`];
     if (handler) return handler.call(this, node);
   }
 
   subTransformConditionalExpression(node) {
-    var callConsequent = this.subTransform(node.consequent);
-    var callAlternate = this.subTransform(node.alternate);
+    let callConsequent = this.subTransform(node.consequent);
+    let callAlternate = this.subTransform(node.alternate);
     if (!callConsequent && !callAlternate) {
       return;
     }
@@ -255,12 +255,12 @@ class TailCallTransformer {
 
   subTransformLogicalExpression(node) {
     // only call in right-value of can be optimized
-    var callRight = this.subTransform(node.right);
+    let callRight = this.subTransform(node.right);
     if (!callRight) return;
 
     // cache left value as it might have side-effects
-    var leftId = this.getLeftId();
-    var testExpr = t.assignmentExpression(
+    let leftId = this.getLeftId();
+    let testExpr = t.assignmentExpression(
       "=",
       leftId,
       node.left
@@ -274,10 +274,10 @@ class TailCallTransformer {
   }
 
   subTransformSequenceExpression(node) {
-    var seq = node.expressions;
+    let seq = node.expressions;
 
     // only last element can be optimized
-    var lastCall = this.subTransform(seq[seq.length - 1]);
+    let lastCall = this.subTransform(seq[seq.length - 1]);
     if (!lastCall) {
       return;
     }
@@ -292,8 +292,8 @@ class TailCallTransformer {
   }
 
   subTransformCallExpression(node) {
-    var callee = node.callee;
-    var thisBinding, args;
+    let callee = node.callee;
+    let thisBinding, args;
 
     if (t.isMemberExpression(callee, { computed: false }) && t.isIdentifier(callee.property)) {
       switch (callee.property.name) {
@@ -323,7 +323,7 @@ class TailCallTransformer {
 
     if (this.hasDeopt()) return;
 
-    var body = [];
+    let body = [];
 
     if (this.needsThis && !t.isThisExpression(thisBinding)) {
       body.push(t.expressionStatement(t.assignmentExpression(
@@ -337,8 +337,8 @@ class TailCallTransformer {
       args = t.arrayExpression(node.arguments);
     }
 
-    var argumentsId = this.getArgumentsId();
-    var params      = this.getParams();
+    let argumentsId = this.getArgumentsId();
+    let params      = this.getParams();
 
     if (this.needsArguments) {
       body.push(t.expressionStatement(t.assignmentExpression(
@@ -349,7 +349,7 @@ class TailCallTransformer {
     }
 
     if (t.isArrayExpression(args)) {
-      var elems = args.elements;
+      let elems = args.elements;
 
       // pad out the args so all the function args are reset - https://github.com/babel/babel/issues/1938
       while (elems.length < params.length) {
diff --git a/packages/babel-core/src/transformation/transformers/es7/async-functions.js b/packages/babel-core/src/transformation/transformers/es7/async-functions.js
index 83435eaaaa..18eb858c21 100644
--- a/packages/babel-core/src/transformation/transformers/es7/async-functions.js
+++ b/packages/babel-core/src/transformation/transformers/es7/async-functions.js
@@ -1,3 +1,3 @@
-export var metadata = {
+export let metadata = {
   stage: 2
 };
diff --git a/packages/babel-core/src/transformation/transformers/es7/class-properties.js b/packages/babel-core/src/transformation/transformers/es7/class-properties.js
index ca95104fab..6d65970ef0 100644
--- a/packages/babel-core/src/transformation/transformers/es7/class-properties.js
+++ b/packages/babel-core/src/transformation/transformers/es7/class-properties.js
@@ -1,4 +1,4 @@
-export var metadata = {
+export let metadata = {
   stage: 0,
   dependencies: ["es6.classes"]
 };
diff --git a/packages/babel-core/src/transformation/transformers/es7/comprehensions.js b/packages/babel-core/src/transformation/transformers/es7/comprehensions.js
index 0936429cef..b9dd2a9e8e 100644
--- a/packages/babel-core/src/transformation/transformers/es7/comprehensions.js
+++ b/packages/babel-core/src/transformation/transformers/es7/comprehensions.js
@@ -3,21 +3,21 @@ import traverse from "babel-traverse";
 import * as util from  "../../../util";
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   stage: 0
 };
 
-export var visitor = {
+export let visitor = {
   ComprehensionExpression(node, parent, scope) {
-    var callback = array;
+    let callback = array;
     if (node.generator) callback = generator;
     return callback(node, parent, scope);
   }
 };
 
 function generator(node) {
-  var body = [];
-  var container = t.functionExpression(null, [], t.blockStatement(body), true);
+  let body = [];
+  let container = t.functionExpression(null, [], t.blockStatement(body), true);
   container.shadow = true;
 
   body.push(buildComprehension(node, function () {
@@ -28,22 +28,22 @@ function generator(node) {
 }
 
 function array(node, parent, scope) {
-  var uid = scope.generateUidIdentifierBasedOnNode(parent);
+  let uid = scope.generateUidIdentifierBasedOnNode(parent);
 
-  var container = util.template("array-comprehension-container", {
+  let container = util.template("array-comprehension-container", {
     KEY: uid
   });
   container.callee.shadow = true;
 
-  var block = container.callee.body;
-  var body  = block.body;
+  let block = container.callee.body;
+  let body  = block.body;
 
   if (traverse.hasType(node, scope, "YieldExpression", t.FUNCTION_TYPES)) {
     container.callee.generator = true;
     container = t.yieldExpression(container, true);
   }
 
-  var returnStatement = body.pop();
+  let returnStatement = body.pop();
 
   body.push(buildComprehension(node, function () {
     return util.template("array-push", {
diff --git a/packages/babel-core/src/transformation/transformers/es7/decorators.js b/packages/babel-core/src/transformation/transformers/es7/decorators.js
index b0fd12c146..e81465ee44 100644
--- a/packages/babel-core/src/transformation/transformers/es7/decorators.js
+++ b/packages/babel-core/src/transformation/transformers/es7/decorators.js
@@ -2,15 +2,15 @@ import memoiseDecorators from "../../helpers/memoise-decorators";
 import * as defineMap from "../../helpers/define-map";
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   dependencies: ["es6.classes"],
   optional: true,
   stage: 1
 };
 
-export var visitor = {
+export let visitor = {
   ObjectExpression(node, parent, scope, file) {
-    var hasDecorators = false;
+    let hasDecorators = false;
     for (let i = 0; i < node.properties.length; i++) {
       let prop = node.properties[i];
       if (prop.decorators) {
@@ -20,7 +20,7 @@ export var visitor = {
     }
     if (!hasDecorators) return;
 
-    var mutatorMap = {};
+    let mutatorMap = {};
 
     for (let i = 0; i < node.properties.length; i++) {
       let prop = node.properties[i];
@@ -37,7 +37,7 @@ export var visitor = {
       defineMap.push(mutatorMap, prop, "initializer", file);
     }
 
-    var obj = defineMap.toClassObject(mutatorMap);
+    let obj = defineMap.toClassObject(mutatorMap);
     obj = defineMap.toComputedObjectFromClass(obj);
     return t.callExpression(file.addHelper("create-decorated-object"), [obj]);
   }
diff --git a/packages/babel-core/src/transformation/transformers/es7/do-expressions.js b/packages/babel-core/src/transformation/transformers/es7/do-expressions.js
index 3e3be4b82b..e7f01f1102 100644
--- a/packages/babel-core/src/transformation/transformers/es7/do-expressions.js
+++ b/packages/babel-core/src/transformation/transformers/es7/do-expressions.js
@@ -1,13 +1,13 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   optional: true,
   stage: 0
 };
 
-export var visitor = {
+export let visitor = {
   DoExpression(node) {
-    var body = node.body.body;
+    let body = node.body.body;
     if (body.length) {
       return body;
     } else {
diff --git a/packages/babel-core/src/transformation/transformers/es7/exponentiation-operator.js b/packages/babel-core/src/transformation/transformers/es7/exponentiation-operator.js
index 6539ed6860..90e4b214c5 100644
--- a/packages/babel-core/src/transformation/transformers/es7/exponentiation-operator.js
+++ b/packages/babel-core/src/transformation/transformers/es7/exponentiation-operator.js
@@ -3,11 +3,11 @@
 import build from "../../helpers/build-binary-assignment-operator-transformer";
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   stage: 2
 };
 
-export var visitor = build({
+export let visitor = build({
   operator: "**",
 
   build(left, right) {
diff --git a/packages/babel-core/src/transformation/transformers/es7/export-extensions.js b/packages/babel-core/src/transformation/transformers/es7/export-extensions.js
index 2e622c2d56..4cf5d4caa4 100644
--- a/packages/babel-core/src/transformation/transformers/es7/export-extensions.js
+++ b/packages/babel-core/src/transformation/transformers/es7/export-extensions.js
@@ -2,18 +2,18 @@
 
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   stage: 1
 };
 
 function build(node, nodes, scope) {
-  var first = node.specifiers[0];
+  let first = node.specifiers[0];
   if (!t.isExportNamespaceSpecifier(first) && !t.isExportDefaultSpecifier(first)) return;
 
-  var specifier = node.specifiers.shift();
-  var uid = scope.generateUidIdentifier(specifier.exported.name);
+  let specifier = node.specifiers.shift();
+  let uid = scope.generateUidIdentifier(specifier.exported.name);
 
-  var newSpecifier;
+  let newSpecifier;
   if (t.isExportNamespaceSpecifier(specifier)) {
     newSpecifier = t.importNamespaceSpecifier(uid);
   } else {
@@ -26,9 +26,9 @@ function build(node, nodes, scope) {
   build(node, nodes, scope);
 }
 
-export var visitor = {
+export let visitor = {
   ExportNamedDeclaration(node, parent, scope) {
-    var nodes = [];
+    let nodes = [];
     build(node, nodes, scope);
     if (!nodes.length) return;
 
diff --git a/packages/babel-core/src/transformation/transformers/es7/function-bind.js b/packages/babel-core/src/transformation/transformers/es7/function-bind.js
index 30f9ddf621..74bf548bff 100644
--- a/packages/babel-core/src/transformation/transformers/es7/function-bind.js
+++ b/packages/babel-core/src/transformation/transformers/es7/function-bind.js
@@ -2,13 +2,13 @@
 
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   optional: true,
   stage: 0
 };
 
 function getTempId(scope) {
-  var id = scope.path.getData("functionBind");
+  let id = scope.path.getData("functionBind");
   if (id) return id;
 
   id = scope.generateDeclaredUidIdentifier("context");
@@ -16,15 +16,15 @@ function getTempId(scope) {
 }
 
 function getStaticContext(bind, scope) {
-  var object = bind.object || bind.callee.object;
+  let object = bind.object || bind.callee.object;
   return scope.isStatic(object) && object;
 }
 
 function inferBindContext(bind, scope) {
-  var staticContext = getStaticContext(bind, scope);
+  let staticContext = getStaticContext(bind, scope);
   if (staticContext) return staticContext;
 
-  var tempId = getTempId(scope);
+  let tempId = getTempId(scope);
   if (bind.object) {
     bind.callee = t.sequenceExpression([
       t.assignmentExpression("=", tempId, bind.object),
@@ -36,18 +36,18 @@ function inferBindContext(bind, scope) {
   return tempId;
 }
 
-export var visitor = {
+export let visitor = {
   CallExpression(node, parent, scope) {
-    var bind = node.callee;
+    let bind = node.callee;
     if (!t.isBindExpression(bind)) return;
 
-    var context = inferBindContext(bind, scope);
+    let context = inferBindContext(bind, scope);
     node.callee = t.memberExpression(bind.callee, t.identifier("call"));
     node.arguments.unshift(context);
   },
 
   BindExpression(node, parent, scope) {
-    var context = inferBindContext(node, scope);
+    let context = inferBindContext(node, scope);
     return t.callExpression(t.memberExpression(node.callee, t.identifier("bind")), [context]);
   }
 };
diff --git a/packages/babel-core/src/transformation/transformers/es7/object-rest-spread.js b/packages/babel-core/src/transformation/transformers/es7/object-rest-spread.js
index 932adb0118..ea7b9c6a31 100644
--- a/packages/babel-core/src/transformation/transformers/es7/object-rest-spread.js
+++ b/packages/babel-core/src/transformation/transformers/es7/object-rest-spread.js
@@ -2,13 +2,13 @@
 
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   stage: 2,
   dependencies: ["es6.destructuring"]
 };
 
-var hasSpread = function (node) {
-  for (var i = 0; i < node.properties.length; i++) {
+let hasSpread = function (node) {
+  for (let i = 0; i < node.properties.length; i++) {
     if (t.isSpreadProperty(node.properties[i])) {
       return true;
     }
@@ -16,21 +16,21 @@ var hasSpread = function (node) {
   return false;
 };
 
-export var visitor = {
+export let visitor = {
   ObjectExpression(node, parent, scope, file) {
     if (!hasSpread(node)) return;
 
-    var args = [];
-    var props = [];
+    let args = [];
+    let props = [];
 
-    var push = function () {
+    let push = function () {
       if (!props.length) return;
       args.push(t.objectExpression(props));
       props = [];
     };
 
-    for (var i = 0; i < node.properties.length; i++) {
-      var prop = node.properties[i];
+    for (let i = 0; i < node.properties.length; i++) {
+      let prop = node.properties[i];
       if (t.isSpreadProperty(prop)) {
         push();
         args.push(prop.argument);
diff --git a/packages/babel-core/src/transformation/transformers/es7/trailing-function-commas.js b/packages/babel-core/src/transformation/transformers/es7/trailing-function-commas.js
index 5059fdd666..e75a1bd865 100644
--- a/packages/babel-core/src/transformation/transformers/es7/trailing-function-commas.js
+++ b/packages/babel-core/src/transformation/transformers/es7/trailing-function-commas.js
@@ -1,3 +1,3 @@
-export var metadata = {
+export let metadata = {
   stage: 1
 };
diff --git a/packages/babel-core/src/transformation/transformers/filters.js b/packages/babel-core/src/transformation/transformers/filters.js
index dff6631b9b..c34e5ea70e 100644
--- a/packages/babel-core/src/transformation/transformers/filters.js
+++ b/packages/babel-core/src/transformation/transformers/filters.js
@@ -5,17 +5,17 @@ export function internal(transformer) {
 }
 
 export function blacklist(transformer, opts) {
-  var blacklist = opts.blacklist;
+  let blacklist = opts.blacklist;
   if (blacklist.length && includes(blacklist, transformer.key)) return false;
 }
 
 export function whitelist(transformer, opts) {
-  var whitelist = opts.whitelist;
+  let whitelist = opts.whitelist;
   if (whitelist) return includes(whitelist, transformer.key);
 }
 
 export function stage(transformer, opts) {
-  var stage = transformer.metadata.stage;
+  let stage = transformer.metadata.stage;
   if (stage != null && stage >= opts.stage) return true;
 }
 
diff --git a/packages/babel-core/src/transformation/transformers/internal/hoist-directives.js b/packages/babel-core/src/transformation/transformers/internal/hoist-directives.js
index 74c9601515..dc38fc570a 100644
--- a/packages/babel-core/src/transformation/transformers/internal/hoist-directives.js
+++ b/packages/babel-core/src/transformation/transformers/internal/hoist-directives.js
@@ -1,14 +1,14 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-pre"
 };
 
-export var visitor = {
+export let visitor = {
   Block: {
     exit(node) {
-      for (var i = 0; i < node.body.length; i++) {
-        var bodyNode = node.body[i];
+      for (let i = 0; i < node.body.length; i++) {
+        let bodyNode = node.body[i];
         if (t.isExpressionStatement(bodyNode) && t.isLiteral(bodyNode.expression)) {
           bodyNode._blockHoist = Infinity;
         } else {
diff --git a/packages/babel-core/src/transformation/transformers/internal/module-formatter.js b/packages/babel-core/src/transformation/transformers/internal/module-formatter.js
index d767376582..a617385915 100644
--- a/packages/babel-core/src/transformation/transformers/internal/module-formatter.js
+++ b/packages/babel-core/src/transformation/transformers/internal/module-formatter.js
@@ -1,12 +1,12 @@
-export var metadata = {
+export let metadata = {
   group: "builtin-modules"
 };
 
-export var visitor = {
+export let visitor = {
   Program: {
     exit(program, parent, scope, file) {
       // ensure that these are at the top, just like normal imports
-      for (var node of (file.dynamicImports: Array)) {
+      for (let node of (file.dynamicImports: Array)) {
         node._blockHoist = 3;
       }
 
diff --git a/packages/babel-core/src/transformation/transformers/internal/modules.js b/packages/babel-core/src/transformation/transformers/internal/modules.js
index 340cc42893..ff3867981e 100644
--- a/packages/babel-core/src/transformation/transformers/internal/modules.js
+++ b/packages/babel-core/src/transformation/transformers/internal/modules.js
@@ -7,7 +7,7 @@
 import * as t from "babel-types";
 
 function getDeclar(node) {
-  var declar = node.declaration;
+  let declar = node.declaration;
   t.inheritsComments(declar, node);
   t.removeComments(node);
   declar._ignoreUserWhitespace = true;
@@ -19,18 +19,18 @@ function buildExportSpecifier(id) {
 }
 
 function cloneIdentifier({ name, loc }) {
-  var id = t.identifier(name);
+  let id = t.identifier(name);
   id._loc = loc;
   return id;
 }
 
-export var metadata = {
+export let metadata = {
   group: "builtin-pre"
 };
 
-export var visitor = {
+export let visitor = {
   ExportDefaultDeclaration(node, parent, scope) {
-    var declar = node.declaration;
+    let declar = node.declaration;
 
     if (t.isClassDeclaration(declar)) {
       // export default class Foo {};
@@ -39,7 +39,7 @@ export var visitor = {
       return nodes;
     } else if (t.isClassExpression(declar)) {
       // export default class {};
-      var temp = scope.generateUidIdentifier("default");
+      let temp = scope.generateUidIdentifier("default");
       node.declaration = t.variableDeclaration("var", [
         t.variableDeclarator(temp, declar)
       ]);
@@ -58,7 +58,7 @@ export var visitor = {
   },
 
   ExportNamedDeclaration(node) {
-    var declar = node.declaration;
+    let declar = node.declaration;
 
     if (t.isClassDeclaration(declar)) {
       // export class Foo {}
@@ -69,14 +69,14 @@ export var visitor = {
       return nodes;
     } else if (t.isFunctionDeclaration(declar)) {
       // export function Foo() {}
-      var newExport = t.exportNamedDeclaration(null, [buildExportSpecifier(declar.id)]);
+      let newExport = t.exportNamedDeclaration(null, [buildExportSpecifier(declar.id)]);
       newExport._blockHoist = 2;
       return [getDeclar(node), newExport];
     } else if (t.isVariableDeclaration(declar)) {
-      // export var foo = "bar";
-      var specifiers = [];
-      var bindings = this.get("declaration").getBindingIdentifiers();
-      for (var key in bindings) {
+      // export let foo = "bar";
+      let specifiers = [];
+      let bindings = this.get("declaration").getBindingIdentifiers();
+      for (let key in bindings) {
         specifiers.push(buildExportSpecifier(bindings[key]));
       }
       return [declar, t.exportNamedDeclaration(null, specifiers)];
@@ -85,11 +85,11 @@ export var visitor = {
 
   Program: {
     enter(node) {
-      var imports = [];
-      var rest    = [];
+      let imports = [];
+      let rest    = [];
 
-      for (var i = 0; i < node.body.length; i++) {
-        var bodyNode = node.body[i];
+      for (let i = 0; i < node.body.length; i++) {
+        let bodyNode = node.body[i];
         if (t.isImportDeclaration(bodyNode)) {
           imports.push(bodyNode);
         } else {
diff --git a/packages/babel-core/src/transformation/transformers/internal/validation.js b/packages/babel-core/src/transformation/transformers/internal/validation.js
index 7704923a70..8b27442419 100644
--- a/packages/babel-core/src/transformation/transformers/internal/validation.js
+++ b/packages/babel-core/src/transformation/transformers/internal/validation.js
@@ -1,22 +1,22 @@
 import * as messages from "babel-messages";
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-pre"
 };
 
-export var visitor = {
+export let visitor = {
   ForXStatement(node, parent, scope, file) {
-    var left = node.left;
+    let left = node.left;
     if (t.isVariableDeclaration(left)) {
-      var declar = left.declarations[0];
+      let declar = left.declarations[0];
       if (declar.init) throw file.errorWithNode(declar, messages.get("noAssignmentsInForHead"));
     }
   },
 
   Property(node, parent, scope, file) {
     if (node.kind === "set") {
-      var first = node.value.params[0];
+      let first = node.value.params[0];
       if (t.isRestElement(first)) {
         throw file.errorWithNode(first, messages.get("settersNoRest"));
       }
diff --git a/packages/babel-core/src/transformation/transformers/optimisation/flow.for-of.js b/packages/babel-core/src/transformation/transformers/optimisation/flow.for-of.js
index b5eeba6c62..b6b78bc25f 100644
--- a/packages/babel-core/src/transformation/transformers/optimisation/flow.for-of.js
+++ b/packages/babel-core/src/transformation/transformers/optimisation/flow.for-of.js
@@ -1,10 +1,10 @@
 import { _ForOfStatementArray } from "../es6/for-of";
 
-export var metadata = {
+export let metadata = {
   optional: true
 };
 
-export var visitor = {
+export let visitor = {
   ForOfStatement(node, parent, scope, file) {
     if (this.get("right").isGenericType("Array")) {
       return _ForOfStatementArray.call(this, node, scope, file);
diff --git a/packages/babel-core/src/transformation/transformers/optimisation/modules.system.js b/packages/babel-core/src/transformation/transformers/optimisation/modules.system.js
index cf60703d1e..88260235ce 100644
--- a/packages/babel-core/src/transformation/transformers/optimisation/modules.system.js
+++ b/packages/babel-core/src/transformation/transformers/optimisation/modules.system.js
@@ -1,11 +1,11 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   optional: true,
   group: "builtin-trailing"
 };
 
-export var visitor = {
+export let visitor = {
   Program(node, parent, scope, file){
     if (file.moduleFormatter._setters){
       scope.traverse(file.moduleFormatter._setters, optimizeSettersVisitor, {
@@ -24,7 +24,7 @@ export var visitor = {
  * TODO: Ideally this would be optimized during construction of the setters, but the current
  * architecture of the module formatters make that difficult.
  */
-var optimizeSettersVisitor = {
+let optimizeSettersVisitor = {
   FunctionExpression: {
     enter: (node, parent, scope, state) => {
       state.hasExports = false;
@@ -44,7 +44,7 @@ var optimizeSettersVisitor = {
     if (!t.isIdentifier(node.callee, {name: state.exportFunctionIdentifier.name})) return;
 
     state.hasExports = true;
-    var memberNode = t.memberExpression(t.cloneDeep(state.exportObjectIdentifier), node.arguments[0], true);
+    let memberNode = t.memberExpression(t.cloneDeep(state.exportObjectIdentifier), node.arguments[0], true);
     return t.assignmentExpression("=", memberNode, node.arguments[1]);
   }
 };
diff --git a/packages/babel-core/src/transformation/transformers/optimisation/react.inline-elements.js b/packages/babel-core/src/transformation/transformers/optimisation/react.inline-elements.js
index 6013f4772c..9769516a9f 100644
--- a/packages/babel-core/src/transformation/transformers/optimisation/react.inline-elements.js
+++ b/packages/babel-core/src/transformation/transformers/optimisation/react.inline-elements.js
@@ -1,13 +1,13 @@
 import { react } from "babel-types";
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   optional: true
 };
 
 function hasRefOrSpread(attrs) {
-  for (var i = 0; i < attrs.length; i++) {
-    var attr = attrs[i];
+  for (let i = 0; i < attrs.length; i++) {
+    let attr = attrs[i];
     if (t.isJSXSpreadAttribute(attr)) return true;
     if (isJSXAttributeOfName(attr, "ref")) return true;
   }
@@ -17,18 +17,18 @@ function hasRefOrSpread(attrs) {
 function isJSXAttributeOfName(attr, name) {
   return t.isJSXAttribute(attr) && t.isJSXIdentifier(attr.name, { name: name });
 }
-export var visitor = {
+export let visitor = {
   JSXElement(node, parent, scope, file) {
     // filter
-    var open = node.openingElement;
+    let open = node.openingElement;
     if (hasRefOrSpread(open.attributes)) return;
 
     // init
-    var isComponent = true;
-    var props       = t.objectExpression([]);
-    var obj         = t.objectExpression([]);
-    var key         = t.nullLiteral();
-    var type        = open.name;
+    let isComponent = true;
+    let props       = t.objectExpression([]);
+    let obj         = t.objectExpression([]);
+    let key         = t.nullLiteral();
+    let type        = open.name;
 
     if (t.isJSXIdentifier(type) && react.isCompatTag(type.name)) {
       type = t.stringLiteral(type.name);
@@ -48,14 +48,14 @@ export var visitor = {
     pushElemProp("ref", t.nullLiteral());
 
     if (node.children.length) {
-      var children = react.buildChildren(node);
+      let children = react.buildChildren(node);
       children = children.length === 1 ? children[0] : t.arrayExpression(children);
       pushProp(props.properties, t.identifier("children"), children);
     }
 
     // props
-    for (var i = 0; i < open.attributes.length; i++) {
-      var attr = open.attributes[i];
+    for (let i = 0; i < open.attributes.length; i++) {
+      let attr = open.attributes[i];
       if (isJSXAttributeOfName(attr, "key")) {
         key = attr.value;
       } else {
diff --git a/packages/babel-core/src/transformation/transformers/other/async-to-generator.js b/packages/babel-core/src/transformation/transformers/other/async-to-generator.js
index 844bfd0691..a1ca6f2666 100644
--- a/packages/babel-core/src/transformation/transformers/other/async-to-generator.js
+++ b/packages/babel-core/src/transformation/transformers/other/async-to-generator.js
@@ -2,12 +2,12 @@ import remapAsyncToGenerator from "../../helpers/remap-async-to-generator";
 
 export { manipulateOptions } from "./bluebird-coroutines";
 
-export var metadata = {
+export let metadata = {
   optional: true,
   dependencies: ["es7.asyncFunctions", "es6.classes"]
 };
 
-export var visitor = {
+export let visitor = {
   Function(node, parent, scope, file) {
     if (!node.async || node.generator) return;
 
diff --git a/packages/babel-core/src/transformation/transformers/other/bluebird-coroutines.js b/packages/babel-core/src/transformation/transformers/other/bluebird-coroutines.js
index 52cdc1b1e7..1caf3e82cd 100644
--- a/packages/babel-core/src/transformation/transformers/other/bluebird-coroutines.js
+++ b/packages/babel-core/src/transformation/transformers/other/bluebird-coroutines.js
@@ -5,12 +5,12 @@ export function manipulateOptions(opts) {
   opts.blacklist.push("regenerator");
 }
 
-export var metadata = {
+export let metadata = {
   optional: true,
   dependencies: ["es7.asyncFunctions", "es6.classes"]
 };
 
-export var visitor = {
+export let visitor = {
   Function(node, parent, scope, file) {
     if (!node.async || node.generator) return;
 
diff --git a/packages/babel-core/src/transformation/transformers/other/react-compat.js b/packages/babel-core/src/transformation/transformers/other/react-compat.js
index ed065009eb..0e3be1822b 100644
--- a/packages/babel-core/src/transformation/transformers/other/react-compat.js
+++ b/packages/babel-core/src/transformation/transformers/other/react-compat.js
@@ -5,12 +5,12 @@ export function manipulateOptions(opts) {
   opts.blacklist.push("react");
 }
 
-export var metadata = {
+export let metadata = {
   optional: true,
   group: "builtin-advanced"
 };
 
-export var visitor = require("../../helpers/build-react-transformer")({
+export let visitor = require("../../helpers/build-react-transformer")({
   pre(state) {
     state.callee = state.tagExpr;
   },
diff --git a/packages/babel-core/src/transformation/transformers/other/react.js b/packages/babel-core/src/transformation/transformers/other/react.js
index 1345f842f2..7a8267bef6 100644
--- a/packages/babel-core/src/transformation/transformers/other/react.js
+++ b/packages/babel-core/src/transformation/transformers/other/react.js
@@ -1,16 +1,16 @@
 import { react } from "babel-types";
 import * as t from "babel-types";
 
-var JSX_ANNOTATION_REGEX = /^\*\s*@jsx\s+([^\s]+)/;
+let JSX_ANNOTATION_REGEX = /^\*\s*@jsx\s+([^\s]+)/;
 
-export var metadata = {
+export let metadata = {
   group: "builtin-advanced"
 };
 
-export var visitor = require("../../helpers/build-react-transformer")({
+export let visitor = require("../../helpers/build-react-transformer")({
   pre(state) {
-    var tagName = state.tagName;
-    var args    = state.args;
+    let tagName = state.tagName;
+    let args    = state.args;
     if (react.isCompatTag(tagName)) {
       args.push(t.stringLiteral(tagName));
     } else {
@@ -24,11 +24,11 @@ export var visitor = require("../../helpers/build-react-transformer")({
 });
 
 visitor.Program = function (node, parent, scope, file) {
-  var id = file.opts.jsxPragma;
+  let id = file.opts.jsxPragma;
 
-  for (var i = 0; i < file.ast.comments.length; i++) {
-    var comment = file.ast.comments[i];
-    var matches = JSX_ANNOTATION_REGEX.exec(comment.value);
+  for (let i = 0; i < file.ast.comments.length; i++) {
+    let comment = file.ast.comments[i];
+    let matches = JSX_ANNOTATION_REGEX.exec(comment.value);
     if (matches) {
       id = matches[1];
       if (id === "React.DOM") {
diff --git a/packages/babel-core/src/transformation/transformers/other/regenerator.js b/packages/babel-core/src/transformation/transformers/other/regenerator.js
index 91f91dffb8..4d051b5f00 100644
--- a/packages/babel-core/src/transformation/transformers/other/regenerator.js
+++ b/packages/babel-core/src/transformation/transformers/other/regenerator.js
@@ -7,11 +7,11 @@ import * as t from "babel-types";
 // the version that Regenerator depends on. See for example #1958.
 const NodePath = regenerator.types.NodePath;
 
-export var metadata = {
+export let metadata = {
   group: "builtin-advanced"
 };
 
-export var visitor = {
+export let visitor = {
   Function: {
     exit(node) {
       if (node.async || node.generator) {
@@ -33,17 +33,17 @@ export var visitor = {
 // complicated by having to include intermediate objects like blockStatement.body
 // arrays, in addition to Node objects.
 function convertNodePath(path) {
-  var programNode;
-  var keysAlongPath = [];
+  let programNode;
+  let keysAlongPath = [];
 
   while (path) {
-    var pp = path.parentPath;
-    var parentNode = pp && pp.node;
+    let pp = path.parentPath;
+    let parentNode = pp && pp.node;
     if (parentNode) {
       keysAlongPath.push(path.key);
 
       if (parentNode !== path.container) {
-        var found = Object.keys(parentNode).some(listKey => {
+        let found = Object.keys(parentNode).some(listKey => {
           if (parentNode[listKey] === path.container) {
             keysAlongPath.push(listKey);
             return true;
@@ -68,7 +68,7 @@ function convertNodePath(path) {
     throw new Error("Failed to find root Program node");
   }
 
-  var nodePath = new NodePath(programNode);
+  let nodePath = new NodePath(programNode);
 
   while (keysAlongPath.length > 0) {
     nodePath = nodePath.get(keysAlongPath.pop());
diff --git a/packages/babel-core/src/transformation/transformers/other/strict.js b/packages/babel-core/src/transformation/transformers/other/strict.js
index 06832f7061..0902c92fcb 100644
--- a/packages/babel-core/src/transformation/transformers/other/strict.js
+++ b/packages/babel-core/src/transformation/transformers/other/strict.js
@@ -1,6 +1,6 @@
 import * as t from "babel-types";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-pre"
 };
 
@@ -16,12 +16,12 @@ function isUseStrict(node) {
   }
 }
 
-export var visitor = {
+export let visitor = {
   Program: {
     enter(program) {
-      var first = program.body[0];
+      let first = program.body[0];
 
-      var directive;
+      let directive;
       if (t.isExpressionStatement(first) && isUseStrict(first.expression)) {
         directive = first;
       } else {
diff --git a/packages/babel-core/src/transformation/transformers/spec/function-name.js b/packages/babel-core/src/transformation/transformers/spec/function-name.js
index 5b6784c0b2..a4044ce7d2 100644
--- a/packages/babel-core/src/transformation/transformers/spec/function-name.js
+++ b/packages/babel-core/src/transformation/transformers/spec/function-name.js
@@ -1,12 +1,12 @@
 import { bare } from "../../helpers/name-method";
 
-export var metadata = {
+export let metadata = {
   group: "builtin-basic"
 };
 
 // visit Property functions first - https://github.com/babel/babel/issues/1860
 
-export var visitor = {
+export let visitor = {
   "ArrowFunctionExpression|FunctionExpression": {
     exit() {
       if (!this.parentPath.isProperty()) {
@@ -16,11 +16,11 @@ export var visitor = {
   },
 
   ObjectExpression() {
-    var props = this.get("properties");
-    for (var prop of (props: Array)) {
-      var value = prop.get("value");
+    let props = this.get("properties");
+    for (let prop of (props: Array)) {
+      let value = prop.get("value");
       if (value.isFunction()) {
-        var newNode = bare(value.node, prop.node, value.scope);
+        let newNode = bare(value.node, prop.node, value.scope);
         if (newNode) value.replaceWith(newNode);
       }
     }
diff --git a/packages/babel-core/src/util.js b/packages/babel-core/src/util.js
index 878af6f20c..cc7fe4f303 100644
--- a/packages/babel-core/src/util.js
+++ b/packages/babel-core/src/util.js
@@ -24,8 +24,8 @@ export { inherits, inspect } from "util";
  */
 
 export function canCompile(filename: string, altExts?: Array<string>) {
-  var exts = altExts || canCompile.EXTENSIONS;
-  var ext = path.extname(filename);
+  let exts = altExts || canCompile.EXTENSIONS;
+  let ext = path.extname(filename);
   return contains(exts, ext);
 }
 
@@ -68,7 +68,7 @@ export function regexify(val: any): RegExp {
     if (startsWith(val, "./") || startsWith(val, "*/")) val = val.slice(2);
     if (startsWith(val, "**/")) val = val.slice(3);
 
-    var regex = minimatch.makeRe(val, { nocase: true });
+    let regex = minimatch.makeRe(val, { nocase: true });
     return new RegExp(regex.source.slice(1, -1), "i");
   }
 
@@ -142,7 +142,7 @@ function _shouldIgnore(pattern, filename) {
  * A visitor for Babel templates, replaces placeholder references.
  */
 
-var templateVisitor = {
+let templateVisitor = {
   /**
    * 360 NoScope PWNd
    */
@@ -169,7 +169,7 @@ var templateVisitor = {
  */
 
 export function template(name: string, nodes?: Array<Object>, keepExpression?: boolean): Object {
-  var ast = exports.templates[name];
+  let ast = exports.templates[name];
   if (!ast) throw new ReferenceError(`unknown template ${name}`);
 
   if (nodes === true) {
@@ -185,7 +185,7 @@ export function template(name: string, nodes?: Array<Object>, keepExpression?: b
 
   if (ast.body.length > 1) return ast.body;
 
-  var node = ast.body[0];
+  let node = ast.body[0];
 
   if (!keepExpression && t.isExpressionStatement(node)) {
     return node.expression;
@@ -200,7 +200,7 @@ export function template(name: string, nodes?: Array<Object>, keepExpression?: b
 
 export function parseTemplate(loc: string, code: string): Object {
   try {
-    var ast = parse(code, { filename: loc, looseModules: true }).program;
+    let ast = parse(code, { filename: loc, looseModules: true }).program;
     ast = traverse.removeProperties(ast);
     return ast;
   } catch (err) {
@@ -214,19 +214,19 @@ export function parseTemplate(loc: string, code: string): Object {
  */
 
 function loadTemplates(): Object {
-  var templates = {};
+  let templates = {};
 
-  var templatesLoc = path.join(__dirname, "transformation/templates");
+  let templatesLoc = path.join(__dirname, "transformation/templates");
   if (!pathExists.sync(templatesLoc)) {
     throw new ReferenceError(messages.get("missingTemplatesDirectory"));
   }
 
-  for (var name of (fs.readdirSync(templatesLoc): Array)) {
+  for (let name of (fs.readdirSync(templatesLoc): Array)) {
     if (name[0] === ".") continue;
 
-    var key  = path.basename(name, path.extname(name));
-    var loc  = path.join(templatesLoc, name);
-    var code = fs.readFileSync(loc, "utf8");
+    let key  = path.basename(name, path.extname(name));
+    let loc  = path.join(templatesLoc, name);
+    let code = fs.readFileSync(loc, "utf8");
 
     templates[key] = parseTemplate(loc, code);
   }
diff --git a/packages/babel-generator/src/buffer.js b/packages/babel-generator/src/buffer.js
index 1dc67227d9..8cc1760976 100644
--- a/packages/babel-generator/src/buffer.js
+++ b/packages/babel-generator/src/buffer.js
@@ -210,12 +210,12 @@ export default class Buffer {
    */
 
   _removeSpacesAfterLastNewline() {
-    var lastNewlineIndex = this.buf.lastIndexOf("\n");
+    let lastNewlineIndex = this.buf.lastIndexOf("\n");
     if (lastNewlineIndex === -1) {
       return;
     }
 
-    var index = this.buf.length - 1;
+    let index = this.buf.length - 1;
     while (index > lastNewlineIndex) {
       if (this.buf[index] !== " ") {
         break;
@@ -236,7 +236,7 @@ export default class Buffer {
   push(str, noIndent) {
     if (!this.format.compact && this._indent && !noIndent && str !== "\n") {
       // we have an indent level and we aren't pushing a newline
-      var indent = this.getIndent();
+      let indent = this.getIndent();
 
       // replace all newlines with newlines with the indentation
       str = str.replace(/\n/g, `\n${indent}`);
@@ -254,10 +254,10 @@ export default class Buffer {
 
   _push(str) {
     // see startTerminatorless() instance method
-    var parenPushNewlineState = this.parenPushNewlineState;
+    let parenPushNewlineState = this.parenPushNewlineState;
     if (parenPushNewlineState) {
-      for (var i = 0; i < str.length; i++) {
-        var cha = str[i];
+      for (let i = 0; i < str.length; i++) {
+        let cha = str[i];
 
         // we can ignore spaces since they wont interupt a terminatorless separator
         if (cha === " ") continue;
@@ -299,8 +299,8 @@ export default class Buffer {
   isLast(cha) {
     if (this.format.compact) return false;
 
-    var buf = this.buf;
-    var last = buf[buf.length - 1];
+    let buf = this.buf;
+    let last = buf[buf.length - 1];
 
     if (Array.isArray(cha)) {
       return includes(cha, last);
diff --git a/packages/babel-generator/src/generators/expressions.js b/packages/babel-generator/src/generators/expressions.js
index d1ec81fd54..598592c4c6 100644
--- a/packages/babel-generator/src/generators/expressions.js
+++ b/packages/babel-generator/src/generators/expressions.js
@@ -5,8 +5,8 @@ import * as t from "babel-types";
 const SCIENTIFIC_NOTATION = /e/i;
 
 export function UnaryExpression(node, print) {
-  var needsSpace = /[a-z]$/.test(node.operator);
-  var arg = node.argument;
+  let needsSpace = /[a-z]$/.test(node.operator);
+  let arg = node.argument;
 
   if (t.isUpdateExpression(arg) || t.isUnaryExpression(arg)) {
     needsSpace = true;
@@ -86,9 +86,9 @@ export function CallExpression(node, print) {
 
   this.push("(");
 
-  var isPrettyCall = node._prettyCall && !this.format.retainLines && !this.format.compact;
+  let isPrettyCall = node._prettyCall && !this.format.retainLines && !this.format.compact;
 
-  var separator;
+  let separator;
   if (isPrettyCall) {
     separator = ",\n";
     this.newline();
@@ -115,15 +115,15 @@ function buildYieldAwait(keyword) {
 
     if (node.argument) {
       this.push(" ");
-      var terminatorState = this.startTerminatorless();
+      let terminatorState = this.startTerminatorless();
       print.plain(node.argument);
       this.endTerminatorless(terminatorState);
     }
   };
 }
 
-export var YieldExpression = buildYieldAwait("yield");
-export var AwaitExpression = buildYieldAwait("await");
+export let YieldExpression = buildYieldAwait("yield");
+export let AwaitExpression = buildYieldAwait("await");
 
 export function EmptyStatement() {
   this.semicolon();
@@ -144,7 +144,7 @@ export function AssignmentExpression(node, print) {
   // todo: add cases where the spaces can be dropped when in compact mode
   print.plain(node.left);
 
-  var spaces = node.operator === "in" || node.operator === "instanceof";
+  let spaces = node.operator === "in" || node.operator === "instanceof";
   spaces = true; // todo: https://github.com/babel/babel/issues/1835
   this.space(spaces);
 
@@ -175,14 +175,14 @@ export {
 };
 
 export function MemberExpression(node, print) {
-  var obj = node.object;
+  let obj = node.object;
   print.plain(obj);
 
   if (!node.computed && t.isMemberExpression(node.property)) {
     throw new TypeError("Got a MemberExpression for MemberExpression property");
   }
 
-  var computed = node.computed;
+  let computed = node.computed;
   if (t.isLiteral(node.property) && isNumber(node.property.value)) {
     computed = true;
   }
@@ -193,7 +193,7 @@ export function MemberExpression(node, print) {
     this.push("]");
   } else {
     if (t.isLiteral(node.object)) {
-      var val = this._Literal(node.object);
+      let val = this._Literal(node.object);
       if (isInteger(+val) && !SCIENTIFIC_NOTATION.test(val) && !this.endsWith(".")) {
         this.push(".");
       }
diff --git a/packages/babel-generator/src/generators/flow.js b/packages/babel-generator/src/generators/flow.js
index 8e0e0cf832..4a42f81d04 100644
--- a/packages/babel-generator/src/generators/flow.js
+++ b/packages/babel-generator/src/generators/flow.js
@@ -38,7 +38,7 @@ export function DeclareModule(node, print) {
 }
 
 export function DeclareVariable(node, print) {
-  this.push("declare var ");
+  this.push("declare let ");
   print.plain(node.id);
   print.plain(node.id.typeAnnotation);
   this.semicolon();
@@ -174,7 +174,7 @@ export { TypeParameterInstantiation as TypeParameterDeclaration };
 
 export function ObjectTypeAnnotation(node, print) {
   this.push("{");
-  var props = node.properties.concat(node.callProperties, node.indexers);
+  let props = node.properties.concat(node.callProperties, node.indexers);
 
   if (props.length) {
     this.space();
diff --git a/packages/babel-generator/src/generators/jsx.js b/packages/babel-generator/src/generators/jsx.js
index b187930a69..a89061e540 100644
--- a/packages/babel-generator/src/generators/jsx.js
+++ b/packages/babel-generator/src/generators/jsx.js
@@ -39,12 +39,12 @@ export function JSXText(node) {
 }
 
 export function JSXElement(node, print) {
-  var open = node.openingElement;
+  let open = node.openingElement;
   print.plain(open);
   if (open.selfClosing) return;
 
   this.indent();
-  for (var child of (node.children: Array)) {
+  for (let child of (node.children: Array)) {
     print.plain(child);
   }
   this.dedent();
diff --git a/packages/babel-generator/src/generators/methods.js b/packages/babel-generator/src/generators/methods.js
index 68f86f67dd..920235f05f 100644
--- a/packages/babel-generator/src/generators/methods.js
+++ b/packages/babel-generator/src/generators/methods.js
@@ -17,9 +17,9 @@ export function _params(node, print) {
 }
 
 export function _method(node, print) {
-  var value = node.value;
-  var kind  = node.kind;
-  var key   = node.key;
+  let value = node.value;
+  let kind  = node.kind;
+  let key   = node.key;
 
   if (kind === "method" || kind === "init") {
     if (value.generator) {
diff --git a/packages/babel-generator/src/generators/modules.js b/packages/babel-generator/src/generators/modules.js
index 951e6bb24b..0864dd58a6 100644
--- a/packages/babel-generator/src/generators/modules.js
+++ b/packages/babel-generator/src/generators/modules.js
@@ -51,10 +51,10 @@ export function ExportDefaultDeclaration(node, print) {
 }
 
 function ExportDeclaration(node, print) {
-  var specifiers = node.specifiers;
+  let specifiers = node.specifiers;
 
   if (node.declaration) {
-    var declar = node.declaration;
+    let declar = node.declaration;
     print.plain(declar);
     if (t.isStatement(declar) || t.isFunction(declar) || t.isClass(declar)) return;
   } else {
@@ -62,8 +62,8 @@ function ExportDeclaration(node, print) {
       this.push("type ");
     }
 
-    var first = specifiers[0];
-    var hasSpecial = false;
+    let first = specifiers[0];
+    let hasSpecial = false;
     if (t.isExportDefaultSpecifier(first) || t.isExportNamespaceSpecifier(first)) {
       hasSpecial = true;
       print.plain(specifiers.shift());
@@ -98,9 +98,9 @@ export function ImportDeclaration(node, print) {
     this.push(node.importKind + " ");
   }
 
-  var specfiers = node.specifiers;
+  let specfiers = node.specifiers;
   if (specfiers && specfiers.length) {
-    var first = node.specifiers[0];
+    let first = node.specifiers[0];
     if (t.isImportDefaultSpecifier(first) || t.isImportNamespaceSpecifier(first)) {
       print.plain(node.specifiers.shift());
       if (node.specifiers.length) {
diff --git a/packages/babel-generator/src/generators/statements.js b/packages/babel-generator/src/generators/statements.js
index d059ead64d..7a67bfdc63 100644
--- a/packages/babel-generator/src/generators/statements.js
+++ b/packages/babel-generator/src/generators/statements.js
@@ -55,7 +55,7 @@ export function WhileStatement(node, print) {
   print.block(node.body);
 }
 
-var buildForXStatement = function (op) {
+let buildForXStatement = function (op) {
   return function (node, print) {
     this.keyword("for");
     this.push("(");
@@ -67,8 +67,8 @@ var buildForXStatement = function (op) {
   };
 };
 
-export var ForInStatement = buildForXStatement("in");
-export var ForOfStatement = buildForXStatement("of");
+export let ForInStatement = buildForXStatement("in");
+export let ForOfStatement = buildForXStatement("of");
 
 export function DoWhileStatement(node, print) {
   this.push("do ");
@@ -84,10 +84,10 @@ function buildLabelStatement(prefix, key = "label") {
   return function (node, print) {
     this.push(prefix);
 
-    var label = node[key];
+    let label = node[key];
     if (label) {
       this.push(" ");
-      var terminatorState = this.startTerminatorless();
+      let terminatorState = this.startTerminatorless();
       print.plain(label);
       this.endTerminatorless(terminatorState);
     }
@@ -96,10 +96,10 @@ function buildLabelStatement(prefix, key = "label") {
   };
 }
 
-export var ContinueStatement = buildLabelStatement("continue");
-export var ReturnStatement   = buildLabelStatement("return", "argument");
-export var BreakStatement    = buildLabelStatement("break");
-export var ThrowStatement    = buildLabelStatement("throw", "argument");
+export let ContinueStatement = buildLabelStatement("continue");
+export let ReturnStatement   = buildLabelStatement("return", "argument");
+export let BreakStatement    = buildLabelStatement("break");
+export let ThrowStatement    = buildLabelStatement("throw", "argument");
 
 export function LabeledStatement(node, print) {
   print.plain(node.label);
@@ -176,10 +176,10 @@ export function DebuggerStatement() {
 export function VariableDeclaration(node, print, parent) {
   this.push(node.kind + " ");
 
-  var hasInits = false;
+  let hasInits = false;
   // don't add whitespace to loop heads
   if (!t.isFor(parent)) {
-    for (var declar of (node.declarations: Array)) {
+    for (let declar of (node.declarations: Array)) {
       if (declar.init) {
         // has an init so let's split it up over multiple lines
         hasInits = true;
@@ -191,15 +191,15 @@ export function VariableDeclaration(node, print, parent) {
   // use a pretty separator when we aren't in compact mode, have initializers and don't have retainLines on
   // this will format declarations like:
   //
-  //   var foo = "bar", bar = "foo";
+  //   let foo = "bar", bar = "foo";
   //
   // into
   //
-  //   var foo = "bar",
+  //   let foo = "bar",
   //       bar = "foo";
   //
 
-  var sep;
+  let sep;
   if (!this.format.compact && !this.format.concise && hasInits && !this.format.retainLines) {
     sep = `,\n${repeating(" ", node.kind.length + 1)}`;
   }
diff --git a/packages/babel-generator/src/generators/template-literals.js b/packages/babel-generator/src/generators/template-literals.js
index 1bd78796dd..9d0b4e694c 100644
--- a/packages/babel-generator/src/generators/template-literals.js
+++ b/packages/babel-generator/src/generators/template-literals.js
@@ -10,10 +10,10 @@ export function TemplateElement(node) {
 export function TemplateLiteral(node, print) {
   this.push("`");
 
-  var quasis = node.quasis;
-  var len    = quasis.length;
+  let quasis = node.quasis;
+  let len    = quasis.length;
 
-  for (var i = 0; i < len; i++) {
+  for (let i = 0; i < len; i++) {
     print.plain(quasis[i]);
 
     if (i + 1 < len) {
diff --git a/packages/babel-generator/src/generators/types.js b/packages/babel-generator/src/generators/types.js
index dc892ab8f0..d1fcfac361 100644
--- a/packages/babel-generator/src/generators/types.js
+++ b/packages/babel-generator/src/generators/types.js
@@ -14,7 +14,7 @@ export function RestElement(node, print) {
 export { RestElement as SpreadElement, RestElement as SpreadProperty };
 
 export function ObjectExpression(node, print) {
-  var props = node.properties;
+  let props = node.properties;
 
   this.push("{");
   print.printInnerComments();
@@ -65,14 +65,14 @@ export function Property(node, print) {
 }
 
 export function ArrayExpression(node, print) {
-  var elems = node.elements;
-  var len   = elems.length;
+  let elems = node.elements;
+  let len   = elems.length;
 
   this.push("[");
   print.printInnerComments();
 
-  for (var i = 0; i < elems.length; i++) {
-    var elem = elems[i];
+  for (let i = 0; i < elems.length; i++) {
+    let elem = elems[i];
     if (elem) {
       if (i > 0) this.space();
       print.plain(elem);
diff --git a/packages/babel-generator/src/index.js b/packages/babel-generator/src/index.js
index 6a7b60e748..b560b7e518 100644
--- a/packages/babel-generator/src/index.js
+++ b/packages/babel-generator/src/index.js
@@ -40,13 +40,13 @@ class CodeGenerator {
    */
 
   static normalizeOptions(code, opts, tokens) {
-    var style = "  ";
+    let style = "  ";
     if (code) {
-      var indent = detectIndent(code).indent;
+      let indent = detectIndent(code).indent;
       if (indent && indent !== " ") style = indent;
     }
 
-    var format = {
+    let format = {
       shouldPrintComment: opts.shouldPrintComment,
       retainLines: opts.retainLines,
       comments: opts.comments == null || opts.comments,
@@ -78,18 +78,18 @@ class CodeGenerator {
    * Determine if input code uses more single or double quotes.
    */
   static findCommonStringDelimiter(code, tokens) {
-    var occurences = {
+    let occurences = {
       single: 0,
       double: 0
     };
 
-    var checked = 0;
+    let checked = 0;
 
-    for (var i = 0; i < tokens.length; i++) {
-      var token = tokens[i];
+    for (let i = 0; i < tokens.length; i++) {
+      let token = tokens[i];
       if (token.type.label !== "string") continue;
 
-      var raw = code.slice(token.start, token.end);
+      let raw = code.slice(token.start, token.end);
       if (raw[0] === "'") {
         occurences.single++;
       } else {
@@ -131,13 +131,13 @@ class CodeGenerator {
    */
 
   generate() {
-    var ast = this.ast;
+    let ast = this.ast;
 
     this.print(ast);
 
     if (ast.comments) {
-      var comments = [];
-      for (var comment of (ast.comments: Array)) {
+      let comments = [];
+      for (let comment of (ast.comments: Array)) {
         if (!comment._displayed) comments.push(comment);
       }
       this._printComments(comments);
@@ -171,7 +171,7 @@ class CodeGenerator {
       return;
     }
 
-    var lines = 0;
+    let lines = 0;
 
     if (node.start != null && !node._ignoreUserWhitespace) {
       // user node
@@ -185,7 +185,7 @@ class CodeGenerator {
       if (!leading) lines++; // always include at least a single line after
       if (opts.addNewlines) lines += opts.addNewlines(leading, node) || 0;
 
-      var needs = n.needsWhitespaceAfter;
+      let needs = n.needsWhitespaceAfter;
       if (leading) needs = n.needsWhitespaceBefore;
       if (needs(node, parent)) lines++;
 
@@ -203,7 +203,7 @@ class CodeGenerator {
       node._compact = true;
     }
 
-    var oldConcise = this.format.concise;
+    let oldConcise = this.format.concise;
     if (node._compact) {
       this.format.concise = true;
     }
@@ -212,7 +212,7 @@ class CodeGenerator {
       throw new ReferenceError(`unknown node of type ${JSON.stringify(node.type)} with constructor ${JSON.stringify(node && node.constructor.name)}`);
     }
 
-    var needsParens = n.needsParens(node, parent);
+    let needsParens = n.needsParens(node, parent);
     if (needsParens) this.push("(");
 
     this.printLeadingComments(node, parent);
@@ -241,11 +241,11 @@ class CodeGenerator {
   printJoin(print, nodes, opts = {}) {
     if (!nodes || !nodes.length) return;
 
-    var len = nodes.length;
+    let len = nodes.length;
 
     if (opts.indent) this.indent();
 
-    var printOpts = {
+    let printOpts = {
       statement: opts.statement,
       addNewlines: opts.addNewlines,
       after: () => {
@@ -259,8 +259,8 @@ class CodeGenerator {
       }
     };
 
-    for (var i = 0; i < nodes.length; i++) {
-      var node = nodes[i];
+    for (let i = 0; i < nodes.length; i++) {
+      let node = nodes[i];
       print.plain(node, printOpts);
     }
 
@@ -268,7 +268,7 @@ class CodeGenerator {
   }
 
   printAndIndentOnComments(print, node) {
-    var indent = !!node.leadingComments;
+    let indent = !!node.leadingComments;
     if (indent) this.indent();
     print.plain(node);
     if (indent) this.dedent();
@@ -284,7 +284,7 @@ class CodeGenerator {
   }
 
   generateComment(comment) {
-    var val = comment.value;
+    let val = comment.value;
     if (comment.type === "CommentLine") {
       val = `//${val}`;
     } else {
@@ -306,8 +306,8 @@ class CodeGenerator {
       return [];
     }
 
-    var comments = [];
-    var nodes    = [node];
+    let comments = [];
+    let nodes    = [node];
 
     if (t.isExpressionStatement(node)) {
       nodes.push(node.argument);
@@ -339,7 +339,7 @@ class CodeGenerator {
   _printComments(comments) {
     if (!comments || !comments.length) return;
 
-    for (var comment of (comments: Array)) {
+    for (let comment of (comments: Array)) {
       if (!this.shouldPrintComment(comment)) continue;
       if (comment._displayed) continue;
       comment._displayed = true;
@@ -349,8 +349,8 @@ class CodeGenerator {
       // whitespace before
       this.newline(this.whitespace.getNewlinesBefore(comment));
 
-      var column = this.position.column;
-      var val    = this.generateComment(comment);
+      let column = this.position.column;
+      let val    = this.generateComment(comment);
 
       if (column && !this.isLast(["\n", " ", "[", "{"])) {
         this._push(" ");
@@ -359,13 +359,13 @@ class CodeGenerator {
 
       //
       if (comment.type === "CommentBlock" && this.format.indent.adjustMultilineComment) {
-        var offset = comment.loc && comment.loc.start.column;
+        let offset = comment.loc && comment.loc.start.column;
         if (offset) {
-          var newlineRegex = new RegExp("\\n\\s{1," + offset + "}", "g");
+          let newlineRegex = new RegExp("\\n\\s{1," + offset + "}", "g");
           val = val.replace(newlineRegex, "\n");
         }
 
-        var indent = Math.max(this.indentSize(), column);
+        let indent = Math.max(this.indentSize(), column);
         val = val.replace(/\n/g, `\n${repeating(" ", indent)}`);
       }
 
@@ -399,7 +399,7 @@ each(CodeGenerator.generators, function (generator) {
 });
 
 module.exports = function (ast, opts, code) {
-  var gen = new CodeGenerator(ast, opts, code);
+  let gen = new CodeGenerator(ast, opts, code);
   return gen.generate();
 };
 
diff --git a/packages/babel-generator/src/node/index.js b/packages/babel-generator/src/node/index.js
index ef60e3a2c8..b6fecc1a9f 100644
--- a/packages/babel-generator/src/node/index.js
+++ b/packages/babel-generator/src/node/index.js
@@ -14,16 +14,16 @@ import * as t from "babel-types";
  * }, node, parent);
  */
 
-var find = function (obj, node, parent) {
+let find = function (obj, node, parent) {
   if (!obj) return;
-  var result;
+  let result;
 
-  var types = Object.keys(obj);
-  for (var i = 0; i < types.length; i++) {
-    var type = types[i];
+  let types = Object.keys(obj);
+  for (let i = 0; i < types.length; i++) {
+    let type = types[i];
 
     if (t.is(type, node)) {
-      var fn = obj[type];
+      let fn = obj[type];
       result = fn(node, parent);
       if (result != null) break;
     }
@@ -61,12 +61,12 @@ export default class Node {
       node = node.expression;
     }
 
-    var linesInfo = find(whitespace.nodes, node, parent);
+    let linesInfo = find(whitespace.nodes, node, parent);
 
     if (!linesInfo) {
-      var items = find(whitespace.list, node, parent);
+      let items = find(whitespace.list, node, parent);
       if (items) {
-        for (var i = 0; i < items.length; i++) {
+        for (let i = 0; i < items.length; i++) {
           linesInfo = Node.needsWhitespace(items[i], node, type);
           if (linesInfo) break;
         }
@@ -102,7 +102,7 @@ export default class Node {
     if (t.isNewExpression(parent) && parent.callee === node) {
       if (t.isCallExpression(node)) return true;
 
-      var hasCall = some(node, function (val) {
+      let hasCall = some(node, function (val) {
         return t.isCallExpression(val);
       });
       if (hasCall) return true;
@@ -119,12 +119,12 @@ export default class Node {
 each(Node, function (fn, key) {
   Node.prototype[key] = function () {
     // Avoid leaking arguments to prevent deoptimization
-    var args = new Array(arguments.length + 2);
+    let args = new Array(arguments.length + 2);
 
     args[0] = this.node;
     args[1] = this.parent;
 
-    for (var i = 0; i < args.length; i++) {
+    for (let i = 0; i < args.length; i++) {
       args[i + 2] = arguments[i];
     }
 
diff --git a/packages/babel-generator/src/node/parentheses.js b/packages/babel-generator/src/node/parentheses.js
index cb7642c7a1..11c8eb3502 100644
--- a/packages/babel-generator/src/node/parentheses.js
+++ b/packages/babel-generator/src/node/parentheses.js
@@ -62,11 +62,11 @@ export function Binary(node, parent) {
   }
 
   if (t.isBinary(parent)) {
-    var parentOp  = parent.operator;
-    var parentPos = PRECEDENCE[parentOp];
+    let parentOp  = parent.operator;
+    let parentPos = PRECEDENCE[parentOp];
 
-    var nodeOp = node.operator;
-    var nodePos = PRECEDENCE[nodeOp];
+    let nodeOp = node.operator;
+    let nodePos = PRECEDENCE[nodeOp];
 
     if (parentPos > nodePos) {
       return true;
@@ -80,7 +80,7 @@ export function Binary(node, parent) {
 
 export function BinaryExpression(node, parent) {
   if (node.operator === "in") {
-    // var i = (1 in []);
+    // let i = (1 in []);
     if (t.isVariableDeclarator(parent)) {
       return true;
     }
diff --git a/packages/babel-generator/src/node/printer.js b/packages/babel-generator/src/node/printer.js
index ce580cae1f..5ee0f4a9a7 100644
--- a/packages/babel-generator/src/node/printer.js
+++ b/packages/babel-generator/src/node/printer.js
@@ -14,7 +14,7 @@ export default class NodePrinter {
 
   printInnerComments() {
     if (!this.parent.innerComments) return;
-    var gen = this.generator;
+    let gen = this.generator;
     gen.indent();
     gen._printComments(this.parent.innerComments);
     gen.dedent();
diff --git a/packages/babel-generator/src/node/whitespace.js b/packages/babel-generator/src/node/whitespace.js
index 6511c35949..2623e1a273 100644
--- a/packages/babel-generator/src/node/whitespace.js
+++ b/packages/babel-generator/src/node/whitespace.js
@@ -64,7 +64,7 @@ exports.nodes = {
    */
 
   AssignmentExpression(node) {
-    var state = crawl(node.right);
+    let state = crawl(node.right);
     if ((state.hasCall && state.hasHelper) || state.hasFunction) {
       return {
         before: state.hasFunction,
@@ -125,12 +125,12 @@ exports.nodes = {
    */
 
   VariableDeclaration(node) {
-    for (var i = 0; i < node.declarations.length; i++) {
-      var declar = node.declarations[i];
+    for (let i = 0; i < node.declarations.length; i++) {
+      let declar = node.declarations[i];
 
-      var enabled = isHelper(declar.id) && !isType(declar.init);
+      let enabled = isHelper(declar.id) && !isType(declar.init);
       if (!enabled) {
-        var state = crawl(declar.init);
+        let state = crawl(declar.init);
         enabled = (isHelper(declar.init) && state.hasCall) || state.hasFunction;
       }
 
diff --git a/packages/babel-generator/src/position.js b/packages/babel-generator/src/position.js
index 9dc707b5d2..a6787fd3b2 100644
--- a/packages/babel-generator/src/position.js
+++ b/packages/babel-generator/src/position.js
@@ -13,7 +13,7 @@ export default class Position {
    */
 
   push(str) {
-    for (var i = 0; i < str.length; i++) {
+    for (let i = 0; i < str.length; i++) {
       if (str[i] === "\n") {
         this.line++;
         this.column = 0;
@@ -28,7 +28,7 @@ export default class Position {
    */
 
   unshift(str) {
-    for (var i = 0; i < str.length; i++) {
+    for (let i = 0; i < str.length; i++) {
       if (str[i] === "\n") {
         this.line--;
       } else {
diff --git a/packages/babel-generator/src/source-map.js b/packages/babel-generator/src/source-map.js
index 51cb5665a7..f1b3ee3727 100644
--- a/packages/babel-generator/src/source-map.js
+++ b/packages/babel-generator/src/source-map.js
@@ -27,7 +27,7 @@ export default class SourceMap {
    */
 
   get() {
-    var map = this.map;
+    let map = this.map;
     if (map) {
       return map.toJSON();
     } else {
@@ -40,22 +40,22 @@ export default class SourceMap {
    */
 
   mark(node, type) {
-    var loc = node.loc;
+    let loc = node.loc;
     if (!loc) return; // no location info
 
-    var map = this.map;
+    let map = this.map;
     if (!map) return; // no source map
 
     if (t.isProgram(node) || t.isFile(node)) return; // illegal mapping nodes
 
-    var position = this.position;
+    let position = this.position;
 
-    var generated = {
+    let generated = {
       line: position.line,
       column: position.column
     };
 
-    var original = loc[type];
+    let original = loc[type];
 
     map.addMapping({
       source: this.opts.sourceFileName,
diff --git a/packages/babel-generator/src/whitespace.js b/packages/babel-generator/src/whitespace.js
index 24ff017962..05816b1b73 100644
--- a/packages/babel-generator/src/whitespace.js
+++ b/packages/babel-generator/src/whitespace.js
@@ -38,14 +38,14 @@ export default class Whitespace {
    */
 
   getNewlinesBefore(node) {
-    var startToken;
-    var endToken;
-    var tokens = this.tokens;
+    let startToken;
+    let endToken;
+    let tokens = this.tokens;
 
-    for (var j = 0; j < tokens.length; j++) {
+    for (let j = 0; j < tokens.length; j++) {
       // optimize for forward traversal by shifting for loop index
-      var i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
-      var token = tokens[i];
+      let i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
+      let token = tokens[i];
 
       // this is the token this node starts with
       if (node.start === token.start) {
@@ -65,14 +65,14 @@ export default class Whitespace {
    */
 
   getNewlinesAfter(node) {
-    var startToken;
-    var endToken;
-    var tokens = this.tokens;
+    let startToken;
+    let endToken;
+    let tokens = this.tokens;
 
-    for (var j = 0; j < tokens.length; j++) {
+    for (let j = 0; j < tokens.length; j++) {
       // optimize for forward traversal by shifting for loop index
-      var i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
-      var token = tokens[i];
+      let i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
+      let token = tokens[i];
 
       // this is the token this node ends with
       if (node.end === token.end) {
@@ -88,7 +88,7 @@ export default class Whitespace {
     if (endToken && endToken.type.label === "eof") {
       return 1;
     } else {
-      var lines = this.getNewlinesBetween(startToken, endToken);
+      let lines = this.getNewlinesBetween(startToken, endToken);
       if (node.type === "CommentLine" && !lines) {
         // line comment
         return 1;
@@ -105,11 +105,11 @@ export default class Whitespace {
   getNewlinesBetween(startToken, endToken) {
     if (!endToken || !endToken.loc) return 0;
 
-    var start = startToken ? startToken.loc.end.line : 1;
-    var end   = endToken.loc.start.line;
-    var lines = 0;
+    let start = startToken ? startToken.loc.end.line : 1;
+    let end   = endToken.loc.start.line;
+    let lines = 0;
 
-    for (var line = start; line < end; line++) {
+    for (let line = start; line < end; line++) {
       if (typeof this.used[line] === "undefined") {
         this.used[line] = true;
         lines++;