From ed40ec03d1e9d63f5b36c434308f8718d49b7f67 Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Tue, 3 Mar 2015 22:31:49 +1100
Subject: [PATCH] more es6, template literals and modules

---
 src/babel/api/browser.js                      |  2 +-
 src/babel/api/register/node.js                |  2 +-
 src/babel/api/register/resolve-rc.js          |  2 +-
 src/babel/generation/buffer.js                |  2 +-
 .../generation/generators/expressions.js      | 11 ++--
 src/babel/generation/generators/methods.js    |  7 ++-
 src/babel/generation/generators/modules.js    |  2 +-
 src/babel/generation/generators/playground.js |  2 +-
 src/babel/generation/generators/statements.js |  4 +-
 src/babel/generation/generators/types.js      | 24 ++++----
 src/babel/generation/index.js                 |  8 +--
 src/babel/helpers/code-frame.js               |  3 +-
 src/babel/helpers/parse.js                    |  2 +-
 src/babel/messages.js                         |  6 +-
 .../transformation/helpers/define-map.js      |  2 +-
 .../helpers/explode-assignable-expression.js  |  2 +-
 src/babel/transformation/helpers/strict.js    |  2 +-
 src/babel/transformation/index.js             |  4 +-
 src/babel/transformation/transformer-pass.js  |  2 +-
 .../transformers/es6/block-scoping-tdz.js     |  6 +-
 .../transformers/es6/block-scoping.js         |  2 +-
 .../transformers/es6/classes.js               |  2 +-
 .../transformers/es6/tail-call.js             |  2 +-
 .../utility/dead-code-elimination.js          |  8 +--
 .../utility/inline-expressions.js             |  8 ++-
 src/babel/traversal/index.js                  |  2 +-
 src/babel/traversal/scope.js                  | 37 +++++++++++-
 src/babel/types/index.js                      | 57 +++++++++++++++----
 src/babel/util.js                             |  6 +-
 29 files changed, 146 insertions(+), 73 deletions(-)

diff --git a/src/babel/api/browser.js b/src/babel/api/browser.js
index 248d668e37..82671a3712 100644
--- a/src/babel/api/browser.js
+++ b/src/babel/api/browser.js
@@ -25,7 +25,7 @@ transform.load = function (url, callback, opts = {}, hold) {
       if (!hold) transform.run.apply(transform, param);
       if (callback) callback(param);
     } else {
-      throw new Error("Could not load " + url);
+      throw new Error(`Could not load ${url}`);
     }
   };
 
diff --git a/src/babel/api/register/node.js b/src/babel/api/register/node.js
index e344ca0ab6..3dcc25499a 100644
--- a/src/babel/api/register/node.js
+++ b/src/babel/api/register/node.js
@@ -46,7 +46,7 @@ var compile = function (filename) {
   var opts = extend({}, transformOpts);
   resolveRc(filename, opts);
 
-  var cacheKey = filename + ":" + JSON.stringify(opts);
+  var cacheKey = `${filename}:${JSON.stringify(opts)}`;
 
   if (cache) {
     var cached = cache[cacheKey];
diff --git a/src/babel/api/register/resolve-rc.js b/src/babel/api/register/resolve-rc.js
index 4728f214ba..ae63083e88 100644
--- a/src/babel/api/register/resolve-rc.js
+++ b/src/babel/api/register/resolve-rc.js
@@ -23,7 +23,7 @@ export default function (loc, opts = {}) {
       try {
         json = JSON.parse(content);
       } catch (err) {
-        err.message = file + ": " + err.message;
+        err.message = `${file}: ${err.message}`;
         throw err;
       }
 
diff --git a/src/babel/generation/buffer.js b/src/babel/generation/buffer.js
index f47199d51d..2e909cf525 100644
--- a/src/babel/generation/buffer.js
+++ b/src/babel/generation/buffer.js
@@ -140,7 +140,7 @@ export default class Buffer {
       var indent = this.getIndent();
 
       // replace all newlines with newlines with the indentation
-      str = str.replace(/\n/g, "\n" + indent);
+      str = str.replace(/\n/g, `\n${indent}`);
 
       // we've got a newline before us so prepend on the indentation
       if (this.isLast("\n")) this._push(indent);
diff --git a/src/babel/generation/generators/expressions.js b/src/babel/generation/generators/expressions.js
index e13ddba8f0..650d3d9b68 100644
--- a/src/babel/generation/generators/expressions.js
+++ b/src/babel/generation/generators/expressions.js
@@ -109,16 +109,19 @@ export function ExpressionStatement(node, print) {
   this.semicolon();
 }
 
-exports.BinaryExpression =
-exports.LogicalExpression =
-exports.AssignmentPattern =
-exports.AssignmentExpression = function (node, print) {
+export function AssignmentExpression(node, print) {
   // todo: add cases where the spaces can be dropped when in compact mode
   print(node.left);
   this.push(" ");
   this.push(node.operator);
   this.push(" ");
   print(node.right);
+}
+
+export {
+  AssignmentExpression as BinaryExpression,
+  AssignmentExpression as LogicalExpression,
+  AssignmentExpression as AssignmentPattern
 };
 
 var SCIENTIFIC_NOTATION = /e/i;
diff --git a/src/babel/generation/generators/methods.js b/src/babel/generation/generators/methods.js
index 3c170ab785..d20bcf7add 100644
--- a/src/babel/generation/generators/methods.js
+++ b/src/babel/generation/generators/methods.js
@@ -44,8 +44,7 @@ export function _method(node, print) {
   print(value.body);
 }
 
-exports.FunctionDeclaration =
-exports.FunctionExpression = function (node, print) {
+export function FunctionExpression(node, print) {
   if (node.async) this.push("async ");
   this.push("function");
   if (node.generator) this.push("*");
@@ -60,7 +59,9 @@ exports.FunctionExpression = function (node, print) {
   this._params(node, print);
   this.space();
   print(node.body);
-};
+}
+
+export { FunctionExpression as FunctionDeclaration };
 
 export function ArrowFunctionExpression(node, print) {
   if (node.async) this.push("async ");
diff --git a/src/babel/generation/generators/modules.js b/src/babel/generation/generators/modules.js
index a051127446..b1299335d0 100644
--- a/src/babel/generation/generators/modules.js
+++ b/src/babel/generation/generators/modules.js
@@ -5,7 +5,7 @@ export function ImportSpecifier(node, print) {
   if (t.isSpecifierDefault(node)) {
     print(t.getSpecifierName(node));
   } else {
-    return exports.ExportSpecifier.apply(this, arguments);
+    return ExportSpecifier.apply(this, arguments);
   }
 }
 
diff --git a/src/babel/generation/generators/playground.js b/src/babel/generation/generators/playground.js
index 6c26bc34d1..f3a7e5bf8c 100644
--- a/src/babel/generation/generators/playground.js
+++ b/src/babel/generation/generators/playground.js
@@ -2,6 +2,6 @@ import each from "lodash/collection/each";
 
 each(["BindMemberExpression", "BindFunctionExpression"], function (type) {
   exports[type] = function () {
-    throw new ReferenceError("Trying to render non-standard playground node " + JSON.stringify(type));
+    throw new ReferenceError(`Trying to render non-standard playground node ${JSON.stringify(type)}`);
   };
 });
diff --git a/src/babel/generation/generators/statements.js b/src/babel/generation/generators/statements.js
index 7ffdbd015f..86edb6005e 100644
--- a/src/babel/generation/generators/statements.js
+++ b/src/babel/generation/generators/statements.js
@@ -60,7 +60,7 @@ var buildForXStatement = function (op) {
     this.keyword("for");
     this.push("(");
     print(node.left);
-    this.push(" " + op + " ");
+    this.push(` ${op} `);
     print(node.right);
     this.push(")");
     print.block(node.body);
@@ -192,7 +192,7 @@ export function VariableDeclaration(node, print, parent) {
 
   var sep = ",";
   if (!this.format.compact && hasInits) {
-    sep += "\n" + repeating(" ", node.kind.length + 1);
+    sep += `\n${repeating(" ", node.kind.length + 1)}`;
   } else {
     sep += " ";
   }
diff --git a/src/babel/generation/generators/types.js b/src/babel/generation/generators/types.js
index 87b6d5da4a..dcc685e3f7 100644
--- a/src/babel/generation/generators/types.js
+++ b/src/babel/generation/generators/types.js
@@ -4,12 +4,12 @@ export function Identifier(node) {
   this.push(node.name);
 }
 
-exports.RestElement =
-exports.SpreadElement =
-exports.SpreadProperty = function (node, print) {
+export function RestElement(node, print) {
   this.push("...");
   print(node.argument);
-};
+}
+
+export { RestElement as SpreadElement, RestElement as SpreadProperty };
 
 export function VirtualPropertyExpression(node, print) {
   print(node.object);
@@ -17,8 +17,7 @@ export function VirtualPropertyExpression(node, print) {
   print(node.property);
 }
 
-exports.ObjectExpression =
-exports.ObjectPattern = function (node, print) {
+export function ObjectExpression(node, print) {
   var props = node.properties;
 
   if (props.length) {
@@ -32,7 +31,9 @@ exports.ObjectPattern = function (node, print) {
   } else {
     this.push("{}");
   }
-};
+}
+
+export { ObjectExpression as ObjectPattern };
 
 export function Property(node, print) {
   if (node.method || node.kind === "get" || node.kind === "set") {
@@ -53,8 +54,7 @@ export function Property(node, print) {
   }
 }
 
-exports.ArrayExpression =
-exports.ArrayPattern = function (node, print) {
+export function ArrayExpression(node, print) {
   var elems = node.elements;
   var len   = elems.length;
 
@@ -76,7 +76,9 @@ exports.ArrayPattern = function (node, print) {
   });
 
   this.push("]");
-};
+}
+
+export { ArrayExpression as ArrayPattern };
 
 export function Literal(node) {
   var val  = node.value;
@@ -89,7 +91,7 @@ export function Literal(node) {
   } else if (type === "boolean") {
     this.push(val ? "true" : "false");
   } else if (node.regex) {
-    this.push("/" + node.regex.pattern + "/" + node.regex.flags);
+    this.push(`/${node.regex.pattern}/${node.regex.flags}`);
   } else if (val === null) {
     this.push("null");
   }
diff --git a/src/babel/generation/index.js b/src/babel/generation/index.js
index 6fdadfe589..6c0394d8e8 100644
--- a/src/babel/generation/index.js
+++ b/src/babel/generation/index.js
@@ -187,7 +187,7 @@ class CodeGenerator {
 
       this.printTrailingComments(node, parent);
     } else {
-      throw new ReferenceError("unknown node of type " + JSON.stringify(node.type) + " with constructor " + JSON.stringify(node && node.constructor.name));
+      throw new ReferenceError(`unknown node of type ${JSON.stringify(node.type)} with constructor ${JSON.stringify(node && node.constructor.name)}`);
     }
 
     this.format.concise = oldConcise;
@@ -238,9 +238,9 @@ class CodeGenerator {
   generateComment(comment) {
     var val = comment.value;
     if (comment.type === "Line") {
-      val = "//" + val;
+      val = `//${val}`;
     } else {
-      val = "/*" + val + "*/";
+      val = `/*${val}*/`;
     }
     return val;
   }
@@ -319,7 +319,7 @@ class CodeGenerator {
         }
 
         var indent = Math.max(this.indentSize(), column);
-        val = val.replace(/\n/g, "\n" + repeating(" ", indent));
+        val = val.replace(/\n/g, `\n${repeating(" ", indent)}`);
       }
 
       if (column === 0) {
diff --git a/src/babel/helpers/code-frame.js b/src/babel/helpers/code-frame.js
index 9c8505ae85..caafa4e87d 100644
--- a/src/babel/helpers/code-frame.js
+++ b/src/babel/helpers/code-frame.js
@@ -81,8 +81,7 @@ module.exports = function (lines, lineNumber, colNumber) {
         return;
       }
       if (colNumber) {
-        params.line += "\n" + params.before + repeating(" ", params.width) +
-                       params.after + repeating(" ", colNumber - 1) + "^";
+        params.line += `\n${params.before}${repeating(" ", params.width)}${params.after}${repeating(" ", colNumber - 1)}^`;
       }
       params.before = params.before.replace(/^./, ">");
     }
diff --git a/src/babel/helpers/parse.js b/src/babel/helpers/parse.js
index 858f01e083..20997af4f5 100644
--- a/src/babel/helpers/parse.js
+++ b/src/babel/helpers/parse.js
@@ -32,7 +32,7 @@ module.exports = function (opts, code, callback) {
   } catch (err) {
     if (!err._babel) {
       err._babel = true;
-      var message = opts.filename + ": " + err.message;
+      var message = `${opts.filename}: ${err.message}`;
 
       var loc = err.loc;
       if (loc) {
diff --git a/src/babel/messages.js b/src/babel/messages.js
index b3f0ebe155..73c8848ddf 100644
--- a/src/babel/messages.js
+++ b/src/babel/messages.js
@@ -22,14 +22,14 @@ export var messages = {
 };
 
 export function get(key) {
-  var msg = exports.messages[key];
-  if (!msg) throw new ReferenceError("Unknown message `" + key + "`");
+  var msg = messages[key];
+  if (!msg) throw new ReferenceError(`Unknown message ${JSON.stringify(key)}`);
 
   var args = [];
   for (var i = 1; i < arguments.length; i++) {
     args.push(arguments[i]);
   }
-  args = exports.parseArgs(args);
+  args = parseArgs(args);
 
   return msg.replace(/\$(\d+)/g, function (str, i) {
     return args[--i];
diff --git a/src/babel/transformation/helpers/define-map.js b/src/babel/transformation/helpers/define-map.js
index fdc2099ba2..a951974cf5 100644
--- a/src/babel/transformation/helpers/define-map.js
+++ b/src/babel/transformation/helpers/define-map.js
@@ -10,7 +10,7 @@ export function push(mutatorMap, key, kind, computed, value) {
 
   if (t.isIdentifier(key)) {
     alias = key.name;
-    if (computed) alias = "computed:" + alias;
+    if (computed) alias = `computed:${alias}`;
   } else if (t.isLiteral(key)) {
     alias = String(key.value);
   } else {
diff --git a/src/babel/transformation/helpers/explode-assignable-expression.js b/src/babel/transformation/helpers/explode-assignable-expression.js
index 52f79ead83..bfecd0f26b 100644
--- a/src/babel/transformation/helpers/explode-assignable-expression.js
+++ b/src/babel/transformation/helpers/explode-assignable-expression.js
@@ -23,7 +23,7 @@ var getObjRef = function (node, nodes, file, scope) {
       return ref;
     }
   } else {
-    throw new Error("We can't explode this node type " + node.type);
+    throw new Error(`We can't explode this node type ${node.type}`);
   }
 
   var temp = scope.generateUidBasedOnNode(ref);
diff --git a/src/babel/transformation/helpers/strict.js b/src/babel/transformation/helpers/strict.js
index a5c959fe20..d5dbe31b41 100644
--- a/src/babel/transformation/helpers/strict.js
+++ b/src/babel/transformation/helpers/strict.js
@@ -7,7 +7,7 @@ export function has(node) {
 
 export function wrap(node, callback) {
   var useStrictNode;
-  if (exports.has(node)) {
+  if (has(node)) {
     useStrictNode = node.body.shift();
   }
 
diff --git a/src/babel/transformation/index.js b/src/babel/transformation/index.js
index b56db8ab88..2aa34a2008 100644
--- a/src/babel/transformation/index.js
+++ b/src/babel/transformation/index.js
@@ -30,7 +30,7 @@ transform._ensureTransformerNames = function (type, rawKeys) {
       keys.push(aliasKey);
     } else if (deprecatedKey) {
       // deprecated key, remap it to the new one
-      console.error("The transformer " + key + " has been renamed to " + deprecatedKey);
+      console.error(`The transformer ${key} has been renamed to ${deprecatedKey}`);
       rawKeys.push(deprecatedKey);
     } else if (transform.transformers[key]) {
       // valid key
@@ -40,7 +40,7 @@ transform._ensureTransformerNames = function (type, rawKeys) {
       keys = keys.concat(transform.namespaces[key]);
     } else {
       // invalid key
-      throw new ReferenceError("Unknown transformer " + key + " specified in " + type);
+      throw new ReferenceError(`Unknown transformer ${key} specified in ${type}`);
     }
   }
 
diff --git a/src/babel/transformation/transformer-pass.js b/src/babel/transformation/transformer-pass.js
index d691b42ebc..6282aa7a48 100644
--- a/src/babel/transformation/transformer-pass.js
+++ b/src/babel/transformation/transformer-pass.js
@@ -56,7 +56,7 @@ export default class TransformerPass {
 
     var file = this.file;
 
-    file.debug("Running transformer " + this.transformer.key);
+    file.debug(`Running transformer ${this.transformer.key}`);
 
     file.scope.traverse(file.ast, this.handlers, file);
   }
diff --git a/src/babel/transformation/transformers/es6/block-scoping-tdz.js b/src/babel/transformation/transformers/es6/block-scoping-tdz.js
index 563923beaf..5fbb842573 100644
--- a/src/babel/transformation/transformers/es6/block-scoping-tdz.js
+++ b/src/babel/transformation/transformers/es6/block-scoping-tdz.js
@@ -32,12 +32,10 @@ export function BlockStatement(node, parent, scope, file) {
   var letRefs = node._letReferences;
   if (!letRefs) return;
 
-  var state = {
+  scope.traverse(node, visitor, {
     letRefs: letRefs,
     file:    file
-  };
-
-  scope.traverse(node, visitor, state);
+  });
 }
 
 export { BlockStatement as Program, BlockStatement as Loop };
diff --git a/src/babel/transformation/transformers/es6/block-scoping.js b/src/babel/transformation/transformers/es6/block-scoping.js
index 39d43664d7..0e2aa04581 100644
--- a/src/babel/transformation/transformers/es6/block-scoping.js
+++ b/src/babel/transformation/transformers/es6/block-scoping.js
@@ -189,7 +189,7 @@ var loopVisitor = {
           return;
         }
 
-        loopText = loopText + "|" + node.label.name;
+        loopText = `${loopText}|${node.label.name}`;
       } else {
         // we shouldn't be transforming these statements because
         // they don't refer to the actual loop we're scopifying
diff --git a/src/babel/transformation/transformers/es6/classes.js b/src/babel/transformation/transformers/es6/classes.js
index e77857fa53..2386c5b995 100644
--- a/src/babel/transformation/transformers/es6/classes.js
+++ b/src/babel/transformation/transformers/es6/classes.js
@@ -182,7 +182,7 @@ class ClassTransformer {
     }
 
     // we have no constructor, we have a super, and the super doesn't appear to be falsy
-    if (!this.hasConstructor && this.hasSuper && t.evaluateTruthy(superName) !== false) {
+    if (!this.hasConstructor && this.hasSuper && t.evaluateTruthy(superName, this.scope) !== false) {
       var helperName = "class-super-constructor-call";
       if (this.isLoose) helperName += "-loose";
       constructor.body.body.push(util.template(helperName, {
diff --git a/src/babel/transformation/transformers/es6/tail-call.js b/src/babel/transformation/transformers/es6/tail-call.js
index 439b3f1712..7cb201b002 100644
--- a/src/babel/transformation/transformers/es6/tail-call.js
+++ b/src/babel/transformation/transformers/es6/tail-call.js
@@ -229,7 +229,7 @@ class TailCallTransformer {
   subTransform(node) {
     if (!node) return;
 
-    var handler = this["subTransform" + node.type];
+    var handler = this[`subTransform${node.type}`];
     if (handler) return handler.call(this, node);
   }
 
diff --git a/src/babel/transformation/transformers/utility/dead-code-elimination.js b/src/babel/transformation/transformers/utility/dead-code-elimination.js
index a8e4430dbe..486b31edee 100644
--- a/src/babel/transformation/transformers/utility/dead-code-elimination.js
+++ b/src/babel/transformation/transformers/utility/dead-code-elimination.js
@@ -19,8 +19,8 @@ function toStatements(node) {
 
 export var optional = true;
 
-export function ConditionalExpression(node) {
-  var evaluateTest = t.evaluateTruthy(node.test);
+export function ConditionalExpression(node, parent, scope) {
+  var evaluateTest = t.evaluateTruthy(node.test, scope);
   if (evaluateTest === true) {
     return node.consequent;
   } else if (evaluateTest === false) {
@@ -29,12 +29,12 @@ export function ConditionalExpression(node) {
 }
 
 export var IfStatement = {
-  exit(node) {
+  exit(node, parent, scope) {
     var consequent = node.consequent;
     var alternate  = node.alternate;
     var test       = node.test;
 
-    var evaluateTest = t.evaluateTruthy(test);
+    var evaluateTest = t.evaluateTruthy(test, scope);
 
     // we can check if a test will be truthy 100% and if so then we can inline
     // the consequent and completely ignore the alternate
diff --git a/src/babel/transformation/transformers/utility/inline-expressions.js b/src/babel/transformation/transformers/utility/inline-expressions.js
index 6dada3a898..b95ee1585c 100644
--- a/src/babel/transformation/transformers/utility/inline-expressions.js
+++ b/src/babel/transformation/transformers/utility/inline-expressions.js
@@ -2,7 +2,11 @@ import t from "../../../types";
 
 export var optional = true;
 
-export function Expression(node) {
-  var res = t.evaluate(node);
+export function Expression(node, parent, scope) {
+  var res = t.evaluate(node, scope);
   if (res.confident) return t.valueToNode(res.value);
 }
+
+export function Identifier() {
+  // override Expression
+}
diff --git a/src/babel/traversal/index.js b/src/babel/traversal/index.js
index 31a45c6462..062dd40459 100644
--- a/src/babel/traversal/index.js
+++ b/src/babel/traversal/index.js
@@ -9,7 +9,7 @@ function traverse(parent, opts, scope, state) {
 
   if (!opts.noScope && !scope) {
     if (parent.type !== "Program" && parent.type !== "File") {
-      throw new Error("Must pass a scope unless traversing a Program/File got a " + parent.type + " node");
+      throw new Error(`Must pass a scope unless traversing a Program/File got a ${parent.type} node`);
     }
   }
 
diff --git a/src/babel/traversal/scope.js b/src/babel/traversal/scope.js
index e24dbc5f27..2585102bbc 100644
--- a/src/babel/traversal/scope.js
+++ b/src/babel/traversal/scope.js
@@ -143,7 +143,7 @@ export default class Scope {
   _generateUid(name, i) {
     var id = name;
     if (i > 1) id += i;
-    return "_" + id;
+    return `_${id}`;
   }
 
   /*
@@ -415,7 +415,7 @@ export default class Scope {
   registerVariableDeclaration(declar) {
     var declars = declar.declarations;
     for (var i = 0; i < declars.length; i++) {
-      this.registerBinding(declars[i], declar.kind);
+      this.registerBinding(declar.kind, declars[i]);
     }
   }
 
@@ -538,7 +538,7 @@ export default class Scope {
         init: opts.init
       };
     } else {
-      throw new TypeError("cannot add a declaration here in node type " + block.type);
+      throw new TypeError(`cannot add a declaration here in node type ${block.type}`);
     }
   }
 
@@ -626,6 +626,37 @@ export default class Scope {
     return binding && binding.identifier;
   }
 
+
+  getOwnImmutableBindingValue(name) {
+    return this._immutableBindingInfoToValue(this.getOwnBindingInfo(name));
+  }
+
+  getImmutableBindingValue(name) {
+    return this._immutableBindingInfoToValue(this.getBindingInfo(name));
+  }
+
+  _immutableBindingInfoToValue(info) {
+    if (!info) return;
+
+    // can't guarantee this value is the same
+    if (info.reassigned) return;
+
+    var node = info.node;
+    if (t.isVariableDeclarator(node)) {
+      if (t.isIdentifier(node.id)) {
+        node = node.init;
+      } else {
+        // otherwise it's probably a destructuring like:
+        // var { foo } = "foo";
+        return;
+      }
+    }
+
+    if (t.isImmutable(node)) {
+      return node;
+    }
+  }
+
   // has
 
   hasOwnBinding(name) {
diff --git a/src/babel/types/index.js b/src/babel/types/index.js
index 931d1a8aad..91c36ed9e3 100644
--- a/src/babel/types/index.js
+++ b/src/babel/types/index.js
@@ -21,14 +21,14 @@ export default t;
  */
 
 function registerType(type, skipAliasCheck) {
-  var is = t["is" + type] = function (node, opts) {
+  var is = t[`is${type}`] = function (node, opts) {
     return t.is(type, node, opts, skipAliasCheck);
   };
 
-  t["assert" + type] = function (node, opts) {
+  t[`assert${type}`] = function (node, opts) {
     opts ||= {};
     if (!is(node, opts)) {
-      throw new Error("Expected type " + JSON.stringify(type) + " with option " + JSON.stringify(opts));
+      throw new Error(`Expected type ${JSON.stringify(type)} with option ${JSON.stringify(opts)}`);
     }
   };
 }
@@ -395,7 +395,7 @@ t.toIdentifier = function (name) {
   });
 
   if (!t.isValidIdentifier(name)) {
-    name = "_" + name;
+    name = `_${name}`;
   }
 
   return name || "_";
@@ -506,7 +506,7 @@ t.toStatement = function (node, ignore) {
     if (ignore) {
       return false;
     } else {
-      throw new Error("cannot turn " + node.type + " to a statement");
+      throw new Error(`cannot turn ${node.type} to a statement`);
     }
   }
 
@@ -536,7 +536,7 @@ t.toExpression = function (node) {
   if (t.isExpression(node)) {
     return node;
   } else {
-    throw new Error("cannot turn " + node.type + " to an expression");
+    throw new Error(`cannot turn ${node.type} to an expression`);
   }
 };
 
@@ -807,6 +807,35 @@ t.isScope = function (node, parent) {
   return t.isScopable(node);
 };
 
+/**
+ * Description
+ *
+ * @param {Node} node
+ * @returns {Boolean}
+ */
+
+t.isImmutable = function (node) {
+  if (t.isLiteral(node)) {
+    if (node.regex) {
+      // regexes are mutable
+      return false;
+    } else {
+      // immutable!
+      return true;
+    }
+  } else if (t.isIdentifier(node)) {
+    if (node.name === "undefined") {
+      // immutable!
+      return true;
+    } else {
+      // no idea...
+      return false;
+    }
+  }
+
+  return false;
+};
+
 /**
  * Walk the input `node` and statically evaluate if it's truthy.
  *
@@ -824,11 +853,12 @@ t.isScope = function (node, parent) {
  *   if (!t.evaluateTruthy(node)) falsyLogic();
  *
  * @param {Node} node
+ * @param {Scope} scope
  * @returns {Boolean}
  */
 
-t.evaluateTruthy = function (node) {
-  var res = t.evaluate(node);
+t.evaluateTruthy = function (node, scope) {
+  var res = t.evaluate(node, scope);
   if (res.confident) return !!res.value;
 };
 
@@ -846,10 +876,11 @@ t.evaluateTruthy = function (node) {
  *   t.evaluate(parse("foo + foo")) // { confident: false, value: undefined }
  *
  * @param {Node} node
+ * @param {Scope} scope
  * @returns {Object}
  */
 
-t.evaluate = function (node) {
+t.evaluate = function (node, scope) {
   var confident = true;
 
   var value = evaluate(node);
@@ -882,8 +913,12 @@ t.evaluate = function (node) {
       }
     }
 
-    if (t.isIdentifier(node, { name: "undefined" })) {
-      return undefined;
+    if (t.isIdentifier(node)) {
+      if (node.name === "undefined") {
+        return undefined;
+      } else {
+        return evaluate(scope.getImmutableBindingValue(node.name));
+      }
     }
 
     if (t.isUnaryExpression(node, { prefix: true })) {
diff --git a/src/babel/util.js b/src/babel/util.js
index d6140bd490..9b7230ff67 100644
--- a/src/babel/util.js
+++ b/src/babel/util.js
@@ -20,7 +20,7 @@ export { inherits, inspect } from "util";
 export var debug = buildDebug("babel");
 
 export function canCompile(filename, altExts) {
-  var exts = altExts || exports.canCompile.EXTENSIONS;
+  var exts = altExts || canCompile.EXTENSIONS;
   var ext = path.extname(filename);
   return contains(exts, ext);
 }
@@ -50,7 +50,7 @@ export function regexify(val) {
 export function arrayify(val) {
   if (!val) return [];
   if (isBoolean(val)) return [val];
-  if (isString(val)) return exports.list(val);
+  if (isString(val)) return list(val);
   if (Array.isArray(val)) return val;
   throw new TypeError("illegal type for arrayify");
 }
@@ -75,7 +75,7 @@ var templateVisitor = {
 
 export function template(name, nodes, keepExpression) {
   var ast = exports.templates[name];
-  if (!ast) throw new ReferenceError("unknown template " + name);
+  if (!ast) throw new ReferenceError(`unknown template ${name}`);
 
   if (nodes === true) {
     keepExpression = true;