fix let variable declaration hoisting bug. All let variable declarators now have a default initializer of undefined unless they're the left of a ForIn or ForOf

lib/6to5/transformation/transformers/es6-let-scoping.js

@@ -3,18 +3,25 @@ var util     = require("../../util");
 var t        = require("../../types");
 var _        = require("lodash");
 
-var isLet = function (node) {
+var isLet = function (node, parent) {
   if (!t.isVariableDeclaration(node)) return false;
   if (node._let) return true;
   if (node.kind !== "let") return false;
 
+  // https://github.com/6to5/6to5/issues/255
+  if (!t.isFor(parent) || t.isFor(parent) && parent.left !== node) {
+    _.each(node.declarations, function (declar) {
+      declar.init = declar.init || t.identifier("undefined");
+    });
+  }
+
   node._let = true;
   node.kind = "var";
   return true;
 };
 
-var isVar = function (node) {
-  return t.isVariableDeclaration(node, { kind: "var" }) && !isLet(node);
+var isVar = function (node, parent) {
+  return t.isVariableDeclaration(node, { kind: "var" }) && !isLet(node, parent);
 };
 
 var standardiseLets = function (declars) {
@@ -23,13 +30,13 @@ var standardiseLets = function (declars) {
   }
 };
 
-exports.VariableDeclaration = function (node) {
-  isLet(node);
+exports.VariableDeclaration = function (node, parent) {
+  isLet(node, parent);
 };
 
 exports.Loop = function (node, parent, file, scope) {
   var init = node.left || node.init;
-  if (isLet(init)) {
+  if (isLet(init, node)) {
     t.ensureBlock(node);
     node.body._letDeclars = [init];
   }
@@ -91,9 +98,6 @@ LetScoping.prototype.run = function () {
   // remap all let references that exist in upper scopes to their uid
   this.remap();
 
-  // add default initializer to let variables in loop bodys
-  this.initialiseLoopLets();
-
   // this is a block within a `Function` so we can safely leave it be
   if (t.isFunction(this.parent)) return this.noClosure();
 
@@ -242,7 +246,7 @@ LetScoping.prototype.getInfo = function () {
 
   for (var i in block.body) {
     var declar = block.body[i];
-    if (!isLet(declar)) continue;
+    if (!isLet(declar, block)) continue;
 
     _.each(t.getIds(declar, true), function (id, key) {
       duplicates(id, key);
@@ -253,31 +257,6 @@ LetScoping.prototype.getInfo = function () {
   return opts;
 };
 
-/**
- * Any let variable declared within a loop body need to have an initializer or
- * else they'll be hoisted and subsequent iterations of the loop will have a
- * previous state. This function adds a default initializer of `undefined` to
- * those variables.
- */
-
-LetScoping.prototype.initialiseLoopLets = function () {
-  var loopParent = this.loopParent;
-  if (!loopParent) return;
-
-  traverse(this.block, function (node) {
-    if (t.isFunction(node) || t.isLoop(node)) {
-      return false;
-    }
-
-    if (isLet(node)) {
-      for (var i in node.declarations) {
-        var declar = node.declarations[i];
-        declar.init = declar.init || t.identifier("undefined");
-      }
-    }
-  });
-};
-
 /**
  * If we're inside of a loop then traverse it and check if it has one of
  * the following node types `ReturnStatement`, `BreakStatement`,
@@ -333,16 +312,16 @@ LetScoping.prototype.checkLoop = function () {
 
 LetScoping.prototype.hoistVarDeclarations = function () {
   var self = this;
-  traverse(this.block, function (node) {
+  traverse(this.block, function (node, parent) {
     if (t.isForStatement(node)) {
-      if (isVar(node.init)) {
+      if (isVar(node.init, node)) {
         node.init = t.sequenceExpression(self.pushDeclar(node.init));
       }
     } else if (t.isFor(node)) {
-      if (isVar(node.left)) {
+      if (isVar(node.left, node)) {
         node.left = node.left.declarations[0].id;
       }
-    } else if (isVar(node)) {
+    } else if (isVar(node, parent)) {
       return self.pushDeclar(node).map(t.expressionStatement);
     } else if (t.isFunction(node)) {
       return false;