implement block scoping TDZ

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

@@ -57,6 +57,7 @@ exports.Loop = function (node, parent, scope, context, file) {
   }
 };
 
+exports.Program =
 exports.BlockStatement = function (block, parent, scope, context, file) {
   if (!t.isLoop(parent)) {
     var letScoping = new LetScoping(false, block, parent, scope, file);
@@ -95,10 +96,12 @@ LetScoping.prototype.run = function () {
   if (block._letDone) return;
   block._letDone = true;
 
-  // this is a block within a `Function` so we can safely leave it be
-  if (t.isFunction(this.parent)) return;
-
   var needsClosure = this.getLetReferences();
+  this.checkTDZ();
+
+  // this is a block within a `Function` so we can safely leave it be
+  if (t.isFunction(this.parent) || t.isProgram(this.block)) return;
+
   if (needsClosure) {
     this.needsClosure();
   } else {
@@ -106,6 +109,46 @@ LetScoping.prototype.run = function () {
   }
 };
 
+/**
+ * Description
+ */
+
+LetScoping.prototype.checkTDZ = function () {
+  var state = {
+    letRefs: this.letReferences,
+    file: this.file
+  };
+
+  traverse(this.block, {
+    enter: function (node, parent, scope, context, state) {
+      if (!t.isIdentifier(node)) return;
+      if (!t.isReferenced(node, parent)) return;
+
+      var declared = state.letRefs[node.name];
+      if (!declared) return;
+
+      // declared node is different in this scope
+      if (scope.get(node.name, true) !== declared) return;
+
+      var declaredLoc  = declared.loc.start;
+      var referenceLoc = node.loc.start;
+
+      // does this reference appear on a line before the declaration?
+      var before = referenceLoc.line < declaredLoc.line;
+
+      if (referenceLoc.line === declaredLoc.line) {
+        // this reference appears on the same line
+        // check it appears before the declaration
+        before = referenceLoc.col < declaredLoc.col;
+      }
+
+      if (before) {
+        throw state.file.errorWithNode(node, "Temporal dead zone - accessing a variable before it's initialized");
+      }
+    }
+  }, this.scope, state);
+};
+
 /**
  * Description
  */

test/fixtures/transformation/es6-let-scoping/temporal-dead-zone/actual.js

@@ -0,0 +1,2 @@
+qux;
+let qux = 456;

test/fixtures/transformation/es6-let-scoping/temporal-dead-zone/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Temporal dead zone - accessing a variable before it's initialized"
+}