From 74f831bd39ffb0674ca86161c4843fc47c23d153 Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Mon, 29 Sep 2014 12:24:21 +1000
Subject: [PATCH] implement constants

---
 README.md                              |  2 +-
 lib/6to5/transformers/block-binding.js | 24 +++++++++------
 lib/6to5/transformers/constants.js     | 41 ++++++++++++++++++++++++++
 3 files changed, 57 insertions(+), 10 deletions(-)

diff --git a/README.md b/README.md
index 0d2369e792..21e9943d38 100644
--- a/README.md
+++ b/README.md
@@ -43,7 +43,7 @@
 | [Modules](FEATURES.md#modules)                                       | ✓           |
 | [Property name shorthand](FEATURES.md#property-name-shorthand)       | ✓           |
 | [Computed property names](FEATURES.md#computed-property-names)       | ✓           |
-| [Constants](FEATURES.md#constants)                                   |             |
+| [Constants](FEATURES.md#constants)                                   | ✓           |
 | [Destructuring assignment](FEATURES.md#destructuring-assignment)     |             |
 | [Array comprehension](FEATURES.md#array-comprehension)               |             |
 | [Generators](FEATURES.md#generators)                                 |             |
diff --git a/lib/6to5/transformers/block-binding.js b/lib/6to5/transformers/block-binding.js
index 84c33e8690..bd1a8e00a5 100644
--- a/lib/6to5/transformers/block-binding.js
+++ b/lib/6to5/transformers/block-binding.js
@@ -9,20 +9,26 @@ var isLet = function (node) {
   }
 };
 
+var hasLet = function (nodes) {
+  var has = false;
+
+  _.each(nodes, function (node) {
+    if (isLet(node)) has = true;
+  });
+
+  return has;
+};
+
 exports.Program = function (node) {
-  _.each(node.body, isLet);
+  if (hasLet(node.body)) {
+    node.body = [buildNode(node.body)];
+  }
 };
 
 exports.BlockStatement = function (node, parent) {
-  var hasLet = false;
+  if (!hasLet(node.body)) return;
 
-  _.each(node.body, function (bodyNode) {
-    if (isLet(bodyNode)) hasLet = true;
-  });
-
-  if (!hasLet) return;
-
-  // ignore if we're direct children of a closure already
+  // ignore if we're the body of a closure already
   if (parent.type === "FunctionExpression") return;
 
   node.body = [buildNode(node.body)];
diff --git a/lib/6to5/transformers/constants.js b/lib/6to5/transformers/constants.js
index e69de29bb2..485285d047 100644
--- a/lib/6to5/transformers/constants.js
+++ b/lib/6to5/transformers/constants.js
@@ -0,0 +1,41 @@
+var traverse = require("../traverse");
+var util     = require("../util");
+var _        = require("lodash");
+
+exports.Program =
+exports.BlockStatement =
+exports.ForInStatement =
+exports.ForStatement = function (node) {
+  var constants = [];
+
+  var check = function (node, name) {
+    if (constants.indexOf(name) >= 0) {
+      throw util.errorWithNode(node, name + " is read-only");
+    }
+  };
+
+  _.each(node.body, function (child) {
+    if (child.type === "VariableDeclaration" && child.kind === "const") {
+      _.each(child.declarations, function (declar) {
+        var name = declar.id.name;
+        check(declar, name);
+
+        declar._ignoreConstant = true;
+        constants.push(name);
+      });
+      child.kind = "let";
+    }
+  });
+
+  if (!constants.length) return;
+
+  traverse(node, function (child) {
+    if (child._ignoreConstant) return;
+
+    if (child.type === "VariableDeclarator") {
+      check(child, child.id.name);
+    } else if (child.type === "AssignmentExpression") {
+      check(child, child.left.name);
+    }
+  });
+};