implement constants

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)];

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);
+    }
+  });
+};