implement temporal dead zone for default parameters - fixes #169

lib/6to5/transformation/transformers/default-parameters.js

@@ -1,21 +1,70 @@
-var util = require("../../util");
-var t    = require("../../types");
-var _    = require("lodash");
+var traverse = require("../../traverse");
+var util     = require("../../util");
+var t        = require("../../types");
+var _        = require("lodash");
 
-exports.Function = function (node) {
+exports.Function = function (node, parent, file, scope) {
   if (!node.defaults || !node.defaults.length) return;
   t.ensureBlock(node);
 
+  var ids = node.params.map(function (param) {
+    return t.getIds(param);
+  });
+
+  var closure = false;
+
   _.each(node.defaults, function (def, i) {
     if (!def) return;
 
     var param = node.params[i];
 
-    node.body.body.unshift(util.template("if-undefined-set-to", {
-      VARIABLE: param,
-      DEFAULT: def
+    // temporal dead zone check - here we prevent accessing of params that
+    // are to the right - ie. uninitialized parameters
+    _.each(ids.slice(i), function (ids, i) {
+      var check = function (node, parent) {
+        if (!t.isIdentifier(node) || !t.isReferenced(node, parent)) return;
+
+        if (_.contains(ids, node.name)) {
+          throw file.errorWithNode(node, "Temporal dead zone - accessing a variable before it's initialized");
+        }
+
+        if (scope.has(node.name)) {
+          closure = true;
+        }
+      };
+
+      check(def, node);
+      traverse(def, check);
+    });
+
+    // we're accessing a variable that's already defined within this function
+    var has = scope.get(param.name);
+    if (has && !_.contains(node.params, has)) {
+      closure = true;
+    }
+  });
+
+  var body = [];
+
+  _.each(node.defaults, function (def, i) {
+    if (!def) return;
+
+    body.push(util.template("if-undefined-set-to", {
+      VARIABLE: node.params[i],
+      DEFAULT:  def
     }, true));
   });
 
+  if (closure) {
+    var container = t.functionExpression(null, [], node.body, node.generator);
+    container._aliasFunction = true;
+
+    body.push(t.returnStatement(t.callExpression(container, [])));
+
+    node.body = t.blockStatement(body);
+  } else {
+    node.body.body = body.concat(node.body.body);
+  }
+
   node.defaults = [];
 };

test/fixtures/transformation/default-parameters/multiple/expected.js

@@ -1,8 +1,8 @@
 "use strict";
 
 var t = function (t, f) {
-  if (f === undefined) f = 5;
   if (t === undefined) t = "foo";
+  if (f === undefined) f = 5;
   return t + " bar " + f;
 };
 

test/fixtures/transformation/default-parameters/tdz-2/actual.js

@@ -0,0 +1 @@
+function foo(a=b, b) {}

test/fixtures/transformation/default-parameters/tdz-2/options.json

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

test/fixtures/transformation/default-parameters/tdz-3/exec.js

@@ -0,0 +1,4 @@
+assert.equal((function(a, b=a++){
+  function b(){}
+  return a;
+})(1), 2);

test/fixtures/transformation/default-parameters/tdz/actual.js

@@ -0,0 +1 @@
+function foo(a=a) {}

test/fixtures/transformation/default-parameters/tdz/options.json

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