Merge pull request #402 from EB-Forks/fix/use-define-for-tostringtag

runtime: Use `[[Define]]` for the `Symbol.toStringTag` property

Author: benjamn

Diff for: packages/regenerator-runtime/runtime.js

@@ -16,6 +16,24 @@ var runtime = (function (exports) {
   var asyncIteratorSymbol = $Symbol.asyncIterator || "@@asyncIterator";
   var toStringTagSymbol = $Symbol.toStringTag || "@@toStringTag";
 
+  function define(obj, key, value) {
+    Object.defineProperty(obj, key, {
+      value: value,
+      enumerable: true,
+      configurable: true,
+      writable: true
+    });
+    return obj[key];
+  }
+  try {
+    // IE 8 has a broken Object.defineProperty that only works on DOM objects.
+    define({}, "");
+  } catch (err) {
+    define = function(obj, key, value) {
+      return obj[key] = value;
+    };
+  }
+
   function wrap(innerFn, outerFn, self, tryLocsList) {
     // If outerFn provided and outerFn.prototype is a Generator, then outerFn.prototype instanceof Generator.
     var protoGenerator = outerFn && outerFn.prototype instanceof Generator ? outerFn : Generator;
@@ -82,29 +100,23 @@ var runtime = (function (exports) {
     IteratorPrototype = NativeIteratorPrototype;
   }
 
-  function ensureDefaultToStringTag(object, defaultValue) {
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1644581#c6
-    return toStringTagSymbol in object
-      ? object[toStringTagSymbol]
-      : object[toStringTagSymbol] = defaultValue;
-  }
-
   var Gp = GeneratorFunctionPrototype.prototype =
     Generator.prototype = Object.create(IteratorPrototype);
   GeneratorFunction.prototype = Gp.constructor = GeneratorFunctionPrototype;
   GeneratorFunctionPrototype.constructor = GeneratorFunction;
-  GeneratorFunction.displayName = ensureDefaultToStringTag(
+  GeneratorFunction.displayName = define(
     GeneratorFunctionPrototype,
+    toStringTagSymbol,
     "GeneratorFunction"
   );
 
   // Helper for defining the .next, .throw, and .return methods of the
   // Iterator interface in terms of a single ._invoke method.
   function defineIteratorMethods(prototype) {
     ["next", "throw", "return"].forEach(function(method) {
-      prototype[method] = function(arg) {
+      define(prototype, method, function(arg) {
         return this._invoke(method, arg);
-      };
+      });
     });
   }
 
@@ -123,7 +135,7 @@ var runtime = (function (exports) {
       Object.setPrototypeOf(genFun, GeneratorFunctionPrototype);
     } else {
       genFun.__proto__ = GeneratorFunctionPrototype;
-      ensureDefaultToStringTag(genFun, "GeneratorFunction");
+      define(genFun, toStringTagSymbol, "GeneratorFunction");
     }
     genFun.prototype = Object.create(Gp);
     return genFun;
@@ -393,7 +405,7 @@ var runtime = (function (exports) {
   // unified ._invoke helper method.
   defineIteratorMethods(Gp);
 
-  ensureDefaultToStringTag(Gp, "Generator");
+  define(Gp, toStringTagSymbol, "Generator");
 
   // A Generator should always return itself as the iterator object when the
   // @@iterator function is called on it. Some browsers' implementations of the

Diff for: test/non-writable-tostringtag-property.js

@@ -5,7 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-var hasOwn = Object.prototype.hasOwnProperty;
+var assert = require("assert");
+
 var getProto = Object.getPrototypeOf;
 var NativeIteratorPrototype =
   typeof Symbol === "function" &&
@@ -20,8 +21,16 @@ Object.defineProperty(NativeIteratorPrototype, Symbol.toStringTag, {
   value: "Iterator",
 });
 
-describe("Symbol.toStringTag safety (#399, #400)", function () {
+(NativeIteratorPrototype ? describe : describe.skip)("Symbol.toStringTag safety (#399, #400)", function () {
   it("regenerator-runtime doesn't fail to initialize when native iterator prototype has a non-writable @@toStringTag property", function() {
     require("./runtime.js");
   });
+
+  it("regenerator-runtime's polyfilled generator prototype has the correct @@toStringTag value", function() {
+    require("./runtime.js");
+    function foo() {}
+    regeneratorRuntime.mark(foo);
+
+    assert.strictEqual(foo.prototype[Symbol.toStringTag], "Generator");
+  });
 });