Add tests for Subclassing the built-in WeakMap Objects

test/language/subclassing/WeakMap/regular-subclassing.js

@@ -0,0 +1,28 @@
+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+es6id: 23.3.1
+description: Subclassing the WeakMap object
+info: >
+  23.3.1 The WeakMap Constructor
+
+  ...
+
+  The WeakMap constructor is designed to be subclassable. It may be used as the
+  value in an extends clause of a class definition. Subclass constructors that
+  intend to inherit the specified WeakMap behaviour must include a super call to
+  the WeakMap constructor to create and initialize the subclass instance with
+  the internal state necessary to support the WeakMap.prototype built-in
+  methods.
+---*/
+
+class WM extends WeakMap {}
+
+var map = new WM();
+var obj = {};
+
+assert.sameValue(map.has(obj), false);
+
+map.set(obj, 42);
+assert.sameValue(map.has(obj), true);
+assert.sameValue(map.get(obj), 42);

test/language/subclassing/WeakMap/super-must-be-called.js

@@ -0,0 +1,33 @@
+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+es6id: 23.3.1
+description: Super need to be called to initialize internals
+info: >
+  23.3.1 The WeakMap Constructor
+
+  ...
+
+  The WeakMap constructor is designed to be subclassable. It may be used as the
+  value in an extends clause of a class definition. Subclass constructors that
+  intend to inherit the specified WeakMap behaviour must include a super call to
+  the WeakMap constructor to create and initialize the subclass instance with
+  the internal state necessary to support the WeakMap.prototype built-in
+  methods.
+---*/
+
+class M1 extends WeakMap {
+  constructor() {}
+}
+
+assert.throws(ReferenceError, function() {
+  new M1();
+});
+
+class M2 extends WeakMap {
+  constructor() {
+    super();
+  }
+}
+
+new M2();