Implement well-known symbol `Symbol.toStringTag`

test/mjsunit/es6/symbol-tostringtag.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+assertEquals(typeof Symbol.toStringTag, "symbol");
+
+var toStringTag = Symbol.toStringTag;
+
+function tagIs(value, tag){
+  assertEquals(({}).toString.call(value), "[object " + tag + "]");
+}
+
+(function testBuiltinTags() {
+  tagIs(undefined, "Undefined");
+  tagIs(null, "Null");
+  tagIs([], "Array");
+  tagIs("", "String");
+  tagIs(function(){return arguments}(), "Arguments");
+  tagIs(Function(), "Function");
+  tagIs(Error(), "Error");
+  tagIs(!1, "Boolean");
+  tagIs(1, "Number");
+  tagIs(new Date, "Date");
+  tagIs(/./, "RegExp");
+  tagIs({}, "Object");
+})();
+
+(function testBuiltinToStringTags() {
+  tagIs(Math, "Math");
+  tagIs(JSON, "JSON");
+  tagIs(Symbol(), "Symbol");
+  tagIs(new Promise(function(){}), "Promise");
+  tagIs(new Map, "Map");
+  tagIs(new WeakMap, "WeakMap");
+  tagIs(new Set, "Set");
+  tagIs(new WeakSet, "WeakSet");
+  tagIs(new ArrayBuffer, "ArrayBuffer");
+  tagIs(new DataView(new ArrayBuffer), "DataView");
+})();
+
+(function testChangingBuiltinToStringTags() {
+  changedBuiltinToStringTagIs(Math, "foo");
+  changedBuiltinToStringTagIs(JSON, "foo");
+  changedBuiltinToStringTagIs(Symbol(), "foo");
+  changedBuiltinToStringTagIs(new Promise(function(){}), "foo");
+  changedBuiltinToStringTagIs(new Map, "foo");
+  changedBuiltinToStringTagIs(new WeakMap, "foo");
+  changedBuiltinToStringTagIs(new Set, "foo");
+  changedBuiltinToStringTagIs(new WeakSet, "foo");
+  changedBuiltinToStringTagIs(new ArrayBuffer, "foo");
+  changedBuiltinToStringTagIs(new DataView(new ArrayBuffer), "foo");
+
+  function changedBuiltinToStringTagIs(value, tag){
+    changeBuiltinToStringTag(value);
+    tagIs(value, tag);
+  }
+
+  function changeBuiltinToStringTag(value){
+    if(toStringTag in value.__proto__){
+      var value = value.__proto__;
+    }
+    Object.defineProperty(value, toStringTag, {
+      value: "foo",
+      configurable: true
+    });
+  }
+})();
+
+(function testDeletingBuiltinToStringTags() {
+  deletedBuiltinToStringTagIs(Math, "Object");
+  deletedBuiltinToStringTagIs(JSON, "Object");
+  deletedBuiltinToStringTagIs(Symbol(), "Object");
+  deletedBuiltinToStringTagIs(new Promise(function(){}), "Object");
+  deletedBuiltinToStringTagIs(new Map, "Object");
+  deletedBuiltinToStringTagIs(new WeakMap, "Object");
+  deletedBuiltinToStringTagIs(new Set, "Object");
+  deletedBuiltinToStringTagIs(new WeakSet, "Object");
+  deletedBuiltinToStringTagIs(new ArrayBuffer, "Object");
+  deletedBuiltinToStringTagIs(new DataView(new ArrayBuffer), "Object");
+
+  function deletedBuiltinToStringTagIs(value, tag){
+    deleteBuiltinToStringTag(value);
+    tagIs(value, tag);
+  }
+
+  function deleteBuiltinToStringTag(value){
+    if(toStringTag in value.__proto__){
+      var value = value.__proto__;
+    }
+    delete value[toStringTag];
+  }
+})();
+
+(function testUserDefinedToStringTags() {
+  // `toStringTag` is an own property
+  var o = {};
+  o[toStringTag] = "foo";
+  tagIs(o, "foo");
+
+  // `toStringTag` is an inherited property
+  var proto = {};
+  proto[toStringTag] = "foo";
+  var o = {__proto__: proto};
+  tagIs(o, "foo");
+
+  // The type of `toStringTag` property's value isn't string
+  var o = {};
+  o[toStringTag] = 1;
+  tagIs(o, "???");
+
+  // The value of `toStringTag` property is one of
+  // ["Arguments", "Array", "Boolean", "Date",
+  // "Error", "Function", "Number", "RegExp", "String"]
+  var o = {};
+  ["Arguments", "Array", "Boolean", "Date", "Error",
+  "Function", "Number", "RegExp", "String"].forEach(function(tag){
+    o[toStringTag] = tag;
+    tagIs(o, "~" + tag);
+  });
+})();