javascript - 是否可以更改 JavaScript 对象的内部 [[Class]] 属性？

Question

假设我已经创建了一个函数构造函数，例如

    function Bar(){
    }
    var obj = new Bar();

是否可以让 Object.prototype.toString.call(obj) 返回类似 [object Date] 的内容？

Answer 1

ECMAScript5不提供任何修改 [[Class]] 的方法:

This specification defines no ECMAScript language operators or built-in functions that permit a program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of [[Extensible]] from false to true. Implementation specific extensions that modify [[Class]], [[Prototype]] or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

ECMAScript6 删除了 [[Class]] 内部插槽，但您可以使用 @@toStringTag Well-Known Symbol :

• Specification Name
  @@toStringTag
• [[Description]]
  "Symbol.toStringTag"
• Value and Purpose
  A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method Object.prototype.toString.

Bar.prototype[Symbol.toStringTag] = 'Date';
Object.prototype.toString.call(new Bar()); // "[object Date]"