有没有改变JavaScript对象的内部[[Class]]属性？ [英] Is there away to change the internal [[Class]] property of a JavaScript Object?

问题描述

假设我已经创建了一个函数构造函数，例如

Suppose that I have created a function constructor such as

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

是否需要制作 Object.prototype.toString.call（obj） 返回类似 [object Date] ？

Is there away to make Object.prototype.toString.call(obj) return something like [object Date] ?

推荐答案

ECMAScript5 不提供任何方式修改[[Class]]：

ECMAScript5 doesn't provide any way to modify [[Class]]:

此规范不定义ECMAScript语言运算符或允许程序的
内置函数修改对象的
[[Class]]或[[Prototype]]内部属性或将[[Extensible]]的值
从false更改为true。修改[[Class]]，[[Prototype]]或[[Extensible]]的实现特定
扩展必须
不违反前一段中定义的不变量。

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，但你可以使用@@ toStringTag 众所周知的符号：

ECMAScript6 removed the [[Class]] internal slot, but you can use the @@toStringTag Well-Known Symbol:

• 规格名称
  
  @@ toStringTag


• [[描述]]
  
  Symbol.toStringTag


• 值和目的
  
  一个字符串值属性，用于创建默认字符串描述宾语。通过内置方法访问 Object.prototype.toString 。

• 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]"

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