javascript - JavaScript 保证输出 "[object X]"吗？

Question

当您在某个对象上调用 Object.prototype.toString.apply(object) 时，您会以 [object X] 的形式返回某个类型名称的字符串X(例如，Array、Function、Null、Boolean、Number ，依此类推)。

是否有任何 ECMAScript 标准保证这种行为？我知道未指定宿主对象的结果是什么，但是 ECMA 标准中的常规对象呢？

Answer 1

根据 ECMA 5.1 Specifications ,

When the toString method is called, the following steps are taken:

1. If the this value is undefined, return "[object Undefined]".
2. If the this value is null, return "[object Null]".
3. Let O be the result of calling ToObject passing the this value as the argument.
4. Let class be the value of the [[Class]] internal property of O.
5. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

还有 [[Class]]定义为，

A String value indicating a specification defined classification of objects.

The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The value of the [[Class]] internal property of a host object may be any String value except one of "Arguments", "Array", "Boolean", "Date", "Error", "Function", "JSON", "Math", "Number", "Object", "RegExp", and "String". The value of a [[Class]] internal property is used internally to distinguish different kinds of objects. Note that this specification does not provide any means for a program to access that value except through Object.prototype.toString (see 15.2.4.2).