为什么Boolean原语不能调用原型toString()？

Question

说我有这个代码:

Boolean.prototype.toString = function toString() {
  return this.valueOf() ? '1' : '0';
};

var object = {
  true: 'true',
  false: 'false',
  1: '1',
  0: '0'
};

// "true" - this doesn't work
console.log('primitive', object[true]);
// "1" - but these do
console.log('primitive.toString()', object[true.toString()]);
console.log('instance', object[new Boolean(true)]);

为什么原语不使用类的toString定义？对象键是字符串或符号,它们不能只是原始的布尔值.这就是为什么我很困惑.

Answer 1

因为规格说明了. http://www.ecma-international.org/ecma-262/6.0/index.html#sec-tostring 在此表中定义了基元的String值.仅用于对象 ToPrimitive.

该表告诉我们,ToString对象o是ToString( ToPrimitive(o, "string"))

规范告诉我们,如果ToPrimitive使用Object调用,我们必须遵循以下步骤:

1. If PreferredType was not passed, let hint be "default".
2. Else if PreferredType is hint String, let hint be "string".
3. Else PreferredType is hint Number, let hint be "number".
4. Let exoticToPrim be GetMethod(input, @@toPrimitive).
5. ReturnIfAbrupt(exoticToPrim).
6. If exoticToPrim is not undefined, then
  a. Let result be Call(exoticToPrim, input, «hint»).
  b. ReturnIfAbrupt(result).
  c. If Type(result) is not Object, return result.
  d. Throw a TypeError exception.
7. If hint is "default", let hint be "number".
8. Return OrdinaryToPrimitive(input,hint).

@@toPrimitive beeing set是一个特例,所以我们现在要看一下 OrdinaryToPrimitive

1. Assert: Type(O) is Object
2. Assert: Type(hint) is String and its value is either "string" or "number".
3. If hint is "string", then
  a. Let methodNames be «"toString", "valueOf"».
4. Else,
  a. Let methodNames be «"valueOf", "toString"».
5. For each name in methodNames in List order, do
  a. Let method be Get(O, name).
  b. ReturnIfAbrupt(method).
  c. If IsCallable(method) is true, then
    i. Let result be Call(method, O).
    ii. ReturnIfAbrupt(result).
    iii. If Type(result) is not Object, return result.
6. Throw a TypeError exception.

所以这意味着返回值ToPrimitive(o, "string")是o.toString()和toString(o.toString())是一样的o.toString().