为什么布尔原语不调用原型 toString()？

假设我有这段代码：

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？对象键可以是字符串或符号，它们不能只是原始布尔值。这就是我感到困惑的原因。

因为规格是这么说的。http://www.ecma-international.org/ecma-262/6.0/index.html#sec-tostring http://www.ecma-international.org/ecma-262/6.0/index.html#sec-tostring在此表中定义了基元的字符串值。仅适用于对象到原语 http://www.ecma-international.org/ecma-262/6.0/index.html#sec-toprimitive用来。

该表告诉我们ToString对于一个对象o is ToString( ToPrimitive(o, "string"))

规范告诉我们，如果ToPrimitive使用对象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).

@@toPrimitivebeeing set 是一个特殊情况，所以我们现在必须看看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") is o.toString() and toString(o.toString())是相同的o.toString().