javascript – 为什么布尔基元不会调用原型toString()？

Boolean.prototype.toString=function toString(){
  return this?'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)]);

因为规格这样说.
http://www.ecma-international.org/ecma-262/6.0/index.html#sec-tostring
在这个表中,定义了基元的String值.仅使用对象 ToPrimitive.

该表告诉我们,ToString对象o是ToString(ToPrimitive(o,“String”))

规范告诉我们,如果使用Object调用ToPrimitive,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ToPrimitive(o,“String”)的返回值是o.toString()和toString(o.toString())与o.toString()相同.