Why does Boolean primitive not call prototype toString()?

问题

Say I have this code:

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)]);

Why doesn't the primitive use the class's toString definition? Object keys are either strings or symbols, they cannot just be raw booleans. This is why I'm confused.

回答1:

Because the specifications says so. http://www.ecma-international.org/ecma-262/6.0/index.html#sec-tostring In this table the String values of primitives are defined. Only for Objects ToPrimitive is used.

---

The table tells us thatToString for an Object o is ToString( ToPrimitive(o, "string"))

The Specification tells us that if ToPrimitive is called with an Object we have to follow these steps:

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 is a special case so we now have to look at 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.

So this means that the return value of ToPrimitive(o, "string") is o.toString() and toString(o.toString()) is the same as o.toString().

回答2:

• "true" is a boolean.
• A "Boolean" instance is an object, not a boolean.
• So the instance is not obliged to use the prototype.
• "Boolean" object is just an abstraction in js for using boolean type.

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

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

console.log('primitive', object[true]);
console.log('instance', object[new Boolean(true)]);
console.log('bool type:', typeof(true));
console.log('Boolean type:', typeof(new Boolean(true)));

来源：https://stackoverflow.com/questions/37659354/why-does-boolean-primitive-not-call-prototype-tostring