javascript - 为什么从绑定(bind)函数返回的 `this` 不严格等于传递的原始值？

Question

我注意到从绑定(bind)函数返回的 this 并不严格等于绑定(bind)时传递的原始值。

根据MDN当调用原始函数时 [[BoundThis]] 被传递。

从绑定(bind)函数返回的this是什么？是[[BoundThis]]吗？

const fn = function() { return this }
const strBound = fn.bind('111')
const numBound = fn.bind(111)
const boolBound = fn.bind(true)

console.log(strBound()) // [String: '111']
console.log(strBound() == '111') // true
console.log(strBound() === '111') // false

console.log(numBound()) // [Number: 111]
console.log(numBound() == 111) // true
console.log(numBound() === 111) // false

console.log(boolBound()) // [Boolean: true]
console.log(boolBound() == true) // true
console.log(boolBound() === true) // false

Answer 1

与往常一样，此行为已由 ECMAScript 规范定义。在这种情况下，实际遵循不同的规范需要一段时间，但如果您对细节感兴趣，就来吧。

19.2.3.2 Function.prototype.bind ( thisArg , ...args)

[...]

4. Let F be BoundFunctionCreate(Target, thisArg, args).

[...]

16. Return F.

---

9.4.1.3 BoundFunctionCreate (targetFunction, boundThis, boundArgs)

[...]

6. Set the [[Call]] internal method of obj as described in 9.4.1.1.

---

9.4.1.1 [[Call]] ( thisArgument, argumentsList)

[...]

5. Return Call(target, boundThis, args).

---

7.3.12 Call(F, V, [argumentsList])

[...]

4. Return F.[[Call]](V, argumentsList).

---

9.2.1 [[Call]] ( thisArgument, argumentsList)

[...]

6. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).

---

9.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument )

[...]

5. If thisMode is strict, let thisValue be thisArgument.
6. Else
   • if thisArgument is null or undefined, then
     Let thisValue be calleeRealm.[[globalThis]].
   • Else
     Let thisValue be ToObject(thisArgument).

---

7.1.13 ToObject ( argument )

[...]

Number: Return a new Number object whose [[NumberData]] internal slot is set to the value of argument. See 20.1 for a description of Number objects.

String: Return a new String object whose [[StringData]] internal slot is set to the value of argument. See 21.1 for a description of String objects.

• 原始字符串并不严格等于具有相同值的字符串对象。
• 原始数字并不严格等于具有相同值的数字对象。