我不明白这个 javascript 函数是如何工作的

Question

我正在阅读function definition of bind，但我无法 100% 理解所写的代码：

if (!Function.prototype.bind) {
    Function.prototype.bind = function(oThis) {
        if (typeof this !== "function") {
            // closest thing possible to the ECMAScript 5 internal IsCallable function
            throw new TypeError("Function.prototype.bind - what is trying to be bound is not callable");
        }

        var aArgs = Array.prototype.slice.call(arguments, 1),
            fToBind = this,
            fNOP = function() {},
            fBound = function() {
                return fToBind.apply(this instanceof fNOP
                                       ? this 
                                       : oThis || window,
                                     aArgs.concat(Array.prototype.slice.call(arguments)));
            };

        fNOP.prototype = this.prototype;
        fBound.prototype = new fNOP();

        return fBound;
    };
}

具体看不懂fNOP的用途，也不明白为什么需要设置fBound的原型。我也挂断了fToBind.apply 部分（我无法弄清楚这在这种情况下代表什么）。

有人能解释一下这里发生了什么吗？

Answer 1

嗯，需要设置fBound 的原型的一个原因是，在函数上调用bind 的结果与该函数具有相同的原型。这也是fNop 的用武之地——它允许您使用new fNop() 设置fBound 的原型，而无需调用可能有副作用的原始函数。

对apply 的调用允许您在函数中设置this 并指定其他参数。由于bind 允许您将参数“curry”到函数中，因此您必须将绑定函数时传入的参数和调用它的参数结合起来。

Answer 2

这是为了确保

• (1) 绑定函数可以用作构造函数，忽略绑定。 （因此是instanceof 检查）
• (2) 同时，您要确保new g() 继承自f 的原型链。 （因此是 .prototype = new fNop 部分）

例子：

function f() {
    this.foo = 'bar';
}
f.prototype = {
    baz: 'yay!'
};

var g = f.bind({});
var o = new g();
console.log(o.foo); // 'bar' - (1)
console.log(o.baz); // 'yay!' - (2)

在您调用new g() 的那一刻，fBound 函数被调用为具有全新对象对象 (this) 的构造函数，该对象对象是fNop 的一个实例。

---

编辑：

ECMAScript5 标准为绑定函数定义了一个复杂的算法。除其他外，以下断言必须成立：

var DateJan2042 = Date.bind(null, 2042, 0);

 /*1*/ console.assert(Function.prototype.bind.length == 1, 'bind should have a length of 1');
 /*2*/ console.assert(typeof DateJan2042 == 'function', 'bind() should return a function');
 /*3*/ console.assert(!DateJan2042.hasOwnProperty('prototype'), 'Bound function must not have a prototype');
 /*4*/ console.assert(DateJan2042.length == Math.max(Date.length - 2, 0), 'Bound function should have a proper length');
 /*5*/ console.assert(typeof DateJan2042() == 'string', 'Function call should return a string');
 /*6*/ console.assert({}.toString.call(new DateJan2042()).indexOf('Date') != -1, 'Constructor call should return a new Date object');
 /*7*/ console.assert(new DateJan2042() instanceof DateJan2042, 'Instanceof check should pass for constructor\'s return value');
 /*8*/ console.assert((new DateJan2042()).getMonth() == 0, 'Constructor should be called with bound arguments');
 /*9*/ console.assert((new DateJan2042(1)).getDate() == 1, 'Constructor should take additional arguments');
/*10*/ console.assert(!/^function *\( *[^ )]/.test(Function.prototype.toString.call(DateJan2042)), 'Bound function should have no formal arguments');

由于正确绑定的函数不是真正的Function 对象，因此使用 polyfill （尤其是数字 2/3 和 4/10）是不可能的，但您可以尝试尽可能多地实现.

有问题的实现试图通过挂钩到原型链来解决第 6 和第 7 问题，但 that's not enough。

这里有一个更好的替代实现，但仍然不完美： http://jsfiddle.net/YR6MJ/

Answer 3

来自之前的评论：

你为什么不能直接说fBound.prototype = this.prototype而不是fNop？

据我所知，主要区别在于，当绑定函数内的this 的值是调用bind 的原始函数的实例时，那么要绑定到的值——最初传递给 bind 的第一个参数 -- 被忽略。

例如这段代码：

function Test(blah) {
    console.log(this.length, blah);
}

Test.prototype.length = 77;
Test.prototype.fn = Test.bind(['a', 'b', 'c'], "testing");
new Test().fn()

...导致fn 打印：

77 testing

换句话说，fn 内的this 的值是调用它的Test 实例。您的建议会将绑定数组提供给bind 内的apply，因此，这样写，相同代码的最后一行将打印：

3 testing

我并不完全清楚为什么这很重要，但它确实强调了您的建议不会产生相同的结果。

Answer 4

// check to see if the native implementation of bind already
// exists in this version of JavaScript. We only define the
// polyfill if it doesn't yet exist.

if (!Function.prototype.bind) {

  // creating the bind function for all Function instances by 
  // assigning it to the `Function.prototype` object. Normally 
  // you would avoid assigning to builtin prototypes because you
  // may cause a conflict with new features, but here this is a
  // known feature that is already in the spec that we're adding
  // to a JavaScript runtime that is not up to spec, so its ok

  Function.prototype.bind = function (oThis) {

    // if you attempt to call this function from a non-function object
    // for example if you assign this bind function to a normal object
    // or use `call`/`apply` to change the context of this function call to
    // a non function value (e.g. `Function.prototype.bind.call({})`), we
    // throw an error because bind can only work on functions, and we
    // require that `this` in this call is a function

    if (typeof this !== "function") {
      throw new TypeError("Function.prototype.bind - what is trying to be bound is not callable");
    }

    // bind does two things, it binds a context (`this` value) to a
    // function for when its called, and it provides a way to bake in
    // some pre-defined arguments that are automatically passed into 
    // that function when called. Those arguments can be passed into
    // the bind call and get picked up here as `aArgs` pulling them
    // from `arguments` making sure to lop off the `oThis` value

    var aArgs = Array.prototype.slice.call(arguments, 1),

      fToBind = this, // this is the function we're binding
      fNOP = function () {}, // a constructor used for `new` usage (see below)

      // `fBound` is the bound function - the result that bind is going to
      // return to represent the current function (`this` or `fToBind`) with
      // a new context.  The idea behind this function is that it will simply
      // take the original function and call it through `apply` with the
      // new context specified.

      fBound = function () {

        // call the original function with a new context using `apply`.
        // however if the function is called with `new`, it needs to be called
        // with the context of, and return, a new object instance and not the
        // bound version of this.  In that case, binding gets ignored in favor
        // of using the `this` of the new instance rather than the `oThis` binding.

        // new object instances inherit from the prototype of their constructors.
        // Our `fBound` function is supposed to mimic the original with the
        // exception of a change in context.  So if new objects are created with
        // it, they should behave as though they were created from the original.
        // But at the same time, we can't simply carry over the prototype of the
        // original into `fBound` because it is a separate function and needs its
        // own prototype, just one that also inherits from the original. To
        // accommodate this, the `fNOP` function (constructor) above is used as
        // an intermediary for creating `fBound`'s prototype while allowing it to
        // be unique but also inherit the original.  And because that becomes part
        // of the bound function's prototype chain, it can be used to determine
        // whether `this` in `fBound` is an instance created by `new` or not since
        // `instanceof` works through a prototype chain lookup.

        return fToBind.apply(this instanceof fNOP
               ? this
               : oThis,

               // call the function with arguments that include the added 
               // arguments specified from the original bind call plus
               // the arguments this function was called with

               aArgs.concat(Array.prototype.slice.call(arguments)));
      };

    // `fNOP`'s use to provide an intermediary prototype between `fBound` and
    // the current function instance mimics `Object.create`. But we're assuming
    // if you don't have `bind`, you probably don't have `create` either, so do
    // it the old fashioned way with a constructor.  This works by setting the
    // constructor's prototype to the to-inherit-from constructor's (this)
    // prototype. A check is needed to prevent assinging that prototype to null
    // if it doesn't exist on this function (Function.prototype is technically
    // a valid target for `bind()` because it is a function but one that does not
    // have its own prototype).

    if (this.prototype) {
      fNOP.prototype = this.prototype;
    }

    // here the inheritance is made.  As a new function, `fBound` has no existing
    // inheritance chain to worry about, so we can easily replace it with a new
    // one - that of a new instance `fNOP`.  Since `fNOP`'s prototype is the original
    // function's prototype, `fBound` has a prototype which directly inherits from
    // that, one level between new instances and the original prototype. So
    // `fBound.prototype.__proto__ === this.prototype` and new instances of `fBound`
    // created with `new fBound()` will inherit from `fBound.prototype` as well as
    // the original function's prototype.

    fBound.prototype = new fNOP();

    // return the bound version of the function as
    // the result of the bind call

    return fBound;
  };
}