我能'；我不理解这个javascript函数是如何工作的

我正在阅读，但我不能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;
    };
}

具体来说，我不明白

fNOP

的目的，也不明白为什么需要设置

fBound

的原型。我还在

fToBind.apply

部分挂断了电话（我不知道这在上下文中代表什么）

---

有人能解释一下这里发生了什么吗？

好吧，需要设置

fBound

的原型的一个原因是，对函数调用

bind

的结果与该函数具有相同的原型。这也是

fNop

似乎的用武之地——它允许您使用

new fNop（）

设置

fBound

的原型，而无需调用可能产生副作用的原始函数

---

调用

apply

可以在函数中设置

this

，并指定其他参数。由于

bind

允许将参数“curry”到函数中，因此必须将绑定函数时传入的参数和调用函数时使用的参数结合起来。

前面的注释：

---

与其使用fNop，为什么不能直接说

fBound.prototype=
这个.原型

据我所知，主要区别在于，当绑定函数中的

this

的值是调用

bind

的原始函数的一个实例时，要绑定到的值——最初传递给

bind

的第一个参数——被忽略

例如，此代码：

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

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

…使

fn

打印：

77 testing

换句话说，

this

在

fn

中的值是调用它的

Test

实例。您的建议将绑定数组提供给

apply

内部的

bind

，因此，以这种方式编写，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，但它确实强调了你的建议不会产生相同的结果。

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，

  instanceof

  检查）
• （2） 同时，您要确保

  newg（）

  继承自

  f

  的原型链。（因此，

  .prototype=新fNop

  零件）

例如：

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)

在调用

new g（）

时，

fBound

函数被调用为一个构造函数，其中包含一个全新的对象（

this

），它是

fNop

的实例

---

编辑： ECMAScript 5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

函数

对象，因此不可能使用多边形填充（特别是数字2/3和4/10）使其正常工作，但您可以尝试尽可能多地实现

所讨论的实现试图通过连接到原型链来解决第6个和第7个问题，但是

下面是一个工作稍微好一点但仍不完美的替代实现：

---

我需要更多的伏特加“斯米尔诺夫”来回答这个问题……大约在中途，这是否有任何启发？我认为这不会有多大帮助，但仅供参考：。@Davis这不是bind的定义，它是旧浏览器的部分（！）解决方法。请记住，MDN是一个wiki，可能是错误的。这就是代码的来源：与其使用fNop，为什么不能直接说fbond.prototype=This.prototype？我认为这与为什么要进行fNop检查有关，这是我无法推理的部分。这一点是，

bind

创建了一个函数。因此，它的定义只是为了与声明一个函数保持一致，其中一个新对象被分配给该函数的

prototype

属性。我想我们可以使用

fBound.prototype=Object.create（this.prototype）

，但这没有多大意义，因为

Object.create

可能不存在于不知道

bind

的浏览器中。“对函数调用bind的结果与该函数具有相同的原型。”实际上，它应该根本没有原型，因为作为一个整体，它的行为似乎不符合标准。一旦绑定，

这个

就不能被覆盖（无论调用方是谁，

源代码的实例与否）。因此，在任何情况下都不应不使用
或
（使该条件充其量是不必要的，充其量是非标准的）。
// 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;
  };
}