我無法理解這個javascript函數是如何工作的

Question

我正在閱讀bind的函數定義 ，但我無法100％理解編寫的代碼：

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;
    };
}

具體來說，我沒有達到fNOP的目的，我不明白為什么需要設置fBound的原型。 我也掛在fToBind.apply部分（我無法弄清楚這在這個上下文中代表什么）。

有人可以解釋這里發生了什么嗎？

Answer 1

好吧，需要設置fBound原型的一個原因是，在函數上調用bind的結果bind該函數具有相同的原型。 這也是fNop似乎進來的地方 - 它允許你使用new fNop()設置fBound的原型而不調用可能有副作用的原始函數。

對apply的調用允許您在函數中設置this並指定其他參數。 由於bind允許你“curry”函數的參數，你必須結合綁定函數時傳入的參數和調用它的參數。

Answer 2

這是為了確保

• （1）綁定函數可以用作構造函數，忽略綁定。 （因此檢查的instanceof ）
• （2）同時，你想確保new g()繼承自f的原型鏈。 （因此.prototype = new fNop部分）

例：

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)

在你調用new g()的那一刻， fBound函數被稱為一個fBound ，它有一個全新的對象對象（ this ），它是fNop一個實例。

---

編輯：

ECMAScript5標准定義了一種用於綁定功能的復雜算法。 除其他外，以下斷言必須成立：

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');

由於正確綁定的函數不是真正的Function對象，因此使用polyfill（特別是數字2/3和4/10）是不可能完成的，但您可以嘗試盡可能多地實現。

有問題的實現試圖通過掛鈎原型鏈來解決6號和7號問題，但這還不夠 。

這是一個更好的替代實現，但仍然不完美： http ： //jsfiddle.net/YR6MJ/

Answer 3

// 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;
  };
}

Answer 4

從早先的評論：

而不是使用fNop，為什么你不能只說fBound.prototype = this.prototype ？

據我所知，主要區別在於當綁定函數內的this值是調用bind的原始函數的實例時，則綁定到的值 - 最初傳遞給bind的第一個參數 - - 被忽略了。

例如，這段代碼：

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

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

...導致fn打印：

77 testing

換句話說，價值this里面fn是Test在它被調用的實例。 您的建議將提供綁定數組到apply inside bind ，因此，以這種方式編寫，相同代碼的最后一行將打印：

3 testing

我並不完全清楚為什么這很重要，但它確實強調了你的建議不會產生相同的結果。