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;
};
}
具体来说，我没有达到fNOP的目的，我不明白为什么需要设置fBound的原型。我也挂在fToBind.apply部分（我无法弄清楚在这种情况下代表什么）。
有人可以解释这里发生了什么吗？
答案 0 :(得分：6)
嗯，需要设置fBound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似乎进入的地方 - 它允许您使用fBound设置new fNop()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允许你“晃动”函数的参数，你必须结合函数绑定时传入的参数和调用它的参数。
答案 1 :(得分：1)
确保
instanceof检查）new g()继承自f的原型链。 （因此.prototype = new fNop部分）示例：
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)
当您调用new g()时，fBound函数将被调用为具有全新对象对象（this）的构造函数，该对象是fNop的实例。
ECMAScript5标准定义了一种用于绑定功能的复杂算法。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，使用polyfill（特别是数字2/3和4/10）是不可能的，但是你可以尝试实现尽可能多的尽可能。
有问题的实现试图通过挂钩到原型链来解决6号和7号，但是that's not enough。
这是另一种可以更好地运行的实现，但仍然不完美： http://jsfiddle.net/YR6MJ/
答案 2 :(得分：0)
来自之前的评论：
而不是使用fNop，为什么不能只说
fBound.prototype = this.prototype？
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，则绑定的值to - 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()
...导致fn打印：
77 testing
换句话说，this内fn的值是调用它的Test实例。您的建议会将绑定数组提供给apply内的bind，因此，以这种方式编写，将打印相同代码的最后一行：
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。
答案 3 :(得分：0)
// 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;
};
}