JavaScript的this,call(),apply(),bind()

为了建立一个scope chain, 每个JavaScript的代码执行上下文都提供了this关键字。In its most common usage,

this

serves as an identity function, providing our neighborhoods a way of
referring to themselves. We can’t always rely on that behavior,
however: Depending on how we get into a particular neighborhood,

this

might mean something else entirely. In fact, how we get into the neighborhood is itself exactly what

this

generally refers to. 需要注意特殊的四种情况:

Calling an Object’s Method
在典型的面向对象编程时，我们需要一种方式去指向和引用我们调用的对象.

this

serves the purpose admirably, providing our objects the ability to examine themselves, and point at their own properties.

var deep_thought = {
the_answer: 42,
ask_question: function () {
return this.the_answer;
}
};

var the_meaning = deep_thought.ask_question();

This example builds an object named

deep_thought

, sets its

the_answer

property to 42, and creates an

ask_question

method. When

deep_thought.ask_question()

is executed, JavaScript establishes an execution context for the function call, setting

this

to the object referenced by whatever came before the last ”.”, in this case:

deep_thought

. The method can then look in the mirror via

this

to examine its own properties, returning the value stored in

this.the_answer

: 42.

Constructor
Likewise, when defining a function to be used as a constructor with the

new

keyword,

this

can be used to refer to the object being created. Let’s rewrite the example above to reflect that scenario:

<mce:script type="text/javascript"><!--
function BigComputer(answer) {
this.the_answer = answer;
this.ask_question = function () {
return this.the_answer;
}
}

var deep_thought = new BigComputer(42);
var the_meaning = deep_thought.ask_question();

// --></mce:script>

Instead of explicitly creating the

deep_thought

object, we’ll write a function to create

BigComputer

objects, and instantiate

deep_thought

as an instance variable via the

new

keyword. When

new BigComputer()

is executed, a completely new object is created transparently in the background.

BigComputer

is called, and its

this

keyword is set to reference that new object. The function can set properties and methods on

this

, which is transparently returned at the end of

BigComputer

’s execution.

Notice, though, that

deep_thought.the_question()

still works just as it did before. What’s going on there? Why does

this

mean something different inside

the_question

than it does inside

BigComputer

? Put simply, we entered

BigComputer

via

new

, so

this

meant “the new object.” On the other hand, we entered

the_question

via

deep_thought

, so while we’re executing that method,

this

means “whatever

deep_thought

refers to”.

this

is not read from the scope chain as other variables are, but instead is reset on a context by context basis.

Function Call
What if we just call a normal, everyday function without any of this fancy object stuff? What does

this

mean in that scenario?

<mce:script type="text/javascript"><!--
function test_this() {
return this;
}
var i_wonder_what_this_is = test_this();

// --></mce:script>

In this case, we weren’t provided a context by

new

, nor were we given a context in the form of an object to piggyback off of. Here,

this

defaults to reference the most global thing it can: for web pages, this is the

window

object.

Event Handler
For a more complicated twist on the normal function call, let’s say that we’re using a function to handle an

onclick

event. What does

this

mean when the event triggers our function’s execution? Unfortunately, there’s not a simple answer to this question.

If we write the event handler inline,

this

refers to the global

window

object:

 <mce:script type="text/javascript"><!--
function click_handler() {
alert(this); // alerts the window object
}

// --></mce:script>

...
<button id='thebutton' onclick='click_handler()'>Click me!</button>


However, when we add an event handler via JavaScript, [code]this

refers to the DOM element that generated the event. (Note: The event
handling shown here is short and readable, but otherwise poor. Please
use a real addEvent function instead.):

<script type="text/javascript">
<mce:script type="text/javascript"><!--
function click_handler() {
alert(this); // alerts the button DOM node
}

function addhandler() {
document.getElementById('thebutton').onclick = click_handler;
}

window.onload = addhandler;

// --></mce:script>

...
<button id='thebutton'>Click me!</button>


Complications
Let’s run with that last example for a moment longer. What if instead of running [code]click_handler

, we wanted to ask

deep_thought

a question every time we clicked the button? The code for that seems pretty straightforward; we might try this:

<mce:script type="text/javascript"><!--
function BigComputer(answer) {
this.the_answer = answer;
this.ask_question = function () {
alert(this.the_answer);
}
}

function addhandler() {
var deep_thought = new BigComputer(42),
the_button = document.getElementById('thebutton');

the_button.onclick = deep_thought.ask_question;
}

window.onload = addhandler;
// --></mce:script>

对上面的代码，我们期望点击按钮,

deep_thought.ask_question被执行，我们得到返回结果

“42.” 但为什么得到的结果反而是

undefined？哪里错了

?

The problem is simply this: We’ve passed off a reference to the

ask_question

method, which, when executed as an event handler, runs in a different
context than when it’s executed as an object method. 简而言之，ask_question 中的this关键字是指向产生事件的DOM元素节点，而不是

BigComputer对象

. DOM元素节点并没有

the_answer属性，所以返回结果是

undefined而不是

“42.”

setTimeout

exhibits similar behavior, delaying the execution of a function while at the same time moving it out into a global context.

This issue crops up all over the place in our programs, and it’s a
terribly difficult problem to debug without keeping careful track of
what’s going on in all the corners of your program, especially if your
object has properties that do exist on DOM elements or the

window

object.

Manipulating Context With

.apply()

and

.call()

We really do want to be able to ask

deep_thought

a question when we click the button, and more generally, we do want to be able to call object methods in their native context when responding to things like events and

setTimeout

calls. Two little-known JavaScript methods,

apply

and

call

, indirectly enable this functionality by allowing us to manually override the default value of

this

when we execute a function call. Let’s look at

call

first:

<mce:script type="text/javascript"><!--
var first_object = {
num: 42
};
var second_object = {
num: 24
};

function multiply(mult) {
return this.num * mult;
}

multiply.call(first_object, 5); // returns 42 * 5
multiply.call(second_object, 5); // returns 24 * 5
// --></mce:script>

In this example, we first define two objects,

first_object

and

second_object

, each with a

num

property. Then we define a

multiply

function that accepts a single argument, and returns the product of that argument, and the

num

property of its

this

object. If we called that function by itself, the answer returned would almost certainly be

undefined

, since the global

window

object doesn’t have a

num

property unless we explicitly set one. We need some way of telling

multiply

what its

this

keyword ought refer to; the

call

method of the

multiply

function is exactly what we’re looking for.

call方法的第一个参数定义了this关键字在被调用方法的执行上下文中指向和对象，call方法的剩余参数则是被调用方法的参数。因此当

multiply.call(first_object, 5)被执行，

multiply函数被调用

,

5

为传入方法的第一个参数，

this

执行

first_object对象。

Likewise, when

multiply.call(second_object, 5)

is executed, the

multiply

function is called,

5

is passed in as the first argument, and the

this

keyword is set to refer to object

second_object

.

apply方法和

call方法基本一致

,但是允许你以数组的形式向被调用的函数传递参数,
which can be quite useful when programatically generating function
calls. Replicating the functionality we just talked about using

apply

is trivial:

<mce:script type="text/javascript"><!--
...

multiply.apply(first_object, [5]); // returns 42 * 5
multiply.apply(second_object, [5]); // returns 24 * 5
// --></mce:script>

apply

and

call

are very useful on their
own, and well worth keeping around in your toolkit, but they only get
us halfway to solving the problem of context shifts for event handlers.
It’s easy to think that we could solve the problem by simply using

call

to shift the meaning of

this

when we set up the handler:

function addhandler() {
var deep_thought = new BigComputer(42),
the_button = document.getElementById('thebutton');

the_button.onclick = deep_thought.ask_question.call(deep_thought);
}

上面的代码仍然存在问题:

call是立即执行函数的，因此我们提供的

onclick

handler是函数的执行结果而不是函数本身.我们需要JavaScript的另一个特性来解决这个问题：bind方法。

The Beauty of

.bind()

I’m not a huge fan of the Prototype JavaScript framework, but I am very much impressed with the quality of its code as a whole. In particular, one simple addition it makes to the

Function

object has had a hugely positive impact on my ability to manage the context in which function calls execute:

bind

performs the same general task as

call

, altering the context in which a function executes. The difference is that

bind

returns a function reference that can be used later, rather than the result of an immediate execution that we get with

call

.

If we simplify the

bind

function a bit to get at the
key concepts, we can insert it into the multiplication example we
discussed earlier to really dig into how it works; it’s quite an
elegant solution:

<mce:script type="text/javascript"><!--
var first_object = {
num: 42
};
var second_object = {
num: 24
};

function multiply(mult) {
return this.num * mult;
}

Function.prototype.bind = function(obj) {
var method = this,
temp = function() {
return method.apply(obj, arguments);
};

return temp;
}

var first_multiply = multiply.bind(first_object);
first_multiply(5); // returns 42 * 5

var second_multiply = multiply.bind(second_object);
second_multiply(5); // returns 24 * 5
// --></mce:script>

First, we define

first_object

,

second_object

, and the

multiply

function, just as before. With those taken care of, we move on to creating a

bind

method on the

Function

object’s

prototype

, which has the effect of making

bind

available for all functions in our program. When

multiply.bind(first_object)

is called, JavaScript creates an execution context for the

bind

method, setting

this

to the

multiply

function, and setting the first argument,

obj

, to reference

first_object

. So far, so good.

The real genius of this solution is the creation of

method

, set equal to

this

(the

multiply

function itself). When the anonymous function is created on the next line,

method

is accessible via its scope chain, as is

obj

(

this

couldn’t be used here, because when the newly created function is executed,

this

will be overwritten by a new, local context). This alias to

this

makes it possible to use

apply

to execute the

multiply

function, passing in

obj

to ensure that the context is set correctly. In computer-science-speak,

temp

is a closure that, when returned at the end of the

bind

call, can be used in any context whatsoever to execute

multiply

in the context of

first_object

.

This is exactly what we need for the event handler and

setTimeout

scenarios discussed above. The following code solves that problem completely, binding the

deep_thought.ask_question

method to the

deep_thought

context, so that it executes correctly whenever the event is triggered:

function addhandler() {
var deep_thought = new BigComputer(42),
the_button = document.getElementById('thebutton');

the_button.onclick = deep_thought.ask_question.bind(deep_thought);
}

Beautiful.