爲了建立一個scope chain, 每個JavaScript的代碼執行上下文都提供了this關鍵字。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. 需要注意特殊的四種情況:
- Calling an Object’s Method
在典型的面向對象編程時,我們需要一種方式去指向和引用我們調用的對象.
this
serves the purpose admirably, providing our objects the ability to examine themselves, and point at their own properties.This example builds an object named
deep_thought
, sets itsthe_answer
property to 42, and creates anask_question
method. Whendeep_thought.ask_question()
is executed, JavaScript establishes an execution context for the function call, settingthis
to the object referenced by whatever came before the last ”.”, in this case:deep_thought
. The method can then look in the mirror viathis
to examine its own properties, returning the value stored inthis.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:Instead of explicitly creating the
deep_thought
object, we’ll write a function to createBigComputer
objects, and instantiatedeep_thought
as an instance variable via thenew
keyword. Whennew BigComputer()
is executed, a completely new object is created transparently in the background.BigComputer
is called, and itsthis
keyword is set to reference that new object. The function can set properties and methods onthis
, which is transparently returned at the end ofBigComputer
’s execution.Notice, though, that
deep_thought.the_question()
still works just as it did before. What’s going on there? Why doesthis
mean something different insidethe_question
than it does insideBigComputer
? Put simply, we enteredBigComputer
vianew
, sothis
meant “the new object.” On the other hand, we enteredthe_question
viadeep_thought
, so while we’re executing that method,this
means “whateverdeep_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?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 thewindow
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 doesthis
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 globalwindow
object:
...
However, when we add an event handler via JavaScript,
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">
...
Complications
Let’s run with that last example for a moment longer. What if instead of running 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:
對上面的代碼,我們期望點擊按鈕, deep_thought.ask_question被執行,我們得到返回結果
“42.” 但爲什麼得到的結果反而是undefined?哪裏錯了
?
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. 簡而言之,ask_question 中的this關鍵字是指向產生事件的DOM元素節點,而不是BigComputer對象
. DOM元素節點並沒有the_answer屬性,所以返回結果是
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:
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方法的第一個參數定義了this關鍵字在被調用方法的執行上下文中指向和對象,call方法的剩餘參數則是被調用方法的參數。因此當
multiply.call(first_object, 5)被執行,
multiply函數被調用
, 5
爲傳入方法的第一個參數, this
執行 first_object對象。
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
,但是允許你以數組的形式向被調用的函數傳遞參數,
which can be quite useful when programatically generating function
calls. Replicating the functionality we just talked about using apply
is trivial:
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:
上面的代碼仍然存在問題: call是立即執行函數的,因此我們提供的
onclick
handler是函數的執行結果而不是函數本身.我們需要JavaScript的另一個特性來解決這個問題: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:
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:
Beautiful.