About this keyword in javascript (translation self-understanding)_javascript skills

About 70% of the content below comes from http://www.quirksmode.org/js/this.html, and the other 30% is my own understanding and feelings about it. Hope it can be of some help to those in need. . .

First, let’s look at a very typical question about this keyword:

Copy the code The code is as follows:

var name = 'hong'
var obj = {
name: 'ru',
getName: function(){
return function(){
return this.name;
};
}
}
alert(obj.getName()());

It’s not a big deal here, the execution result is ：hong
Change the code slightly:

Copy the code The code is as follows:

var name = 'hong'
var obj = {
name: 'ru',
getName: function(){
var that = this;
return function(){
return that .name;
};
}
}
alert(obj.getName()());

The execution result is: ru
The execution result is :ru

The reasons for this result will be discussed in detail below.
[Owner of function]
To explain this, we must first explain this concept. In JavaScript, this always points to the "owner" of the function we are currently executing. To be more precise: it points to the object that uses this function as a method.
How to understand this sentence, we can look at the following example:

Copy code The code is as follows:

/* -- test1 -- */
function test1 () {
this.title = 'me';
alert(window['title']);
alert (this === window); //true
}
test1();

The execution result is: me, true
In the above example, this It points to the window object. And write the title attribute of the window object as 'me'. Because test1 is a top-level function, its owner is the window object, or it is a method of the window object. This should not be difficult to understand. For example, when calling test1() above, it can also be written as window.test1(); which is clear.

Next, we create a div and assign test1 as a method to the onclick attribute of the div:

Copy code The code is as follows:

me!

The result of clicking the div is: undefined, false; At the same time, we can use firebug to see that the attribute value of 'me' is actually assigned to the one with the id of 'o' HtmlObject

Obviously, this points to the div at this time, that is to say, the owner of test1() becomes the HtmlObject of the div, or it becomes the onclick method of the div to call. This situation should still be easy to understand.
Next we will change the code, just change one place:

Copy the code The code is as follows:

o.onclick = test1(); // Note: A bracket is added here

After changing the last sentence of the above code to this, the result of clicking the div is: me, true
It becomes the same as the first situation, this points to the window. Some people may wonder and think there is no difference, but in fact there is. This involves the issue of copy and refer functions.

　【Copy of function】
　If you copy the code through

the code is as follows:

o.onclick = test1;

In this way, the function test1() is actually copied to the onclick attribute of object o. Therefore, the owner of test1 becomes the o object.
If you copy the code through

the code is as follows:

o.onclick = test1() ;

In this way, it essentially means that when the handle of the click event is obtained, it is directed to execute the test1() function. Note that it guides execution rather than assigning it to execution. The owner of test1() has not changed, it is still window.

[Refer of function]
Same as above, if we write the call inline to HTML and call it, it is still the refer method

Copy code The code is as follows:

me!

The execution result of clicking the div still indicates that this points to the window.
　[Example of function copy]

Copy code The code is as follows:

element.onclick = doSomething
element.addEventListener('click',doSomething,false)
element.onclick = function () {this.style.color = '#cc0000';}

These methods will change the pointer of this to the currently called object.
　[Example of function refer]

Copy code The code is as follows:

element.onclick = function () {doSomething()}
element.attachEvent('onclick',doSomething)

These methods It will not change the owner of the function. It should be noted that addEventListener and attachEvent are inconsistent, because attachEvent actually establishes a reference to doSomething instead of copying the function.

[Use the call method]
As we just said, writing still cannot make the owner of test() become , then we can do this

Copy code The code is as follows:

function (o) {
o.title = 'me';
}

Such an explicit call is OK. Alternatively, you can use call or apply to impersonate inheritance methods

Copy the code The code is as follows:

function test () {
this.title = 'me';
}

This is also the most typical way of object impersonation.

　[Free variable problem]
After writing this long, let’s go back to the original question:

Copy code The code is as follows:

var name = 'hong'
var obj = {
name: 'ru',
getName: function(){
return function(){
return this.name;
};
}
}
alert(obj.getName()());

Why does the result obtained in this way be: hong? The focus is on

Copy the code The code is as follows:

return function(){
return this .name;
};

Comparing the function refer example written above, it is not difficult to find that the calling method of the returned anonymous function is the same as onclick = function () {doSomething()} same. So this method does not change the owner of this function. Although it is a nested function, its declaration is top-level. Among them, this points to window.
The second way is to force this to be assigned to that in getName(), that is to say, that.name is actually the same as this.name in getName(). In the context of getName, its owner is the object obj, so this will point to obj, so this.name === obj.name;
I have gone through such a big circle, but I don’t know if I have made it clear to you. .

In fact, it can be summarized like this: In the function context where this is located, if this function is not called in the form of a "method", then this will point to the window object, otherwise it will point to the owner of this function.