Learn JavaScript closures

Source: Internet
Author: User
Tags variable scope

The scope of a variable

To understand closures, you must first understand the special variable scope of JavaScript.

The scope of a variable is nothing more than two kinds: global variables and local variables.

The special point of the JavaScript language is that the global variables can be read directly inside the function.

var n=999;

Function F1 () {
alert (n);
}

F1 (); 999

On the other hand, a local variable inside a function cannot be read naturally outside the function.

Function F1 () {
var n=999;
}

alert (n); Error

Here's a place to be aware that when declaring variables inside a function, be sure to use the var command. If not, you're actually declaring a global variable!

Function F1 () {
n=999;
}

F1 ();

alert (n); 999

Second, how to read the local variables from the outside?

For a variety of reasons, we sometimes need to get local variables within the function. However, as already mentioned, under normal circumstances, this can not be done, only through the workaround to achieve.

That is, in the inside of the function, define a function.

Function F1 () {

var n=999;

function F2 () {
alert (n); 999
}

}

In the above code, the function F2 is included inside the function F1, and all local variables inside the F1 are visible to the F2. But the opposite is not possible, F2 internal variables, the F1 is not visible. This is the JavaScript-specific "chain-scoped" structure (chain scope), where child objects look up the variables of all the parent objects one level at a level. Therefore, all the variables of the parent object are visible to the child object, and vice versa.

Since F2 can read the local variables in the F1, we can not read its internal variables outside the F1 as long as the F2 is the return value!

Function F1 () {

var n=999;

function F2 () {
alert (n);
}

return F2;

}

var result=f1 ();

Result (); 999

Three, the concept of closure

The F2 function in the previous section of the code is the closure.

The definition of "closure" (closure) in various professional literature is very abstract and difficult to read. My understanding is that closures are functions that can read other functions ' internal variables.

Because in the JavaScript language, only sub-functions inside the function can read local variables, it is possible to simply interpret the closure as "a function defined inside a function".

So, in essence, a closure is a bridge that connects the inside of the function to the outside of the function.

Iv. use of closures

Closures can be used in many places. Its maximum usefulness is two, one of the previously mentioned variables that can read the inside of a function, and the other is to keep the values of these variables in memory at all times.

How to understand this sentence? Take a look at the following code.

Function F1 () {

var n=999;

Nadd=function () {n+=1}

function F2 () {
alert (n);
}

return F2;

}

var result=f1 ();

Result (); 999

Nadd ();

Result (); 1000

In this code, result is actually the closure F2 function. It runs altogether two times, the first value is 999, the second value is 1000. This proves that the local variable n in the function F1 is kept in memory and is not automatically cleared after the F1 call.

Why is that? The reason is that F1 is the parent function of F2, and F2 is assigned to a global variable, which causes F2 to always be in memory, and F2 's presence depends on F1, so F1 is always in memory and will not be reclaimed by the garbage collection mechanism (garbage collection) after the call ends.

Another notable part of this code is the line "nadd=function () {n+=1}", which first did not use the var keyword in front of Nadd, so Nadd is a global variable, not a local variable. Second, the value of Nadd is an anonymous function (anonymous functions), and the anonymous function itself is a closure, so nadd is equivalent to a setter that can manipulate local variables inside the function outside of the function.

V. Note points for using closures

1) Because the closure will make the variables in the function are stored in memory, memory consumption is very large, so can not abuse closures, otherwise it will cause the performance of the Web page, in IE may cause memory leaks. The workaround is to remove all unused local variables before exiting the function.

2) The closure changes the value of the inner variable of the parent function outside the parent function. So, if you use the parent function as an object, and the closure as its public method, and the internal variable as its private property (private value), be careful not to arbitrarily change the value of the inner variable of the parent function.

Liu, study questions

If you can understand the results of the following two sections of code, you should understand the operation mechanism of the closure.

Code snippet one.

var name = "the window";

var object = {
Name: "My Object",

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

}

};

Alert (Object.getnamefunc () ());


Code snippet two.

var name = "the window";

var object = {
Name: "My Object",

Getnamefunc:function () {
var = this;
return function () {
return that.name;
};

}

};

Alert (Object.getnamefunc () ());

Learn JavaScript closures

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