JavaScript closed-Pack (Closure) Knowledge learning

Closures (closure) are a difficult and unique feature of the JavaScript language, and many advanced applications rely on closure implementations.

Here are my learning notes, which should be useful for JavaScript beginners.

Scope of a variable

To understand closures, you must first understand JavaScript's special variable scope.

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

The special point of the JavaScript language is that the global variable can be read directly from within the function.

var n=999;
    
Function F1 () {
alert (n);
}
    
F1 (); 999

On the other hand, local variables within a function cannot naturally be read 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, you must use the var command. If not, you actually declare a global variable!

Function F1 () {
n=999;
}
    
F1 ();
    
alert (n); 999

How to read local variables from the outside?

For a variety of reasons, we sometimes need to get local variables within a function. However, as mentioned earlier, this is not possible under normal circumstances and can only be achieved by workaround.

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

Function F1 () {
    
var n=999;
    
function F2 () {
alert (n);//999
}
    
}

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In the above code, the function F2 is included within the function F1, when all local variables within F1 are visible to F2. But the reverse is not, F2 internal variables, the F1 is not visible. This is the "chain scoped" structure (chain scope) peculiar to the JavaScript language, where child objects look up the variables of all the parent objects at one level. Therefore, all the variables of the parent object are visible to the child, and the opposite is not true.

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

Function F1 () {
    
var n=999;
    
function F2 () {
alert (n);
return
    
F2;
    
}
    
var result=f1 ();
    
Result (); 999

Third, 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 internal variables of other functions.

Because in a JavaScript language, only a child function within a function can read a local variable, the closure can be simply understood as "a function defined within a function."

So, in essence, closures are a bridge that connects functions inside and outside functions.

Iv. use of closures

Closures can be used in many places. Its maximum use is two, one is the previous mentioned can read the function inside the variable, the other is to keep the values of these variables always in memory.

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

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 piece of code, result is actually the closure F2 function. It runs two times, the first value is 999, and the second value is 1000. This proves that the local variable n in the function F1 is kept in memory and is not automatically purged 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 the presence of F2 depends on F1, so F1 is always in memory and will not be reclaimed by the garbage collection mechanism (garbage collection) after the call has ended.

Another notable part of this code is the "Nadd=function () {n+=1}" line, which first nadd not use the var keyword, 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 the nadd is equivalent to a setter that can manipulate local variables within the function outside of the function.

Five, use the closure of the attention point

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

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

VI. 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 fragment one.

var name = "The window";
    
var object = {
name: ' My object ',
    
getnamefunc:function () {return
function () {return this
.　　　　Name;
};
    
}
    
};
    
Alert (Object.getnamefunc () ());

Code fragment two.

var name = "The window";
    
var object = {
name: ' My object ',
    
getnamefunc:function () {
var = this;
return function () {return
that.name;
};
    
}
    
};
    
Alert (Object.getnamefunc () ());

Original website: http://www.ruanyifeng.com/blog/2009/08/learning_javascript_closures.html