Understanding Closure in Javascript

Closure (closure) is a difficulty and characteristic of the Javascript language. Many advanced applications rely on closures.

1. The scope of variables

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

The scope of variables is nothing more than two types: global variables and local variables.

The special thing about the Javascript language is that global variables can be read directly inside the function.

var n=999;
 
　　function f1(){
　　　　alert(n);
　　}
 
　　f1(); // 999

On the other hand, local variables within the function cannot be read outside the function.

function f1(){
　　　　var n=999;
}
 
alert(n); // error

There is one thing to note here. When declaring variables inside a function, you must use the var command. If you don't use it, you are actually declaring a global variable!

function f1(){
　　　　n=999;
　　}
 
　　f1();
 
　　alert(n); // 999

2. How to read local variables from the outside?

For various reasons, we sometimes need to get local variables within a function. However, as mentioned before, this is not possible under normal circumstances and can only be achieved through workarounds.

That is to define another function inside the function.

function f1(){
 
　　　　var n=999;
 
　　　　function f2(){
　　　　　　alert(n); // 999
　　　　}
 
　　}

In the above code, function f2 is included inside function f1. At this time, all local variables inside f1 are visible to f2. But the reverse doesn't work. The local variables inside f2 are invisible to f1. This is the "chain scope" structure unique to the Javascript language. The child object will search for the variables of all parent objects level by level. Therefore, all variables of the parent object are visible to the child object, but not vice versa.

Since f2 can read the local variables in f1, then as long as f2 is used as the return value, can't we read its internal variables outside f1?

function f1(){
 
　　　　var n=999;
 
　　　　function f2(){
　　　　　　alert(n); 
　　　　}
 
　　　　return f2;
 
　　}
 
　　var result=f1();
 
　　result(); // 999

3. The concept of closure

The f2 function in the code in the previous section is a closure.

The definition of "closure" in various professional literature is very abstract and difficult to understand. My understanding is that a closure is a function that can read the internal variables of other functions.

Since in the Javascript language, only sub-functions inside the function can read local variables, closures can be simply understood as "functions defined inside a function".

So, in essence, closure is a bridge connecting the inside of the function with the outside of the function.

4. The use of closures

Closures can be used in many places. It has two greatest uses. One is to read the variables inside the function as mentioned earlier, and the other is to keep the values ​​of these variables in memory.

How to understand this sentence? Please 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 code, result is actually the closure f2 function. It was run twice, the first time the value was 999, the second time the value was 1000. This proves that the local variable n in function f1 is always stored in memory and is not automatically cleared after f1 is called.

Why is this so? 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 existence of f2 depends on f1, so f1 is always in memory and will not be deleted after the call is completed. , recycled by the garbage collection mechanism (garbage collection).

Another thing worth noting in this code is the line "nAdd=function(){n =1}". First of all, the var keyword is not used before nAdd, so nAdd is a global variable. rather than local variables. Secondly, the value of nAdd is an anonymous function, and this anonymous function itself is also a closure, so nAdd is equivalent to a setter, which can operate on local variables inside the function outside the function.

5. Points to note when using closures

1) Since closures will cause the variables in the function to be stored in memory, the memory consumption is very large, so Closures cannot be abused, otherwise it will cause performance problems on the web page and may cause memory leaks in IE. The solution is to delete all unused local variables before exiting the function.

2) The closure will change the value of the variable inside the parent function outside the parent function. Therefore, if you use the parent function as an object, the closure as its public method, and the internal variables as its private value, you must be careful not to Feel free to change the value of the variable inside the parent function.

6. Questions to think about

If you can understand the results of the following two pieces of code, you should understand the operating mechanism of closures.

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 that = this;
　　　　　　return function(){
　　　　　　　　return that.name;
　　　　　　};
 
　　　　}
 
　　};
 
　　alert(object.getNameFunc()());

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