How to understand closures in js

Closure (closure) is a major difficulty and feature of JavaScript. Many advanced applications rely on closures. This article mainly shares with you how to understand closures in js. Friends who need it can take a look at

1. Variable Scope

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

There are only two scopes of variables: global variables and local variables.

The special thing about the JavaScript language is that global variables can be read directly inside the function, but local variables inside the function cannot be read outside the function.

Note: When declaring variables inside a function, be sure to use the var command. If you don't use it, you are actually declaring a global variable!

2. How to read local variables inside a function from the outside?

For various reasons, we sometimes need to obtain local variables inside a function. However, as mentioned above, under normal circumstances, this is impossible! This is only possible through workarounds.

That is to define a 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 unique "chain scope" structure of 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?

3. The concept of closure

The f2 function in the above code is a closure.

The closure definitions in various professional literature are very abstract. My understanding is: a closure is a function that can read the internal variables of other functions.

Since in JavaScript, only subfunctions inside the function can read local variables, closure can be simply understood as "a function defined inside a function."

So, in essence, closure is a bridge connecting the inside of the function and 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 and not be 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 cause all variables in the function to be stored in memory, memory consumption is very large. Therefore, 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.

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