Talk about javascript closure_javascript skills

The following are my study notes, which should be very useful for Javascript beginners.

1. Scope of variables
To understand closures, you must first understand JavaScript's special variable scope.
There are two types of 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.

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

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

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

There is something 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 local variables in f1, 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 subfunctions within a function can read local variables, closures can be simply understood as "functions defined within a function."
So, in essence, closure is a bridge connecting the inside of the function with the outside of the function.

4. The purpose of closure
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 happening? 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, not a local variable. 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, which consumes a lot of memory, 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
If you can understand the results of the following two pieces of code, you should understand the operation mechanism of closures.
Code snippet 1:

　var name = “The Window”; 
　　var object = { 
　　　　name : “My Object”, 
　　　　getNameFunc : function(){ 
　　　　　　return function(){ 
　　　　　　　　return this.name; 
　　　　　　}; 
　　　　} 
　　}; 
　　alert(object.getNameFunc()());

Code snippet 2:

　var name = “The Window”; 
　　var object = { 
　　　　name : “My Object”, 
　　　　getNameFunc : function(){ 
　　　　　　var that = this; 
　　　　　　return function(){ 
　　　　　　　　return that.name; 
　　　　　　}; 
　　　　} 
　　}; 
　　alert(object.getNameFunc()()); 

One more thing is how does this work in javascript?

var fullname = ‘John Doe'; 
var obj = { 
fullname: ‘Colin Ihrig', 
prop: { 
fullname: ‘Aurelio De Rosa', 
getFullname: function() { 
return this.fullname; 
} 
} 
};

console.log(obj.prop.getFullname());

var test = obj.prop.getFullname;

console.log(test());

Answer
The answer is Aurelio De Rosa and John Doe. The reason is that in a function, the behavior of this depends on how the JavaScript function is called and defined, not just how it is defined.
In the first console.log() call, getFullname() is called as a function of the obj.prop object. Therefore, the context refers to the latter, and the function returns the fullname of the object. In contrast, when getFullname() is assigned to the test variable, the context refers to the global object (window). This is because test is implicitly set as a property of the global object. For this reason, the function returns the fullname of the window, which is the value defined on the first line.

The above is the entire content of this article. I hope it will be helpful for everyone to understand JavaScript closures again.