The importance of functions in JS

This time I will bring you the importance of functions in JS, what are the precautions when using functions in JS, the following are practical cases, one Get up and take a look.

Since it is an object, then it can be:

1. Created through literals

2. Assign values ​​to properties of variables, array elements and other objects (property)

3. Passed to the function as a parameter

4. As the return value of the function

5. Contains attributes that can be dynamically created and assigned

In addition to the functions of the above objects, functions differ from other objects in that they can be called.

In JS, there is variable promotion. When a variable is declared with var, it is immediately promoted to the top of the current scope (variables declared with let and const are not). For example:

1 function a() {2     console.log(b);3     var b = 10;4 }5 a();  //undefined

The running result of the above code is undefined. The reason is that when the b variable is declared through var, a variable promotion occurs and is immediately promoted to the top of the current scope (note that the promotion occurs immediately after the declaration, when b has not yet been defined!) , the "current scope" at this time is the scope of function a. Therefore, the above code is actually like this:

1 function a() {2     var b;  //变量的声明被提升至当前作用域顶部3     console.log(b);4     b = 10;5 }6 a();

The declaration of variable b is promoted to the second line (only the declaration is promoted), and b is not assigned a value at this time, so the code When execution continues to the third line of console.log(b), undefined is output.

In addition to variable promotion, JS also has "function promotion". In the same way, functions will also be promoted, but the function is not just a declared promotion, but an "overall promotion". Please look at the following code:

1 function a() {2     b();3     function b() {4         console.log("yes!");5     }6 }7 a();  //yes!

At this time, function b can be executed normally. This is precisely because the function is promoted and is "overall promoted", so b() can execute normally. This code is actually equivalent to:

1 function a() {2     function b() {3         console.log("yes!");4     }5     b();6 }7 a();  //yes!

It should be noted that only functions declared in ordinary form can be promoted! For example:

1 function a() {2     b();3     var b = function() {        //字面量声明的函数4         console.log("yes!");5     }6 }7 a();  //报错

The anonymous function here is declared through literals, so there is no function promotion, and an error will be reported.

But, when variables and functions are declared at the same time, which one takes precedence?

In JS, one of the reasons why a function is called a "first-class citizen" is that it has priority than a variable. When promotion occurs , the function will be promoted to the top of the scope! For example:

1 function a() {2     var b = 10;3     function b () {4         console.log("yes！");5     }6     console.log(b);7     console.log(typeof b);8 }9 a();    //10  number

In this code, the declaration of variable b is promoted, but the function as a "first-class citizen" is promoted above the declaration of variable b. During the code execution stage, 10 is assigned to b, so the output result is that b is 10 and the type becomes number.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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