JS scope, scope chain, global variables and local variables

Catalog

One 、Javascript Scope

1、 Global scope

2、 Local scope

Two 、 Scope chain

3、 ... and 、 Depending on the scope , Variables can be divided into two types

1、 Global variables

2. local variable

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One 、Javascript Scope

Is that the code name works and works in a certain range . In order to improve the reliability of the program, it is more important to reduce the naming conflict .

es6 Before ： Global scope and local scope .

1、 Global scope

Whole script label , Or a separate js file .

2、 Local scope

Inside the function is the local scope , The name of this code only works inside the function .

Two 、 Scope chain

Just the code , There is at least one scope , The local scope written inside the function , If there are functions in the function , Then a scope can be created in this scope .

According to this mechanism that external function variables can be accessed in internal functions , Use chain search to determine which data can be accessed by internal functions , It's called scope .  The principle is the principle of proximity . 

The following code output 12 , Also note that functions are executed only when called . Only execute to statement f2() when , To execute functions f2() The statement in .

var num = 23;

        function f1() {
            var num = 12;

            function f2() {
                console.log(num);
            }
            f2();

        }
        f1();

3、 ... and 、 Depending on the scope , Variables can be divided into two types

1、 Global variables

Variables that are globally scoped , Variables defined outside functions , You can use... Anywhere in your code .num It's a global variable ,b It's also , Because if inside a function , Variables that do not declare direct assignment are also global variables .

<script>
        
        var num = 20;
        console.log(num);
 function fn() {
            var a = 20;
            b = 30;
            console.log(a);
        }
</script>

2. local variable

Variables inside functions , Can only be used inside a function .a It's a local variable , Formal parameters are also local variables .

 // Local variables are destroyed after the program is executed , Relatively economical

 // Global variables are destroyed only when the browser is closed , More resources