JS scope

1.1 Scope Overview

 Generally speaking , The names used in a piece of program code are not always valid and available , The scope of the code that defines the usability of the name is the scope of the name . The use of scope improves the locality of program logic , Enhance the reliability of the program , Reduced name conflicts .
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JavaScript（es6 front ） There are two kinds of scopes in ：

• Global scope

• Local scope （ Function scope ）

1.2 Global scope

 The environment that acts on all code execution ( Whole  script  The label inside ) Or an independent  js  file .

1.3 Local scope

 Act on the code environment within the function , It's a local scope .  Because it has something to do with functions , So it is also called function scope .

1.4 JS There is no block-level scope

• Block scope is defined by { } Include .

• In other programming languages （ Such as java、c# etc. ）, stay if sentence 、 A variable created in a loop statement , Only in Ben if sentence 、 Used in this loop statement , Like the one below Java Code ：

  java There are block-level scopes ：

  if(true){
    int num = 123;
    system.out.print(num);  // 123
  }
  system.out.print(num);    //  Report errors 

  above java The code will report an error , Because in the code { } That is, a scope , Where the declared variable num, stay “{ }” Cannot be used outside ;

  And something like that JavaScript Code , You can't report a mistake ：

Js There is no block-level scope in （ stay ES6 Before ）

  if(true){
    var num = 123;
    console.log(123); //123
  }
  console.log(123);   //123

2 - Scope of variable

 stay JavaScript in , Depending on the scope , Variables can be divided into two types ：

• Global variables

• local variable

2.1 Global variables

 Variables declared under the global scope are called global variables （ Variables defined outside functions ）.

• Global variables can be used anywhere in the code

• In global scope var Declared variables Global variable

• Under special circumstances , Don't use... In functions var Declared variables are also global variables （ Not recommended ）

2.2 local variable

 Variables declared under local scope are called local variables （ A variable defined inside a function ）

• Local variables can only be used inside the function

• Inside the function var The declared variable is a local variable

• The formal parameters of a function are actually local variables

2.3 The difference between global and local variables

• Global variables ： It can be used anywhere , It will only be destroyed when the browser is closed , So it takes up more memory

• local variable ： Use only inside functions , When its code block is executed , Will be initialized ; After the contemporary code block runs , Will be destroyed , Therefore, it saves more memory space

3 - Scope chain

 As long as the code is in a scope , The local scope written inside the function , It is not written inside any function, that is, in the global scope ; If there are functions in the function , Then a scope can be created in this scope ; Based on **[ Internal functions can access external function variables ]** This mechanism of , Use chain search to determine which data can be accessed by internal functions , It's called the scope chain 
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 case analysis 1：

function f1() {
    var num = 123;
    function f2() {
        console.log( num );
    }
    f2();
}
var num = 456;
f1();

Scope chain ： Take the principle of proximity to find the final value of the variable .

var a = 1;
function fn1() {
    var a = 2;
    var b = '22';
    fn2();
    function fn2() {
        var a = 3;
        fn3();
        function fn3() {
            var a = 4;
            console.log(a); //a Value ?
            console.log(b); //b Value ?
        }
    }
}
fn1();