Variables, scopes, and variable promotion

Variable 、 Scope and memory issues

Global variables ： Global variable definitions can be provided to all code blocks and function calls

 if(true){

        var c=33

    }console.log(c)//33  Access to

Private variables ： stay js Next , Only the variables defined in the function are private

 var a=11

    function fun(){

        var b=12

        console.log(a)//11

        console.log(b)//12

    }

    fun()

    console.log(a)//11

    console.log(b)// Report errors , Cannot call ,b It's a private variable 

Basic data type ：null undefined number string boolean

Reference data type ：object

 

Variable promotion and scope

Pre explanation ( Variable Promotion )

In the current scope ,js Before code execution , The browser will first bring var and function Contents of the declaration , Make a declaration or definition in advance

Difference between declaration and definition

var a // Statement : Tell the browser to store a variable in memory

var b=11// Definition : Not only defined variables , Assignment is also made

The difference between the function of variable promotion and variable

var Only the declaration is completed in memory

function The memory is defined and declared , Only when the function is not called, it stores a string

for example ：

Console.log(a)// Output undefined

Var a=11

fun()// Execute function

function fun(){

Console.log(“ This is a function ”)

}

Precautions for variable Promotion

1、 Does not define var The difference between

   console.log(a)//undefined

    console.log(b)// Report errors , There is no process of variable Promotion

    var a=11

    b=22

2、 Whether or not the conditions are valid , We need to upgrade the variables

console.log(a)// Output undefined Variables exist

    if(false){

        var a=12

    }

1. When executing an anonymous function , There is no pre explanation , The execution of the code is done together with the definition

(function(){

         var a=b=3 //var b=3 a=b

        })()

        console.log(b)//3  Make a statement first b  b The variable goes up 

        console.log(a)// Report errors  a No domain interpretation  a No variable Promotion 

1. When... In a function return Statement execution time , Although the following statement does not execute , But it also requires variable elevation

Be careful ： Function scope , There are private variables. Find private variables , No formal parameters found , No formal parameters , Find the whole situation , If the formal parameters and private variables are promoted , You won't find global variables , Output undefined

function fun(){

                console.log(a)

                return false

                var a=66

            }

            fun()

1. stay js in , The variable and function names are duplicated , There will also be conflict , When the variable is raised , If the name is declared , Will not declare again , But it can be reassigned

  function num(){

         console.log(111)

       }

       num()

var num=66

       console.log(num)// Output ：66;var Will not restate , But you can change the value 

       num()// Report errors