"JavaScript" variable promotion and var for variable promotion

JavaScript's declared variable promotion often affects our normal access to variables, so close this article so that we can flip through it later.

What is variable elevation

//Variable declaration promotionfunction Test(){    varA= "1";    varF= function(){};    varB= "2";    varC= "3";}//The above code is equivalent tofunction Test(){    varA,F,B,C;A= "1";F= function(){};B= "2";C= "3";}

There are two scenarios for defining variables in JS: (Note that you cannot define a variable without var outside of the method, and that the occurrence of XX is not defined)

1. Plus Var, which is a local variable within the method, and a global variable outside the method.
   
2. Within a method, add var to a local variable, and no var is a global variable (after executing the current method)

Variable Promotion Case Study 1

Since the variable A is defined inside the Test1 function,

var='I\'m a in all'functiontest1{    console.log(a)    console.log(window.a)    var='I\'m a in test1'    console.log(a)}    test1()

The above code is equivalent to

var='I\'m a in all'functiontest1{    var a    console.log// undefined    console.log(window.a// I'm a in all（因为window指的是全局环境）    ='I\'m a in test1'    console.log// I'm a in test1}    test1()

Case 2

var='I\'m a in all'functiontest2{    console.log// I'm a in all    ='I\'m a in test2'// 这里本来就是赋值，所以上边的a会输出全局变量    console.log// I'm a in test2}test2()

Case 3

functiontest3{    console.log// 报错（Uncaught ReferenceError: a is not defined）    ='I\'m a in test3'// 这里本来就是赋值，所以上边的a会输出全局变量    console.log// I'm a in test3}test3()console.log(2// I'm a in test3（本来没有全局变量a，当test3运行时，定义了一个全局变量a，所以这里会输出）

"JavaScript" variable promotion and var for variable promotion