1219基础总结__js变量带、不带VAR的区别

//在全局设置变量a，相当于给window全局对象设置了属性a
console.log(a) //undefined
var a = 12
console.log(window.a) //12 — 全局变量和window中的属性，存在映射机制~~~~

console.log(b) //报错  b is not defined
b = 13 //不带var——本质是window增加属性b   window.b=13
console.log(b)

var a = 12; //创建全局变量12
b = 13;     //本质是window.b = 13;  不存在变量提升

var m = 1,
    n = 2; //这样写 n带var的
 
var m = n = 3;//这样写  n不带var