变量提升与匿名函数

变量提升（预解析）

优点

1.提高性能，提前为变量分配空间

2.提升容错率

缺点

1.不规范的定义会使得某些变量的undefined

2.结构混乱，维护麻烦

 a存在变量提升，所以a等于undefined

var a = 1;
let b = 4;
(function() {
    console.log(a);//undefined
    var a = 3;
    let b = 4;
    console.log(a, b);//3,4
})()

var a = 1;
let b = 4;

function try1() {
    var a = 3;
    console.log(a);//undefined
    let b = 4;
    console.log(a, b);//3,4
};
try1()

 作用域内未定义a

var a = 1;
let b = 4;
(function() {
    console.log(a); //1
    let b = 4;
    console.log(a, b); //3,4
})()

var a = 1;
let b = 4;

function try1() {
    console.log(a); //1
    let b = 4;
    console.log(a, b); //3,4
};
try1()

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