Hoisting

关於Hoisting很像是变数被拉升到上层

• 提升变数不会赋值
• var a = 2 LHS
• console.log(a) RHS

尚未宣告变数,即RHS该数值便会出现参考错误

console.log(a) //ReferenceError: a is not defined

这边Hoisting

console.log(a) //undefined
var a = 1;
var a;

Var

在学习了Let之後,尽量少用Var。原因是Var会被提升到全域变数并且被污染,并且可以重复被宣告改赋值,造成程序错误

a = 1;
var a;

console.log(a) //1

compiler 会把程序看成两个叙述句

var a
a = 2

Let

不能重复宣告

let u = 0
let u = 1
console.log(u) //SyntaxError: Identifier 'u' has already been declared

var a = 10

function test(){
    console.log(a)
    let a //ReferenceError: Cannot access 'a' before initialization
}
test()

Const

Const可以看作Let的加强版,并且Const不能改变赋值

var a = 10

function test(){
    console.log(a)
    const a //ReferenceError: Cannot access 'a' before initialization
}
test()