// var
console.log(a) // undefined
var a = 10
// let
console.log(b) // Cannot access 'b' before initialization
let b = 10
// const
console.log(c) // Cannot access 'c' before initialization
const c = 10
暂时性死区 % ~, M8 f: J" Y' H( |' | var不存在暂时性死区,let和const存在暂时性死区,只有等到声明变量的那一行代码出现,才可以获取和使用该变量。5 l' x$ K8 K( ]7 G) u' u
// var
console.log(a) // undefined
var a = 10
// let
console.log(b) // Cannot access 'b' before initialization
let b = 10
// const
console.log(c) // Cannot access 'c' before initialization
const c = 10
块级作用域 0 S) ]- _% D" ~ var不存在块级作用域,let和const存在块级作用域。; C$ E) K. e r2 o- ]) ? ?2 x4 m
// var
{
var a = 20
}
console.log(a) // 20
// let
{
let b = 20
}
console.log(b) // Uncaught ReferenceError: b is not defined
// const
{
const c = 20
}
console.log(c) // Uncaught ReferenceError: c is not defined
// var
var a = 10
var a = 20 // 20
// let
let b = 10
let b = 20 // Identifier 'b' has already been declared
// const
const c = 10
const c = 20 // Identifier 'c' has already been declared
修改声明的变量 $ B' x) Y8 ?3 B. y3 R var和let可以,const声明一个只读的常量。一旦声明,常量的值就不能改变。. S1 T7 I b9 f4 |- @7 F3 G
// var
var a = 10
a = 20
console.log(a) // 20
//let
let b = 10
b = 20
console.log(b) // 20
// const
const c = 10
c = 20
console.log(c) // Uncaught TypeError: Assignment to constant variable
使用% C/ n7 l% C9 \% d
能用const的情况尽量使用const,其他情况下大多数使用let,避免使用var。