// 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 7 e1 Y$ U, B2 s, g+ t0 R 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。, x. D, c3 r" B( c7 ]
// 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 7 C& i- `% K5 M) S N var不存在块级作用域,let和const存在块级作用域。 6 p# h( G" G& q. n* y) J
// 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
重复声明9 R% `: p) j3 I2 d/ T; E) h3 g
var允许重复声明变量,let和const在同一作用域不允许重复声明变量。6 i, _ R [# `/ r8 A' p0 p
// 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+ v) Z$ h! ?$ r0 ~1 y3 g
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。8 f7 S9 k# M4 O5 L
// 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( o1 e' Y2 T0 |% d/ X$ n
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。