// 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6 H% e" m5 e" \! C; b) O
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。 ! y# d+ z' J0 b; |' S
// 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
// 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8 o' k& s. z6 L- s/ S% ~
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。 J5 D2 m% y, @
// 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1 ^5 [5 m O6 V, @9 M' q0 {
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。5 A. X0 U& `' U% m
// 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( W% p# Z' k, I- m
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。