// 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) V/ z* i, m# i8 {% g
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。. c' v9 L" K D3 b7 c( Q0 M2 k
// 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+ [# g0 O5 A4 v& b3 C) I
var不存在块级作用域,let和const存在块级作用域。 6 x W4 h# I/ q6 F
// 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 ! f7 W8 O' n4 A% c. B; \ 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。 : R* \( k& l! R! o, F/ }. u' ^- @
// 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 6 n' A# R4 v# G3 w5 }2 k& w; J: ` 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。 $ }/ l/ X/ H5 C7 Q, 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