// 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; Q# L5 e! G- R/ E
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。( a" b, M' k: o* l8 Z4 v! P# {$ v
// 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" P2 o5 \6 c$ T
var不存在块级作用域,let和const存在块级作用域。; ]; ?/ ~3 c% h2 l1 {3 f; d
// 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; O6 y: x% U& A q
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。6 g( `7 [2 S2 e3 O' t
// 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; g6 e1 c! B) ]
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。