// 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- i& K( q+ ^. u; l
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。 ) Y5 d# C$ b# m+ v& o! k4 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 - u7 y" h$ x: V, O9 E var不存在块级作用域,let和const存在块级作用域。2 t) { ~' c) _3 ?; |
// 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 1 D0 q1 U3 t' R4 @7 }! k* D: e [7 F 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。; g4 @: z0 U& D% x/ a
// 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2 X7 v0 m$ k. f* T5 J
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。