// 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. ]1 ]" \2 P2 }
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。' B4 H: n) L* A& X' h v" L" v$ E
// 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 # E/ O% K- Y" Q/ X# m7 Q4 l var不存在块级作用域,let和const存在块级作用域。 : D; {8 ]5 o+ g. f6 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. |2 \8 Z+ ~$ Z$ ~8 b5 W
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。3 n4 {9 C4 h1 a. E" 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 % I! N6 f/ |' ~& E% l4 b6 y( L 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。1 w8 |6 E8 c5 ]$ w2 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