// 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
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) S D9 Q8 ?4 C6 U* ~* ~# H
var不存在块级作用域,let和const存在块级作用域。; o, E- B0 @2 l3 `# O8 ?
// 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/ q6 b7 d2 ~; a5 W, R) q8 {
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。 G) o9 m s5 d/ V5 G$ z
// 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 ) t T, Y# d' s( h 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 b6 i O/ J$ t' ?6 o# B0 y, g* Y$ 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