// 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* j+ x9 `8 S* W, 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。: l0 J9 t! Q* j% ?% Y$ ?5 R
// 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 . J4 c) G3 a9 b1 K var不存在块级作用域,let和const存在块级作用域。% U: u2 R# P: }) `
// 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 b/ b/ A) L# a( J7 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。 : g0 f% T8 h7 w
// 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9 `; G/ {4 l5 I& c T0 t
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。