// 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 u9 H. s' E3 X- r9 {
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。, N& Q+ @) D" r. {2 k; T
// 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 6 l; ^7 r$ G/ d# p b var不存在块级作用域,let和const存在块级作用域。 3 }- |3 o$ _7 ?' ~, Z% [
// 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& h! _9 M! u! o* b
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。 ' n0 G @, d6 D4 r& O. E4 T
// 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 5 X" r4 O/ e4 s0 H" u 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。: m5 h5 D, ^! V; {. s* d
// 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