// 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 b: N" h# ^, H7 K' j+ H% }/ 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。7 K+ T6 F2 z$ m
// 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 . l9 Q; h b& ~ var不存在块级作用域,let和const存在块级作用域。) S0 G- n8 e* s5 j$ { `3 w
// 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# \& g# i3 F8 G- C+ k
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。- X a+ U5 K; P% [: Q4 q4 y
// 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' X7 j! G8 \4 z! Y# i
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。 0 \6 F# q. j: g$ Z# f
// 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