// 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 7 g$ K9 o* i4 j6 z _5 Y% r 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。 ( V* T+ }" R- W( ~# T* 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 + U* c5 C+ g$ h var不存在块级作用域,let和const存在块级作用域。 , q+ v9 X& ?/ Z; {& J6 d2 a
// 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 0 D" o H( u& k% U1 N% O 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。 * Z R3 C$ C f( O4 h2 ~2 w
// 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 , D2 {, f' T# d 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。 , i. _6 T- n- k
// 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