一、var + q* m' K7 C: u: L. ~5 v H 在ES5中,顶层对象的属性和全局变量是等价的,用var声明的变量既是全局变量,也是顶层变量。: ]3 r5 }' n( S8 b! b1 P# ]
注意:顶层对象,在浏览器环境指的是window对象,在 Node 指的是global对象。 ! F. E7 e u- r. s# p7 d6 Z' C
// 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 + n5 p& V7 l5 i, X 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。* m9 R+ O; }: S) b8 Y- M' P; O
// 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 4 C' P" G2 ?, l# Q; v var不存在块级作用域,let和const存在块级作用域。; Q( M$ A5 r: h4 o& s$ Y4 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
// 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4 I5 ?- n' @1 l4 t3 e
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。6 ~7 \1 `# r/ o/ O* C b& Z
// 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