What is the meaning of void 0? Is undefined changeable?

Preface

hello , Good afternoon, everyone ！ Today we're going to talk about JavaScript A very interesting phenomenon in ,

void 0  What's the point ？undefined It turns out to be variable ？

explain

First , We need to void Operators have a basic understanding ：

void The correct way to use is to follow the expression , because void Will execute all the expression statements after itself , But its return value is always undefined.

We need to understand three things , One is void The action of the operator , One is void The return value of , The other one is undefined Can be redefined .

1,

void

  It needs to be followed by an expression , All statements in the expression will be executed .

void

  Operator

  Evaluate a given expression , Then return  

undefined

for example ：

void console.log(" Hello everyone , I'm the Antarctic ice ") //  Console printing “ Hello everyone , I'm the Antarctic ice ”
void alert(" Hello everyone , I'm the Antarctic ice ") //  The browser pops up “ Hello everyone , I'm the Antarctic ice ”
void 1+2 //  return  NAN, Think about why ？？？
//  This is because 1+2 It's not added outside (), By default, execute from left to right ,void 1 by undefined,undefined+2 by NAN
void (1+2) //  The return value is undefined


2,

void

The return value is always

undefined

for example ：


let a = void 0
console.log(a) //  The result is undefined

let a = console.log(" Hello everyone , I'm the Antarctic ice ")
console.log(a) //  The result is undefined

let a = void alert(" Hello everyone , I'm the Antarctic ice ")
console.log(a) //  The result is undefined

let a = void (1+2)
console.log(a) //  The result is undefined


3,

undefined

It can be redefined in local variables

Although in ES5 Start ,undefined Has been set to read only , But within the local scope ,undefined Still variable .

for example ：

function fun(){
 let undefined = 10
 console.log(undefined)
}
fun() //  At this time, the printed undefined by 10 


The reason to use  

void 0

  perhaps  

void(0)

  Instead of  

undefined

, A big reason is

undefined

May be reassigned , You can't guarantee that other people's code

undefined

yes

undefined

. however

void

  The return value of must be

undefined

.

in addition , because

void 0

  Than

undefined

It should be shorter , Takes up less character space , So in js Compressed code , We can often see the use of void 0 Instead of undefined The situation of . Like local variables in compressed code will be a,b,c To replace the same . Of course! , If you will , Write

void 1

Instead of

undefined

It's the same .

Postscript

Hello, wow , I'm the Antarctic ice , A front-end engineer whose technology is proportional to his appearance , Advocate getting to the point to solve the front-end problems , I hope my blog has helped you .

Pay attention to me , Go along the front road . Hey ha ~