Do you really know why you want to use void(0) instead of undefined?

We usually use $undefined$只是 w i n d o w \color{#FF3030}{window} An attribute under the$w$$i$$n$$d$$o$$w$ object.

Object.getOwnPropertyDescriptor(window, undefined);
//{value: undefined, writable: false, enumerable: false, configurable: false}

It is precisely because of this that it can be modified in the function scope.
But the prerequisite is to add a statement, because without a statement, the operating $window.undefined$，而 w i n d o w . u n d e f i n e d \color{#FF3030}{window.undefined} The attribute descriptor of$w$$i$$n$$d$$o$$w$$.$$u$$n$$d$$e$$f$$i$$n$$e$$d$ is:

Attributes	value
value	undefined
writable	false
enumerable	false
configurable	false

$value$ cannot be modified, cannot be traversed, and$Genuspropertiesdescribedabovecharacter$ can not be modified (not by$Object.defineProperty$来修改$writable、enumerable、configurable$的值)。

But we can do it in the $Functionnumberforadomain$ or a$Blocklevelforadomain$ ofundefined \ color {# FF3030} {"Modify"the value of$u$$n$$d$$e$$f$$i$$n$$e$$d$$"Repairchange"$ , such as:

function fn(){
    
    
    undefined = 1111;
    console.log('undefined : ' + undefined);
}
fn();//undefined : undefined

function fn(){
    
    
    var undefined = 1111;
    console.log('undefined : ' + undefined);
}
fn();//undefined : 1111

{
    
    
    let undefined = 1111;
    console.log('undefined : ' + undefined);//undefined : 1111
}

This is because in the $Functionnumberforadomain$ or a$Blocklevelforadomain$ is also defined in a$undefined$ variables. In this scope, newly defined variables will be used first instead of$window.undefined$ , thus creating a kind ofwindow. undefined \color{#FF3030}{window.undefined}The illusion that the value of$w$$i$$n$$d$$o$$w$$.$$u$$n$$d$$e$$f$$i$$n$$e$$d$ has changed.

So when we judge the type of variable, it is best to use:

var obj;
typeof obj ===  'undefined';

Don't use:

var obj;
obj === undefined;

Use the $VOIDOFFkeyword$ is possible, because$VOIDOFFkeyword$ in any case will return$undefined$：

var obj;
obj === void(0);

This is also often used in actual development $void(0)$来代替 u n d e f i n e d \color{#FF3030}{undefined} The reason for$u$$n$$d$$e$$f$$i$$n$$e$$d$ .