Clever Geek Handbook

Set variable undefined and null

I just read some code and I see this line:

var foo = null, undefined;

When I check the variable, it is null and undefined.

So my question is: What is the purpose of setting a variable to both null and undefined? I do not understand. Thanks for the explanation.

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3 answers


As mentioned in the comments, you probably won't be testing foo properly, the variable cannot be undefined or undefined .

var foo = null, undefined;
alert(foo); //shows null
alert(typeof foo); //shows object (not undefined)

So what's going on? The comma means that you are declaring an additional variable. Since undefined is already a keyword, this part of the statement has no effect. However, if you did it like this:



var foo = null, undefined1;
alert(foo); //shows null
alert(typeof foo); //shows object (not undefined)
alert(undefined1); //shows undefined
alert(typeof undefined1); //shows undefined

You can see that you are actually declaring a new variable undefined1

that has no initial value.

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The purpose of this statement is to have a local declaration undefined

in a variable of the same name.

For example:

// declare two local variables
var foo = null, undefined;

console.log(foo === undefined); // false

He looks like:

function test(foo, undefined)
{
    console.log(foo === undefined); // false
}
test(null); // only called with a single argument

This is usually not required because a sane browser won't let anyone override what it means undefined

, and jslint will complain about this:



The reserved name is 'undefined'.

Basically, I would recommend not doing this at all:

var foo = null;

Btw, the above expression should not be confused with using a comma like this:

var foo;

foo = 1, 2;
console.log(foo); // 2
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Short: It's useless

without assigning anything to the variable undefined

. You can assign null

to make it null. However, your comparison also matters

Screenshot Demo

if(foo == null) //true
    alert('1');
if(foo == undefined) //true
    alert('2');

Now a strict comparison with === 

if(foo === null) //false............can be true if assigned to null
    alert('3');
if(foo === undefined) //true.......can be flase if assigned to null
    alert('4');
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