'Undefined' or 'Null' Variable in JavaScript

Question

While trying to learn JavaScript I came across a few snippets of code that has confused me.

My original code that I was messing around with was:

var username = prompt("Choose a username:");

if (username === null) {
    alert("Username is required!");
} else {
    console.log("Your username is: " + username);
}

This didn't work. So I came here and found this question: How to determine if variable is 'undefined' or 'null'. Even though I found my solution it raised more questions that I couldn't find the answer for within the sea of "answers".

The first solution that has 910 upvotes uses typeof, but this didn't work for me:

var username = prompt("Choose a username:");

if (typeof username === 'undefined'){
    alert("Username is required!");
} else {
    console.log("Your username is: " + username);
}

It also states a shorter version, which did work for me:

var username = prompt("Choose a username:");

if (!username){
    alert("Username is required!");
} else {
    console.log("Your username is: " + username);
}

Another answer caught my eye as it included a part of my personal code username === null, which has 319 upvotes, but this didn't work for me either:

var username = prompt("Choose a username:");

if (username === undefined || username === null) {
    alert("Username is required!");
} else {
    console.log("Your username is: " + username);
}

An edit to the answer also states that using just username === null is sufficient.

So my question is why does !username work with that specific code but the others did not?

First one doesn't work at all. Nothing gets alerted, because username does not equal null. It equals an empty string. — Richard Hamilton, Oct 01 '15 at 16:18
@nevermind Your jsfiddle still only logs `Your username is: ` when no value is entered (on chrome 45). — xlecoustillier, Oct 01 '15 at 16:18
@X.L.Ant, i see null logged, and see alert? firefox and chrome?! — sinisake, Oct 01 '15 at 16:19
@X.L.Ant, huh, forget it - it works as intended on 'cancel' lol - misunderstanding.... — sinisake, Oct 01 '15 at 16:23
You were looking for the wrong question, [prompt](https://developer.mozilla.org/en-US/docs/Web/API/Window/prompt) always return a string. So there's no point in checking if it's `null` or `undefined`. — MinusFour, Oct 01 '15 at 16:41
I'm not too sure this deserves downvotes since I'm exceptionally new to JavaScript. — phoenixlaef, Oct 01 '15 at 16:55

Richard Hamilton · Accepted Answer · 2015-10-01T16:19:45.557

2

When you are prompted for a username and don't type in anything, the value stored is an empty string, ''. This equals the value false in JavaScript.

The reason why the typeof(username) == undefined one didn't work is because the username variable was declared in the previous line. As I mentioned above, the type of username is a string, even if nothing was typed in.

And yes, to check if a variable is undefined, use typeof varName === undefined.

edited Oct 01 '15 at 16:19

answered Oct 01 '15 at 16:10

Richard Hamilton

25,478
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So would this be the same reasoning for `null`? – phoenixlaef Oct 01 '15 at 16:16
I already explained the value for `username` is `''`, an empty string. That means it has a value and is defined. So it's neither `null` nor `undefined`. – Richard Hamilton Oct 01 '15 at 16:17
so you would simply use `if (!username)`? Or is there something else you would use? – phoenixlaef Oct 01 '15 at 16:31
It is incorrect to say that an empty string is equal to `false`. – Oct 01 '15 at 16:35
Yes it is, try it for yourself. `'' == false` will return true – Richard Hamilton Oct 01 '15 at 16:43

score 0 · Answer 2 · answered Oct 01 '15 at 16:33

0

in javaScript if a variable is undefined then that means it is != null. if one variable is null and second is undefined than result for == will be true and for != will be false

answered Oct 01 '15 at 16:33

dinesh.kumar

167
1
10

Sergeon · Answer 3 · 2015-10-01T16:24:25.023

-1

Try:

typeof username === 'undefined'

Instead of:

username === undefined

if you try:

typeof username === undefined

Will fail too.

Javascript :-)

edited Oct 01 '15 at 16:24

answered Oct 01 '15 at 16:19

Sergeon

6,638
2
23
43

1

That won't work. `typeof` always returns a string. The second one isn't valid syntax – Richard Hamilton Oct 01 '15 at 16:21
Providing a list of things that also fail does not provide an answer to the question. – xlecoustillier Oct 01 '15 at 16:25

'Undefined' or 'Null' Variable in JavaScript

3 Answers3