CODEWITHSUNDEEP
javascriptcompare
David
David

Reputation: 2223

Comparing to null - !== vs != in JavaScript

Ok, so I installed Linter on my Sublime editor while working on my node.js app. One of the things that it caught said that I should always use !== to compare an object to null (I usually use != ).

So I changed it...but then I noticed that the !== wasn't working.

UPDATE Ok, so it may be a bit more complicated than I originally thought. In my real code I was using !== with the node.js GLOBAL object.

console.log('Global User: ' + GLOBAL.User);

if (GLOBAL.User != null)
{
    console.log('User is not null');
}

The console line prints even when GLOBAL.User is null...

Perhaps this object is special?

Ok, so after reading through the comments and looking at my code, I have learned that !== can have issues if the object is undefined rather than null (see this post: Why is null an object and what's the difference between null and undefined?).

So in my case, my global variable could be, depending on when this method is called, undefined, null, or full of data. I am going to go back and update my code so that it is never undefined and then !== will work consistently.

Upvotes: 33

Views: 101956

Answers (3)

oshribr
oshribr

Reputation: 666

It is even more simple 

var x = null;
    if (x) 6 
    if (!x) 7

the result is 

undefined
7

Upvotes: 4

plalx
plalx

Reputation: 43718

The only value that doesn't equal itself in JavaScript is NaN. If null === null is false, then your JavaScript engine has serious problems ;)

To make sure your conditional statement is well written, always use the braces.

var x = null;
if (x !== null) {
    console.log('x is not equal to null');
}

Upvotes: 15

loganfsmyth
loganfsmyth

Reputation: 161467

Your global.User is undefined, not null. When using == they evaluate to equal, but with === the items you are comparing need to be the same type. undefined has the type undefined and null has the type object.

undefined and null are very similar, but they generally mean two very different things. Usually undefined is the result when something has had no value assigned to it, whereas null has a value, and the value is explicitly set to "nothing".

Upvotes: 41

Related Questions