nickb
nickb

Reputation: 9276

How to check if a variable is not null?

I know that below are the two ways in JavaScript to check whether a variable is not null, but I’m confused which is the best practice to use.

Should I do:

if (myVar) {...}

or

if (myVar !== null) {...}

Upvotes: 373

Views: 787609

Answers (10)

Fury
Fury

Reputation: 4776

Rather than using multiple condition statements you can use below solution.

if(![false, 0, "", null, undefined, NaN].includes(myVar)){

   // It's not a null value

}

Upvotes: -2

hoogw
hoogw

Reputation: 5535

if (0) means false, if (-1, or any other number than 0) means true. following value are not truthy, null, undefined, 0, "" (empty string), false, NaN

never use number type like id as

if (id) {}

for id type with possible value 0, we can not use if (id) {}, because if (0) will means false, invalid, which we want it means valid as true id number.

So for id type, we must use following:

if ((Id !== undefined) && (Id !== null) && (Id !== "")) {

} else {

}

for other string type, we can use if (string) {}, because null, undefined, empty string all will evaluate at false, which is correct.

if (string_type_variable) { }

Upvotes: 5

Kamil Kiełczewski
Kamil Kiełczewski

Reputation: 92657

  • code inside your if(myVar) { code } will be NOT executed only when myVar is equal to: false, 0, "", null, undefined, NaN or you never defined variable myVar (then additionally code stop execution and throw exception).
  • code inside your if(myVar !== null) {code} will be NOT executed only when myVar is equal to null or you never defined it (throws exception).

Here you have all (src)

if

enter image description here

== (its negation !=)

enter image description here

=== (its negation !==)

enter image description here

Upvotes: 60

OpenUser
OpenUser

Reputation: 69

Sometimes if it was not even defined is better to be prepared. For this I used typeof

if(typeof(variable) !== "undefined") {
    //it exist
    if(variable !== null) {
        //and is not null
    }
    else {
        //but is null
    }
}
else {
    //it doesn't
}

Upvotes: 3

Sanjay Kumaar
Sanjay Kumaar

Reputation: 48

if myVar is null then if block not execute other-wise it will execute.

if (myVar != null) {...}

Upvotes: 1

Flat Cat
Flat Cat

Reputation: 916

There is another possible scenario I have just come across.

I did an ajax call and got data back as null, in a string format. I had to check it like this:

if(value != 'null'){}

So, null was a string which read "null" rather than really being null.

EDIT: It should be understood that I'm not selling this as the way it should be done. I had a scenario where this was the only way it could be done. I'm not sure why... perhaps the guy who wrote the back-end was presenting the data incorrectly, but regardless, this is real life. It's frustrating to see this down-voted by someone who understands that it's not quite right, and then up-voted by someone it actually helps.

Upvotes: 9

Tolgahan Albayrak
Tolgahan Albayrak

Reputation: 1779

Here is how you can test if a variable is not NULL:

if (myVar !== null) {...}

the block will be executed if myVar is not null.. it will be executed if myVar is undefined or false or 0 or NaN or anything else..

Upvotes: 100

Tim Down
Tim Down

Reputation: 324657

They are not equivalent. The first will execute the block following the if statement if myVar is truthy (i.e. evaluates to true in a conditional), while the second will execute the block if myVar is any value other than null.

The only values that are not truthy in JavaScript are the following (a.k.a. falsy values):

  • null
  • undefined
  • 0
  • "" (the empty string)
  • false
  • NaN

Upvotes: 484

THE DOCTOR
THE DOCTOR

Reputation: 4555

The two conditional statements you list here are not better than one another. Your usage depends on the situation. You have a typo by the way in the 2nd example. There should be only one equals sign after the exclamation mark.

The 1st example determines if the value in myVar is true and executes the code inside of the {...}

The 2nd example evaluates if myVar does not equal null and if that case is true it will execute your code inside of the {...}

I suggest taking a look into conditional statements for more techniques. Once you are familiar with them, you can decide when you need them.

Upvotes: -5

Jonathon Bolster
Jonathon Bolster

Reputation: 15961

Have a read at this post: http://enterprisejquery.com/2010/10/how-good-c-habits-can-encourage-bad-javascript-habits-part-2/

It has some nice tips for JavaScript in general but one thing it does mention is that you should check for null like:

if(myvar) { }

It also mentions what's considered 'falsey' that you might not realise.

Upvotes: 53

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