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Learn all about null,undefined and NaN


We all come across null, undefined and NaN at some point in our coding journey. As a beginner, I'd always wanted to know how and why these special values behave the way they do. So I'm going to illustrate just that in this article.

What is null?

Null represents an empty value and carry no meaningful information.
If you use the typeof operator on null, it shows that null is an object. You can read all about it over here. In fact, null is one of the primitive values in JavaScript.

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What is undefined?

Undefined represents any value that has not been defined anywhere in the program.
If any variable or operation isn't able to produce a meaningful value, it returns undefined.

let a = [1,2]
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In the above example, there is no meaningful value for a[2] because it doesn't exist, so it returns undefined

What is NaN?

NaN stands for Not a Number. You get this value when you try to do some operation that can't yield a meaningful result. If you use the typeof operator on NaN, it shows it is a number.

console.log("five" * 2)
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Behavior of null,undefined,NaN

Let's have a look at how these values behave with each other, and why.

null and undefined

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null and undefined return false when compared with any value. Or does it? Let's take a look.

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Before going into the explanation, let's take a look at how "==" and "===" work.
"==" performs a loose equality operation between its operands and does type coercion.
This means that if you use "==" between two values of different types, JavaScript tries to convert one operand's type into the other's to provide a result.

Whereas "===" performs a strict equality and does not try to convert the type of either value. It checks if the type of the values are the same. If not it returns false.

Now let's get back to the code. Let's look at the two cases.

  • ==

    null and undefined are both falsy values and that is why it returns true in the first line. Also == operator needs its operands to contain some value. Since null and undefined don't contain any value, it returns true.

  • ===
    Since null and undefined are of different types, it returns false.

So null and undefined return true only when they're loosely compared with each other.


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NaN is in itself the result of a nonsensical computation, so equating it with any other nonsensical computation will return false. This is the only value in JavaScript that isn't equal to itself

Let's make it a little interesting.

console.log(NaN === true)   
console.log(NaN === false)    
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Since NaN is a falsy value !NaN returns true.
NaN is not equal to any value. It always returns false.


  • null and undefined are values that contain nothing.
  • null and undefined are loosely equal to each other but are strictly not equal to each other.
  • NaN is a result of nonsensical computation which cannot produce a meaningful result.
  • NaN is the only value which isn't equal to itself.

I hope this clears some of your doubts about these special values. And I hope you enjoyed reading this!

Top comments (1)

techspace profile image

Thanks, great explanation for beginners