a !== a

too good to be true?

The actual heading should be: A short story about the weird parts of NaN

NaN in JavaScript stands for “Not a Number”. The type of NaN is surprisingly Number. Which is weird because it is used to define something which isn’t a number.

typeOf(NaN); // "number"

It is used for instance for a result of a bad calculation.

var a = 3 * "jalal"; // NaN

The most confusing part aboutNaN is that NaN compared to anything else is always false!

NaN == false; // false

NaN == true; // false

An even compared to itself equals always false.

NaN == NaN; // false

NaN === NaN; // false

This raises one question:

How to check if a variable is NaN?

There is a function in JavaScript called isNaN(), but it has, unfortunately, some issues on its own.

Let’s go through some examples:

isNaN(NaN); // true

That worked!

isNaN(1); // false because 1 is a number

This is also not surprising!

isNaN('1'); // false

This worked too because isNaN coerces the passed in parameter and then checks if it is not a number.

isNaN('a'); // true because 'a' is not a number

Again, an implicit coercion is happening here. isNaN tries to convert the parameter into a number. The result of this effort having ‘a’ as a parameter is NaN:

Number('a'); // NaN

parseInt('a'); // NaN

Number('1'); // 1

This is why isNaN('a') returns true. It’s similar to isNaN(Number('a')) .

So basically the isNaN function is useless unless you are 100% certain that the passed in parameter is supposed to be a number.

Again, how to check if a variable is NaN?

We mentioned before that NaN is the only JavaScript variable that returns false if it is compared to itself. We can make use of that!

var a = 3 * 'jalal'; // a = NaN

if (a !== a) console.log('A is NaN'); // a !== a equals to true

This is the only way (I am aware of) to certainly check if a variable is NaN.

You can imagine that those weird parts are important to know because they are fruitful soil for bugs of any kind.

Jalal Fathi is freelance software developer and startup founder. Passionate about web technologies and in love with JavaScript. Learn more.