How do you check that a number is NaN in JavaScript?

Question

I’ve only been trying it in Firefox’s JavaScript console, but neither of the following statements return true:

parseFloat(\'geoff\') == NaN;

parseFloat(\'ge         


        

Answer 1

I wrote this answer to another question on StackOverflow where another checks when NaN == null but then it was marked as duplicate so I don't want to waste my job.

Look at Mozilla Developer Network about NaN.

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Short answer

Just use distance || 0 when you want to be sure you value is a proper number or isNaN() to check it.

Long answer

The NaN (Not-a-Number) is a weirdo Global Object in javascript frequently returned when some mathematical operation failed.

You wanted to check if NaN == null which results false. Hovewer even NaN == NaN results with false.

A Simple way to find out if variable is NaN is an global function isNaN().

Another is x !== x which is only true when x is NaN. (thanks for remind to @raphael-schweikert)

But why the short answer worked?

Let's find out.

When you call NaN == false the result is false, same with NaN == true.

Somewhere in specifications JavaScript has an record with always false values, which includes:

• NaN - Not-a-Number
• "" - empty string
• false - a boolean false
• null - null object
• undefined - undefined variables
• 0 - numerical 0, including +0 and -0