JavaScript - Check if a variable is a number : typeof doesnt work, isNaN does

I currently face a problem, it may be very simple but I found nothing in my research...

I want to check (in JavaScript) that a variable is a number, and if it is higher than 100 or lower than 0.

I tried this :

if ((returnValue > 100) || (returnValue < 0) || (typeof returnValue != 'number')) { 
    //not correct
} else {
 // let's do some stuff here
}

But it doesn't work... When I put for example the number 50 (which is good), happening in the "if" and not in the "else" as expected !

So I did :

if ((returnValue > 100) || (returnValue < 0) || (isNaN(returnValue)))

and then I noticed that it perfectly worked !

I know too I could split that in two parts , firstly testing if the variable is a number and then if it is greater than 100 or less than 0, but this is not what I am currently looking for ;)

Could you please explain me why the first try doesn't work (and/or make it work) ?

Thanks !

Solution

The isNaN() function implicitly coerces its argument to be a number. If you pass it the string "12", it returns false. Thus in your code the value of the variable could be a string representation of a number, and it'll make it through the tests if it's a "nice" string.

The concept of NaN does not really mean, generally, "not a number". It's not really about data types. It's a feature of the IEEE floating point format. There are bit patterns that are "not numbers", and the pseudo-value NaN represents those. It happens that the language designers decided to have failed numeric conversions result in NaN as a sort of marker.

In newer JavaScript environments, there's another isNaN function on the Number constructor. It differs from the global isNaN() in that it performs no type coercion at all, and it only checks to see if its argument is NaN. If you pass it a string — even a string like "banana", which isn't anything like a number — it returns false, because no string value can be the number NaN.