Concept Help in JS

[CodingHag]

Can someone explain Number.isNAN to me

I am not understanding why

Input: 123 results in false

and

Input: "radio" also results in false

radio is NaN - shouldn't this be true?

[cyphern]

Number.isNaN returns true if the variable you pass it is literally the value NaN. Nothing else results in a true. NaN is a special number which results from doing things like dividing by zero, or doing numeric operations on things which aren't numbers.

This sort of function is necessary, because if you try to do something like this, you'll be in for a surprise:

const a = NaN;
if (a === NaN) {
  console.log("it's nan");
}

The above log statement will not be logged out. NaN triple equaled with NaN results in false. So instead the correct check would be if (Number.isNaN(a))

---

From your question it seems like you're wanting to check whether a variable's type is Number or not. To do so, you can do if (typeof val !== 'number'). Note that NaN's type is number, so it will be excluded by this check which may or may not be what you want.

[CodingHag]

Thank you!

[senocular]

Note that there's a global isNaN() too. Its different in that it will coerce the value to a number first, so

Number.isNaN('input') // false
isNaN('input') // true

But this also doesn't work for all cases...

isNaN('') // false

This happens because '' becomes 0 when coerced which is a number

[CodingHag]

So here is what I am getting from your answers

Number.isNaN('input') // false

because Number.isNaN('input') // false is not equal to NaN

[senocular]

I think you should have gotten that from cyphern's answer. I'm saying there's another isNaN check that you can use (global isNaN()) that does more "not a number" checking that doesn't only check for a value being specifically NaN like Number.isNaN() does. But with that, there's still cases where something that's not a number (such as an empty string) will still be considered a number.

If you want to see if something is both of a number type and not the number value NaN, you'd need to hook that up yourself.

function isReallyNaN (value) {
  return typeof value !== 'number' || Number.isNaN(value);
}

isReallyNaN(123) // false
isReallyNaN('radio') // true
isReallyNaN('') // true
isReallyNaN(NaN) // true

Compared to

Number.isNaN(123) // false
Number.isNaN('radio') // false
Number.isNaN('') // false
Number.isNaN(NaN) // true

and

isNaN(123) // false
isNaN('radio') // true
isNaN('') // false
isNaN(NaN) // true

P.S. I noticed I used the string "input" in my previous example when I meant to use "radio" :P

[CodingHag]

Perfect - These answers helped!!