javascriptnan

Why does typeof NaN return 'number'?

Just out of curiosity.

It doesn't seem very logical that typeof NaN is number. Just like NaN === NaN or NaN == NaN returning false, by the way. Is this one of the peculiarities of JavaScript, or would there be a reason for this?

Edit: thanks for your answers. It's not an easy thing to get ones head around though. Reading answers and the wiki I understood more, but still, a sentence like

A comparison with a NaN always returns an unordered result even when comparing with itself. The comparison predicates are either signaling or non-signaling, the signaling versions signal an invalid exception for such comparisons. The equality and inequality predicates are non-signaling so x = x returning false can be used to test if x is a quiet NaN.

just keeps my head spinning. If someone can translate this in human (as opposed to, say, mathematician) readable language, I would be grateful.

[addendum 2023/06] I've create a small module for checking the type of anything in ES. It may be helpful, check it @GitHub

Solution

  • It means Not a Number. It is not a peculiarity of javascript but common computer science principle.

    From http://en.wikipedia.org/wiki/NaN:

    There are three kinds of operation which return NaN:

    Operations with a NaN as at least one operand

    Indeterminate forms

    • The divisions 0/0, ∞/∞, ∞/−∞, −∞/∞, and −∞/−∞
    • The multiplications 0×∞ and 0×−∞
    • The power 1^∞
    • The additions ∞ + (−∞), (−∞) + ∞ and equivalent subtractions.

    Real operations with complex results:

    • The square root of a negative number
    • The logarithm of a negative number
    • The tangent of an odd multiple of 90 degrees (or π/2 radians)
    • The inverse sine or cosine of a number which is less than −1 or greater than +1.

    All these values may not be the same. A simple test for a NaN is to test value == value is false.