KooiInc - 5 months ago 6
Javascript Question

# 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 gratefull.

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

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.

Source (Stackoverflow)