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rated 0 times [  154] [ 2]  / answers: 1 / hits: 100011  / 14 Years ago, mon, may 10, 2010, 12:00:00

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.

More From » nan

1

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.

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