Why does typeof NaN return 'number'?

Question

Just out of curiosity.

It doesn\'t seem very logical that typeof NaN is number. Just like NaN === NaN or NaN == NaN returning

Answer 1

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.