Why is NaN not equal to NaN? [duplicate]

Answer 1

Well, log(-1) gives NaN, and acos(2) also gives NaN. Does that mean that log(-1) == acos(2)? Clearly not. Hence it makes perfect sense that NaN is not equal to itself.

Revisiting this almost two years later, here's a "NaN-safe" comparison function:

function compare(a,b) {
    return a == b || (isNaN(a) && isNaN(b));
}