Ironically enough, 1/0 is Infinity, which is a valid number, instead you need to use 0/0 to get NaN. Even worse, the rules for NaN aren't specific to JavaScript, but come from the IEEE 754 floating point standard.
Standard NaN is even weirder: http://en.m.wikipedia.org/wiki/NaN
Personally I don't see how this solves much, but then again I'm not a JavaScript guy.