On: NaN does not equal NaN

Yes, but this is bigger than that. JavaScript doesn't define NaN, IEEE-754 defines NaN and JavaScript takes its definition from that. So it's IEEE-754 thinks num == num being false is a good test for NaN.

For the record, isNaN in JavaScript is also not necessarily the fastest way of doing it: http://jsperf.com/nantest

> For the record, isNaN in JavaScript is also not necessarily the fastest way of doing it

Which will absolutely not matter in 100% of the cases after rounding. `isNaN` is simple (your original criteria) and clearly says what it's doing.