Check for NaN, null and >=0 in one condition

Question

I have a var a;

Its value can be NaN, null and any +ve/-ve number including 0.

I require a condition which filters out all the values of a such that

Answer 1

NaN is not >= 0, so the only exclusion you need to make is for null:

if (a !== null && a >= 0) {
    ...
}