It's not true in a normal programming language. If it is true in yours, you should stop using it immediately.

probably not true in most other langauges. although I'm not well versed in the way numbers are represented in code and what makes a number "NaN", something tells me the technical implications of that would be quite bad in a production environment.

the definitive way to check for NaN in JS would probably be something like

// with `num` being an unknown value

// Convert value to a number
const res = Number(num);

/*
 * First check if the number is 0, since 0 is a falsy
 * value in JS, and if it isn't, `NaN` is the only other
 * falsy number value
 */
const isNaN = res !== 0 && !res;