Which of these javascript expressions is false?

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;

  

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