Hi! Are you asking why it can’t look something like this?
if (typeof input === 'NaN') return NaN;
The typeof operator returns one of only eight values, for example 'number' or 'boolean'. It never returns 'NaN'. NaN is not one of the types in JavaScript, but number is, so we use typeof to check if the input to the function is a number, and if not, we return NaN to indicate that we received something that is Not a Number.