Validate a number in JavaScript

Number validation is probably one of the hardest things to do even for intermediate developers. However, it's a necessary evil, especially when working with user input. It's even more so in JavaScript, due to the language's quirks and the fact that it's loosely typed.

While, a simple check that parseFloat() produces a sensible numeric value might seem enough, it's a little more involved than that. You also need to check for NaN, using Number.isNaN(), and for Infinity, using Number.isFinite().

And, as if that wasn't already enough, you also need to check if the coercion of the value to a number is correct, using Number() and the loose equality operator (==). Putting everything together, you get a pretty robust number validation function.

const validateNumber = n => {
  const num = parseFloat(n);
  return !Number.isNaN(num) && Number.isFinite(num) && Number(n) == n;
}

validateNumber('10'); // true
validateNumber('a'); // false