Programming Fundamentals › Programming Basics

Truthy and Falsy

Non-boolean values that act as true or false in conditions.

Also known as: falsy, truthiness, truthy

In many languages an if doesn’t need an actual boolean. Any value can be used as a condition, and the language treats it as truthy (counts as true) or falsy (counts as false).

const name = "";
if (name) { /* skipped: "" is falsy */ }

The falsy values in JavaScript

There are only a few, so memorize them: false, 0 (and -0), 0n (a BigInt zero), "" (empty string), null, undefined and NaN. Everything else is truthy, including some surprises:

Python’s rule is different: empty collections are falsy, which is handy (if not items:). Don’t carry one language’s rules into another.

The classic bug

Using truthiness to check “does this exist?” when 0 or "" are valid values:

function showCount(count) {
  if (!count) return "No data";   // bug: a count of 0 is real data
  return `${count} items`;
}
showCount(0); // "No data"

Check what you actually mean:

if (count === undefined || count === null) return "No data";
// or, shorter: count == null  (matches both null and undefined)

The nullish operator ?? is the safe version of || for defaults: count ?? 0 keeps a real 0, while count || 10 replaces it.

Rule of thumb: truthiness is fine for “is this string/array/object present?” when empty means “nothing”. It’s wrong when zero or an empty string is a legitimate value.