What is nullish in JavaScript?

Nullish-coalescing operator (?? or double question mark) is a handy tool that returns the right-hand operand when the left-hand is nullish. In JavaScript, nullish is defined as null or undefined.

I enjoy using the nullish coalescing operator in my code, especially combined with optional chaining:

const name = response?.user?.name ?? "Anonymous";

This code will return the user's name if user and name exist on the response variable, or fallback to "Anonymous" if not.

However, I tend to get nullish confused with falsy in JavaScript. And while there is some overlap, it is important to know and remember the difference between the two.

Common falsy values are null, undefined, 0, "", false, and NaN.

Nullish values are a subset of that at just null and undefined.

If you try to use the nullish-coalescing operator with a falsy value, it will return the left-hand operand:

false ?? "fallback"; // false

To state it one more time, nullish in JavaScript is null or undefined. All other values, even if they are falsy, are excluded. So reach for nullish coalescing only when dealing with null or undefined. Otherwise use logical OR (||) or a ternary operator (? :).