One piece of JavaScript syntax I’ve come to appreciate is the nullish coalescing (??) operator. It’s easy to overlook until it saves you from a subtle bug.

If you’re still using the logical OR (||) operator to provide default values, it’s worth knowing where the two behave differently.

What each one treats as missing

The logical OR (||) operator treats any falsy value as a signal to fall back to the value on the right. That includes false, 0, NaN, "" (an empty string), null, and undefined.

This can be surprising when values like 0 or "" are perfectly valid and should be preserved:

const value = 0;
const _default = 5;
console.log(value || _default); // Output: 5 (because 0 is falsy)

Here, || treats 0 as falsy, so the default value (5) is returned.

The nullish coalescing (??) operator works a little differently. It only falls back when the value on the left is null or undefined.

Everything else, including 0, false, and an empty string, is treated as a legitimate value:

const value = 0;
const _default = 5;
console.log(value ?? _default); // Output: 0 (because 0 is not null or undefined)

Since 0 isn’t null or undefined, it’s preserved instead of being replaced by the default.

Why I usually turn to ??

When I’m providing default values, ?? is usually the operator I want.

It preserves values that happen to be falsy while still giving me a fallback when something is actually missing (null or undefined). That small distinction prevents bugs that are annoying to track down, especially with numbers, form inputs, and config values.

That doesn’t make || wrong. It still has its place when you want any falsy value treated as “missing.” But if all you want is “use a default only when there’s no value,” ?? is usually the better fit.

It’s a small change that makes defaults behave the way you’d expect.

• JavaScript