One piece of JavaScript syntax I’ve come to appreciate is the nullish coalescing (??) operator. It’s one of those features that’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.

The difference comes down to what each operator considers a value worth replacing.

Breaking it down

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 reach for ??

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 helps avoid bugs that can be frustrating to track down, especially when working with numbers, form inputs, or configuration values.

That’s not to say || is wrong; it still has its place when you intentionally want to treat any falsy value as “missing.” But if your goal is simply “use a default only when no value exists,” ?? is often the better fit.

It’s a small change, but it’s one that can make your code behave more predictably and save you from a few head-scratching moments down the road. 👍🏻

• JavaScript