The nullish coalescing operator, ??, provides a fallback only when the left-hand value is null or undefined.

const username = null;
const name = username ?? 'Guest';

console.log(name); // Guest

?? vs. ||

|| falls back for every falsy value. ?? preserves valid falsy data such as 0, false, and '':

const count = 0;

console.log(count || 10); // 10
console.log(count ?? 10); // 0

Function Defaults

function getPrice(price, discount) {
  discount = discount ?? 0;
  return price - price * discount;
}

This treats null and undefined as missing while allowing a discount of 0.

Combine with Optional Chaining

const user = { profile: { name: 'Alice' } };
const bio = user.profile?.bio ?? 'Bio not available';

Optional chaining safely reads the property and ?? supplies the fallback.

Operator Mixing

JavaScript does not allow ?? to be mixed directly with || or && without grouping:

const value = (input || fallbackA) ?? fallbackB;

Use parentheses to make the intended precedence explicit.

Conclusion

Use ?? when only null and undefined should mean “missing.” It is usually safer than || for configuration values, numeric fields, booleans, and strings where falsy values may be meaningful.

JavaScript