Lessons · JavaScript · ?? only catches null and undefined
A default only for missing
a ?? b gives b only when a is null or undefined. a || b gives b for every falsy a, including a real 0 or ''.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A limit of 0, a volume of 0, an empty string the user typed on purpose: || replaces them with the default and the bug is invisible. ?? was added to fix exactly that.
How to think about it
Use ?? for defaults. Use || only when you want every falsy value replaced. Never mix them in one expression without parentheses; the language refuses.
Worked example
console.log(0 ?? "fallback");0: ?? only replaces null and undefined.
console.log(0 || "fallback");fallback: || replaces every falsy value, including a real 0.
console.log(null ?? "fallback");fallback.
console.log(undefined ?? 10);10.
Your turn
A port with a default.
const port = config.port 3000;
Solve one with the tests running
The trap
a || b ?? c is a syntax error. Mixing || and ?? needs parentheses, on purpose, because their rules differ.
Practise ?? only catches null and undefined on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.