null vs undefined

`undefined` usually means "not provided" or "property missing". `null` usually means deliberate absence. Both are nullish, but they signal different intent in APIs.

`value || fallback` treats every falsy value (0, "", false) as missing. `value ?? fallback` falls back only for `null` or `undefined`. Optional chaining `obj?.a?.b` short-circuits only where `?.` appears.

					const count = 0;
console.log(count || 10); // 10 — wrong if 0 is valid
console.log(count ?? 10); // 0
				

Model API contracts explicitly: which fields are optional, which use null versus undefined, and whether 0 or false are valid states.

On interviews, explain the concept with a concrete example and name the runtime behavior interviewers probe.

Common pitfalls include mixing similar APIs and forgetting edge cases during live coding.

The trade-off is often clarity versus performance or safety versus convenience.

Checklist:

• Distinguish || from ??.
• Use ?. for guarded nested access.
• Document absence semantics in APIs.