What is optional chaining (?.) and nullish coalescing (??)?

// Optional chaining
const city = user?.address?.city;
const name = users?.[0]?.name;
const val = obj?.method?.();

// Nullish coalescing — fallback for null/undefined ONLY
const count = user.count ?? 0;
// if count=0, result is 0 (correct!)
// vs OR operator:
const bad = user.count || 0;
// if count=0, bad=0 but for wrong reason (0 is falsy)