Null Check

A null check is guarding code against the case where a value you expected to exist is actually missing — `null`, `undefined`, `None`, empty string, empty array. It sounds trivial, and that's exactly why AI-generated code skips it. The happy path works: the user has a profile, the API returns data, the list has items. What about first-time users with no profile? What about an API that returned `[]` because the query matched nothing? What about a timeout that resolved to `undefined`? Each of these crashes your code at a different place. A null check is one of the easiest bugs to spot in code review — look at every property access (`user.name`) and every array method (`items.map(...)`) and ask 'what if this is null or empty?'. The modern answer is often optional chaining (`user?.name`) plus a sensible default (`user?.name ?? 'Guest'`). For arrays, check `items.length > 0` before iterating or rendering, and show a friendly empty state. Our checklist puts null/empty handling under edge cases/high because users hit these edges within the first hour of launch.