useEffect in React 19: What's Changed useEffect remains one of the most misunderstood React APIs in 2026, but the mental model has become clearer with React's official guidance and the patterns established by the ecosystem. React 19's compiler (React Forget, now stable) automatically manages many cases where developers previously needed to manually write useEffect with careful dependency arrays. Understanding when useEffect is still needed — and when it's an anti-pattern — is the key to writing correct React components. The Dependency Array: The Source of Most Bugs Every value used inside useEffect that can change over the component's lifetime must be in the dependency array. The React ESLint plugin (react-hooks/exhaustive-deps) enforces this rule and should be enabled in every React project. Violating the rule produces stale closure bugs — the effect reads an outdated value from a previous render.…