React useEffect best practices from official docs. Use when writing/reviewing useEffect, useState for derived values, data fetching, or state synchronization. Teaches when NOT to use Effect and better alternatives.

You Might Not Need an Effect Effects are an escape hatch from React. They let you synchronize with external systems. If there is no external system involved, you shouldn't need an Effect. Quick Reference Situation DON'T DO Derived state from props/state useState + useEffect Calculate during render Expensive calculations useEffect to cache useMemo Reset state on prop change useEffect with setState key prop User event responses useEffect watching state Event handler directly Notify parent of changes useEffect calling onChange Call in event handler Fetch data useEffect without cleanup useEffect with cleanup OR framework When You DO Need Effects Synchronizing with external systems (non React widgets, browser APIs) Subscriptions to external stores (use useSyncExternalStore when possible) Analytics/logging that runs because component displayed Data fetching with proper cleanup (or use framework's built in mechanism) When You DON'T Need Effects 1. Transforming data for rendering Calculate at top level, re runs automatically 2. Handling user events Use event handlers, you know exactly what happened 3. Deriving state Just compute it: const fullName = firstName + ' ' + lastName 4. Chaining state updates Calculate all next state in the event handler Decision Tree Detailed Guidance [Anti Patterns](./anti patterns.md) Common mistakes with fixes [Better Alternatives](./alternatives.md) useMemo, key prop, lifting state, useSyncExternalStore

react-useeffect