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react-useeffect/skill.md

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+---
+name: react-useeffect
+description: 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
+
+```
+Need to respond to something?
+├── User interaction (click, submit, drag)?
+│   └── Use EVENT HANDLER
+├── Component appeared on screen?
+│   └── Use EFFECT (external sync, analytics)
+├── Props/state changed and need derived value?
+│   └── CALCULATE DURING RENDER
+│       └── Expensive? Use useMemo
+└── Need to reset state when prop changes?
+    └── Use KEY PROP on component
+```
+
+## Detailed Guidance
+
+- [Anti-Patterns](./anti-patterns.md) - Common mistakes with fixes
+- [Better Alternatives](./alternatives.md) - useMemo, key prop, lifting state, useSyncExternalStore