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name: skill-128 description: "Use when writing React components with useEffect, seeing useState + useEffect for derived values, resetting state on prop change with Effect, chaining Effects that trigger each other, notifying parent via Effect, data fetching without cleanup (race conditions), or reviewing React code for unnecessary Effects and extra re-renders."

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 - Common mistakes with fixes
• Better Alternatives - useMemo, key prop, lifting state, useSyncExternalStore