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.
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Use the react-useeffect skill to review [React component file] for one small first result. For each useEffect, state whether it is justified for external-system synchronization, a subscription, display-triggered analytics or logging, or data fetching. Suggest a documented alternative only when the effect is instead transforming data for rendering, handling a user event, deriving state, or chaining state updates. Flag any data-fetching effect that lacks proper cleanup. Do not edit the file.
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Path to a React component containing useEffect code
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name: react-useeffectdescription: 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
Transforming data for rendering - Calculate at top level, re-runs automatically
Handling user events - Use event handlers, you know exactly what happened
Deriving state - Just compute it: const fullName = firstName + ' ' + lastName
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