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react-useeffect▌

softaworks/agent-toolkit · updated Apr 8, 2026

$npx skills add https://github.com/softaworks/agent-toolkit --skill react-useeffect
summary

React useEffect best practices guide covering when to use Effects and superior alternatives.

  • ›Teaches the escape-hatch nature of Effects: use only for synchronizing with external systems, not for derived state, expensive calculations, or user event responses
  • ›Provides a decision tree and quick-reference table mapping common situations (data fetching, state derivation, prop changes) to the correct React pattern
  • ›Covers when NOT to use Effects: transforming data for render, handling user
skill.md

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