react-useeffect

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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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NPX Install

npx skill4agent add fluid-tools/claude-skills react-useeffect

SKILL.md Content

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

SituationDON'TDO
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