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

Detailed Guidance

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