React Useeffect

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](./anti-patterns.md) - Common mistakes with fixes
• [Better Alternatives](./alternatives.md) - useMemo, key prop, lifting state, useSyncExternalStore

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: softaworks
• Source: softaworks/agent-toolkit
• License: MIT

Install and usage instructions live in the source repository linked above.