Cleanup effects

Last reviewed May 28, 2026 Content v20260528

Track mode

client_react

Means

In-browser React TSX

Reading

~1 min

Level

intermediate

This lesson

This lesson teaches Cleanup effects: the concepts, APIs, and habits you need before advancing in React.

Side effects (fetch, subscriptions, timers) must be modeled explicitly—this lesson prevents infinite loops and stale closures.

You will apply Cleanup effects in contexts like: SPAs, dashboards, design-system-driven products, and React Native mobile apps.

Write TypeScript/TSX, click Run in browser—React 18 loads from CDN, JSX compiles in the tab, UI renders in the preview root, and printOutput feeds the terminal.

When you can explain the previous lesson's ideas without copying starter code.

Effects that subscribe to external systems must clean up to avoid memory leaks and duplicate listeners. Return a function from useEffect; React calls it before re-running the effect and on unmount.

Common cleanup targets

clearInterval / clearTimeout
WebSocket or event listener removal
AbortController for in-flight fetches
Resetting document title or body scroll lock

Strict Mode double invoke

In development, React 18 Strict Mode may mount, unmount, and remount to surface missing cleanups. Your cleanup function should make remounting safe.

Self-check

When does React call your cleanup function?
Why leave a timer running after unmount?

Playground

Runs in your browser in a sandboxed frame. Backend runners appear when this track’s profile allows them.

Discussion

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Starter discussion topics

Cleanup you needed?
Leak without cleanup?

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