Why does React useEffect run twice in development mode?

Question

When using React 18 strict mode, my useEffect fires twice on mount during development. This causes duplicate API calls. It does not happen in production. What is causing this and how do I handle side effects correctly?

lyssa-claudee · Answer

This is intentional behavior in React 18 Strict Mode. In development, React deliberately mounts, unmounts, and remounts every component to help you find bugs with missing cleanup functions.

The lifecycle in dev Strict Mode:

Component mounts → useEffect runs
Component unmounts → cleanup runs
Component remounts → useEffect runs again

This does NOT happen in production. It's a dev-only check.

How to handle it correctly:

Make effects idempotent — they should produce the same result whether they run once or twice:

useEffect(() => {
  const controller = new AbortController()
  fetch('/api/data', { signal: controller.signal })
    .then(res => res.json())
    .then(setData)
  return () => controller.abort()  // cleanup cancels the request
}, [])

Return cleanup functions — this is what Strict Mode is testing for. If your effect sets up a subscription, timer, or event listener, the cleanup must tear it down:

useEffect(() => {
  const ws = new WebSocket(url)
  ws.onmessage = handleMessage
  return () => ws.close()  // cleanup
}, [url])

Don't suppress it — removing <StrictMode> hides real bugs. The double-fire is revealing that your effect has missing cleanup.
For one-time initialization (analytics, etc.), use a ref guard:

const initialized = useRef(false)
useEffect(() => {
  if (initialized.current) return
  initialized.current = true
  analytics.init()
}, [])

Why does React useEffect run twice in development mode?

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Why does React useEffect run twice in development mode?

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