Question 1

Why does React Strict Mode call my useEffect cleanup twice in development?

Accepted Answer

React Strict Mode intentionally mounts, unmounts, and remounts every component in development to simulate what happens in concurrent rendering. This double-invocation is a deliberate stress test — if your effect doesn't survive a mount/unmount/remount cycle cleanly, it will fail under React 18 concurrent features in production. The cleanup being called 'extra' times is React telling you your teardown logic is missing or broken.

Question 2

Does this useEffect cleanup bug affect production builds?

Accepted Answer

Yes. Strict Mode only runs in development, so the double-invocation won't happen in production — but the underlying bug absolutely will. Without cleanup, every component mount in production accumulates stale subscriptions, zombie timers, and dangling event listeners. In long-lived SPAs this manifests as progressive memory growth, duplicate API calls, and stale UI state that worsens over a user session.

Question 3

Is it safe to disable React.StrictMode to stop the cleanup warnings?

Accepted Answer

No. Disabling StrictMode is treating the symptom by removing the diagnostic tool. Your memory leaks and stale subscriptions still exist in production — you just lose the early warning. The correct fix is always to add proper cleanup return functions to your effects. StrictMode compatibility is a prerequisite for React 18 concurrent features like Suspense and useTransition working correctly.

How to Fix React useEffect Cleanup Not Called on Unmount in Strict Mode

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