React 18 useEffect: Why It Runs Twice and How to Fix It

If you recently upgraded to React 18, you likely noticed a confusing behavior: your console logs show your effects firing two times. React 18 useEffect is designed to mount, unmount, and remount your components during development. While this feels like a bug, it is actually a deliberate feature to help you write more resilient code.

At WeBlogTrips, we focus on modern development standards. Consequently, we created this guide to explain the logic behind the React 18 useEffect double-firing and how you can handle it properly without disabling essential safety checks.

Quick Summary: React 18 useEffect Double-Mounting

1. React 18 useEffect Runs Twice to Surface Bugs

The primary reason React 18 useEffect runs twice is to ensure your components handle mounting and unmounting correctly. In development, StrictMode intentionally simulates a “remount” to verify that your cleanup functions work as intended.

If your application breaks because React 18 useEffect runs twice, it usually reveals a missing cleanup function. React forces this behavior to prepare your app for future features, such as “Offscreen Rendering,” where components might preserve state while hidden.

2. Fixing the React 18 useEffect Double-Fetch Issue

Many developers worry about duplicate API calls. However, the React team suggests that you should not try to prevent the effect from running twice. Instead, you should make your effect resilient.

To fix issues when React 18 useEffect runs twice, you should implement an AbortController. This tool cancels the first request when the component unmounts, ensuring that only the final request updates your state. This approach keeps your UI synchronized even during rapid navigation.

3. Best Practices for React 18 useEffect Cleanup

The key to mastering React 18 useEffect lies in the return function. Every time you set up a subscription, timer, or manual DOM change, you must provide a way to undo it.

• Timers: Use clearTimeout or clearInterval.
• Subscriptions: Unsubscribe from your data stream.
• Event Listeners: Use removeEventListener to prevent memory leaks.

By ensuring your setup and cleanup logic are symmetrical, you stop worrying about React 18 useEffect firing twice because the “teardown” happens automatically.

Frequently Asked Questions (FAQ)

1. Should I just disable Strict Mode?

You can disable it by removing <StrictMode> from your index.js, but we don’t recommend this. Strict Mode helps you find subtle bugs that are much harder to debug in production.

2. Does this affect production performance?

No. The double-firing of React 18 useEffect is a development-only check. Once you build your app for production, your effects will only run once upon mounting.

3. Why am I seeing an “Apple Security Warning” in my dev tools?

If your React 18 useEffect logic attempts to fetch data from an insecure http endpoint during its remounting phase, you might trigger an Apple Security Warning on your iPhone. Always use secure protocols even in local development.

Final Verdict: React 18 useEffect

To summarize the React 18 useEffect behavior:

• React runs effects twice to catch missing cleanup logic.
• Use an AbortController to manage duplicate API calls.
• Always return a cleanup function to keep your app stable.

By embracing this pattern, you ensure your React applications are ready for high-performance concurrent rendering.

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External Links

1. React.dev: Synchronizing with Effects: The official documentation on the effect lifecycle.
2. Sentry: React useEffect running twice: A deep technical explanation of the remounting process.
3. MDN: AbortController: How to cancel web requests in JavaScript.