Why Does My useEffect Hook Run Twice in React 18 Development Mode?

Understanding the Double useEffect Calls in React 18

When using useEffect hooks in React 18 with StrictMode, you may notice that the hook gets called twice on initial mount. This can raise concerns, especially if you observe unexpected behavior in your application.

The Rationale Behind Double Calls

According to the React documentation, this is intended behavior introduced in React 18 to prepare for future improvements involving state preservation and efficient UI updates. By remounting components on mount, React aims to ensure that your code is resilient to effects being mounted and unmounted multiple times.

Development vs. Production Behavior

It's important to note that this behavior is specific to development mode with StrictMode enabled. In production mode, only a single call is made.

Resolving Effects Calling Twice

If you face issues with this behavior, consider the following approaches:

1. Handle the Cleanup:

Ensure your useEffect has a cleanup function to perform necessary operations when the component is unmounted. This prevents issues like canceled HTTP requests or memory leaks.

2. Avoid StrictMode in Production:

If double calls are causing significant issues, you can disable StrictMode in production, as the behavior is limited to development mode.

3. Cache HTTP Requests:

To prevent duplicate HTTP requests in development, utilize libraries that cache and deduplicate requests.

Best Practices for useEffect

While this issue may require attention, it also serves as an opportunity to revisit your useEffect usage and adopt best practices:

• Use useEffect to listen to changes and perform side effects.
• Avoid initializing state inside useEffect.
• Use the cleanup function to clean up resources on unmount.

Remember, the double useEffect calls are intended to uncover potential bugs in your code, allowing you to write robust, production-ready React applications.

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