Why is my useEffect Hook Triggering Twice on Component Mount in React 18 Development Mode?

useEffect Triggering Twice on Component Mount

Issue:

When using useEffect with a dependency array containing only state, the effect function is invoked twice on initial mount in React 18 development mode.

Explanation:

React 18 introduced a feature that supports remounting components with the same state, which allows for better performance out of the box but requires components to handle effects being mounted and destroyed multiple times.

To surface potential issues, React automatically unmounts and remounts every component once during development mode. This remounting triggers the useEffect function a second time.

Solution:

This behavior is intended and designed to uncover existing bugs in effect logic. The correct approach is to adjust the effect implementation to handle multiple mountings correctly.

Recommendations:

1. Use Cleanup Functions:

Implement the useEffect cleanup function to stop or undo the effect's actions when the component unmounts. This ensures that the effect's impact on the component's state and side effects is consistent between production and development modes.

2. Cache HTTP Requests:

Employ techniques for deduplicating and caching HTTP requests to optimize performance and avoid redundant network operations. Consider using data fetching libraries or hooks that implement cache mechanisms.

3. Review Your useEffect Usage:

Ensure that useEffect is used appropriately and not for initializing application state or performing actions that should not be repeated on remount. Refer to the "Not an Effect" principles for guidance:

• Not an Effect: Initializing the application
• Not an Effect: Buying a product

Additional Considerations:

• This behavior only occurs in development mode. In production, useEffect will be called only once.
• Attempting to work around the remounting behavior using useRef and if statements in useEffect or removing StrictMode is discouraged.
• Reading the detailed React documentation on useEffect can provide further insights on the topic.

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