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

useEffect Triggering Multiple Times in React 18: Causes and Resolution

In the development environment of React 18, the useEffect hook can sometimes be invoked twice upon initial component mounting. This behavior stems from React's preparation for future UI state preservation features.

Reason for Double Invocation:

React anticipates a scenario where it needs to remount trees while preserving their state for a seamless user experience. For example, when switching tabs and returning to a page, React aims to display the exact screen as before. To facilitate this, it tests component resilience by unmounting and remounting them during initialization.

Development vs. Production Behavior:

It's important to note that this double invocation occurs exclusively in development mode. In production environments, useEffect behaves as expected, being invoked only once per mount.

Handling the Issue:

While the double invocation may seem unusual, it presents an opportunity to enhance code quality and align it with future React versions. Typically, useEffect should be resilient to multiple invocations and cleanup properly to prevent memory leaks.

Example: Interval Inside useEffect:

Consider an example where an interval is set within useEffect. To prevent potential issues in Strict Mode, the cleanup function should clearly terminate the interval once the component unmounts.

useEffect(() => {
  const id = setInterval(() => setCount((prevCount) => prevCount + 1), 1000);
  return () => clearInterval(id);
}, []);

Alternative Approaches (Not Recommended):

Some may suggest using useRef and conditional statements within useEffect to limit its execution to once. However, React explicitly discourages such practices, emphasizing the importance of implementing the cleanup function instead.

API Calls Inside useEffect:

If useEffect contains API calls, consider using fetch with an AbortController. The controller allows for aborting requests that are no longer relevant when the component unmounts. This prevents state updates on unmounted components and enhances performance.

Conclusion:

Understanding the reason behind the double invocation of useEffect in React 18 and employing appropriate handling techniques can lead to more robust and bug-free code. Regardless of its temporary nature in development mode, it's an excellent opportunity to strengthen component resiliency and ensure seamless operation in future React releases.

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