Why Does My useEffect Run Twice in React 18's StrictMode, and How Can I Fix It?

useEffect Running Twice: Handling It Effectively in React

Why does useEffect get called twice on mount in React 18 when in development mode with StrictMode enabled?

Understanding the Reason

React 18, with StrictMode activated, remounts components during the initial render to enhance performance and aid in bug detection. This remounting ensures that components can successfully handle both mounting and unmounting multiple times.

Effects Run Multiple Times: Is It an Issue?

Most effects function correctly despite multiple mounts and unmounts. However, certain effects may encounter issues if they neglect to properly clean up subscriptions or assume a single mount-unmount lifecycle.

Typical Scenarios Where Multiple Calls Are Problematic

Common situations where multiple effect calls can cause issues include:

• HTTP Requests: Effects that make server requests may send redundant requests and lead to unexpected behavior.
• Interval-Based Effects: Effects that involve setting intervals may keep running after the component unmounts, causing memory leaks and unexpected results.

Solutions for Handling Multiple Calls

Rather than inhibiting StrictMode or using complex workarounds, React encourages developers to implement robust effects that handle multiple calls gracefully. This involves implementing cleanup functions that counteract the effect's setup actions.

Cleanup Functions

Cleanup functions typically:

• Stop or undo the effect's operations.
• Prevent side effects from persisting after component unmount.
• Abort requests, clear intervals, or cancel any subscriptions created by the effect.

Example: Handling Multiple HTTP Requests

Here's an example of handling multiple HTTP requests in useEffect:

useEffect(() => {
  const abortController = new AbortController();

  const fetchUser = async () => {
    try {
      const res = await fetch("/api/user/", {
        signal: abortController.signal,
      });
      // Process the response data
    } catch (error) {
      if (error.name !== "AbortError") {
        // Handle non-aborted errors
      }
    }
  };

  fetchUser();

  return () => abortController.abort();
}, []);

Benefits of Robust Effects

Robust effects that handle multiple calls effectively:

• Enhance performance and prevent memory leaks.
• Ensure bug-free behavior even in development mode when remounting becomes evident.
• Align with React development guidelines and compatibility with future versions.
• Facilitate more reliable and maintainable code.

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