Why useEffect Running Twice and How to Handle It Well in React?

React's useEffect hook is a powerful tool for managing side effects in functional components.

However, one baffling behavior that developers often encounter is useEffect running twice in strict mode during development.

This article will demystify why this happens and provide strategies for handling it effectively.

Understanding useEffect

Before diving into why useEffect runs twice, let's briefly revisit what useEffect is for. useEffect allows you to perform side effects in your components, such as data fetching, subscriptions, or manually changing the DOM.

It's the functional component equivalent of the componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods in class components.

useEffect(() => {
  // Side effect logic here
}, [dependencies]);

The second argument, the dependency array, tells React when to re-run the effect. If the array includes values, the effect runs when those values change. If it's empty, the effect runs once after the initial render.

The Double Invocation Mystery

In React 18 and later, you might notice useEffect running twice during development. This behavior is intentional and occurs only in strict mode.

React's strict mode is a development tool that helps you write better, bug-free code by highlighting potential problems in your application.

Why Does It Happen?

The double invocation of useEffect in strict mode is designed to help developers detect side effects that are not properly cleaned up.

By intentionally unmounting and remounting the component, React ensures that any cleanup functions are correctly implemented. This can catch issues like memory leaks or subscription duplications early in the development process.

It's crucial to understand that this behavior is only present in development mode. In production builds, useEffect runs once as expected, ensuring that there's no performance impact on your live application.

Handling the Double Run

Here are some strategies to handle the double invocation of useEffect effectively:

1. Embrace Idempotency

Ensure that your effects are idempotent. This means that running your effect multiple times does not result in different outcomes.

Idempotent effects are not only safer when useEffect runs twice but are also more predictable and easier to debug.

useEffect(() => {
  const subscription = myAPI.subscribe(data => {
    console.log(data);
  });

  return () => subscription.unsubscribe();
}, []);

2. Conditionally Fire Effects

Sometimes, effects do not need to run on every render. By carefully managing your dependency array, you can control when your effects run.

However, remember that omitting dependencies to prevent double invocation can introduce bugs. Always include all variables used inside useEffect in the dependency array.

3. Use a Development-Only Flag

If a specific effect is problematic when run twice during development, you might consider using a development-only flag to disable it on the second run.

However, use this approach sparingly, as it can mask problems that would otherwise be caught by strict mode.

const hasRun = useRef(false);

useEffect(() => {
  if (process.env.NODE_ENV === 'development' && hasRun.current) {
    return;
  }

  // Effect logic here

  hasRun.current = true;
}, []);

4. Optimize for Strict Mode

Instead of working around the double invocation, optimize your effects to work correctly under these conditions.

This approach ensures that your code is robust and adheres to best practices, reducing the likelihood of bugs.

5. Testing and Debugging

Leverage testing libraries like Jest and React Testing Library to simulate strict mode behavior. Writing tests that account for component mounting and unmounting can help ensure that your components behave correctly.

Conclusion

The double invocation of useEffect in strict mode is a feature, not a bug. It's designed to help developers identify and fix side effects that might lead to problems in their applications.

By understanding why this behavior occurs and adopting strategies to handle it effectively, you can ensure that your React components are both efficient and bug-free.Remember, the goal is not to circumvent React's development mode checks but to write code that naturally handles these scenarios.

Embracing strict mode's constraints can lead to more reliable and maintainable React applications. So, next time you see useEffect running twice, take it as an opportunity to scrutinize your side effects and make your application better.