Next.js Issue: useEffect Hook Running Twice in Client

Understanding the Issue

When using React’s useEffect hook in a Next.js application, you might encounter a behavior where the useEffect function runs twice during the initial render in development mode. This is deliberate and part of the React Strict Mode features, which is automatically enabled in Next.js during development to help you write better React components. Strict Mode double-invoking of lifecycle methods (including hooks like useEffect) is intended to help surface unexpected side effects in the render phase and is only present in development mode, so it will not affect your production build.

Disabling Strict Mode

One simple way to stop useEffect from running twice on initial render is to disable React Strict Mode. While this isn’t generally recommended because Strict Mode provides valuable warnings and checks, sometimes during development it could be distracting or not helpful for your specific use case. You can disable Strict Mode by modifying the next.config.js file. Note that in Next.js Version 13 or newer, you’ll typically work with ‘app’ and ‘page’ components, but this setting is still relevant. Open your next.config.js and update the configuration to turn off React Strict Mode:

module.exports = {
  reactStrictMode: false,
};

After doing this, restart your development server to apply the changes. Keep in mind that while this solves the immediate annoyance, disabling React Strict Mode removes a layer of checks that can help prevent future bugs.

Using React 18 Features

In Next.js 13, which leverages React 18, another way to control how and when effects run is to make use of the new API for transitioning to new states. React 18 introduced startTransition, which can help to ensure certain actions, like setting states that trigger effects, do not lead to urgent updates, helping you dictate the flow of rendering more precisely.

Nonetheless, the true resolution lies in understanding that the behavior you’re observing is intentional in development for identifying pitfalls in your logic. Adjusting the logic to accommodate multiple invocations and ensuring side effects are safe to run more than once is fundamentally the best fix.

Example

Let’s consider a simple example that logs a message to the console upon component mount. Instead of changing settings or leveraging React 18 features, we could simply ensure our effect is idempotent (produces the same result even if run multiple times).

import { useEffect } from 'react';

export default function HomePage() {
  useEffect(() => {
    console.log('Component mounted.');

    // This could be an API call or a subscription, instead, ensure
    // that it can be called multiple times without side effects,
    // or utilize a cleanup function if appropriate.
  }, []);

  return (
     <h1>Sling Academy</h1>
  );
}

Notice that our useEffect hook depends on an empty dependency array, indicating it should run once after the component mounts. Even though in development it runs twice, it produces the same result without issue. It’s essential to make effects safe to run multiple times if they are related to setting up or cleaning up subscriptions and other side effects.

Conclusion

To sum up, the useEffect running twice in Next.js during development does not indicate a bug but rather React exercising components under stringent conditions to identify potential issues. It can be circumvented by disabling React Strict Mode, but the key response is to assure any side effects are safe to execute multiple times or resort to cleanup functions and dependencies correctly. Testing and habituating your components under these conditions ultimately ensures more resilient and robust component implementation for your Next.js project.