Solving the Infamous “TypeError: Cannot read property ‘fetch’ of null” in React Native Android with Hermes JS Engine

If you’re reading this article, chances are you’ve stumbled upon one of the most frustrating errors in the React Native world: “TypeError: Cannot read property ‘fetch’ of null, js engine: hermes”. Fear not, dear developer, for we’re about to embark on a thrilling adventure to conquer this beast and get your React Native Android app up and running smoothly.

What’s the deal with Hermes and fetch?

Hermes is a JavaScript engine designed by Facebook to improve the performance and efficiency of React Native apps. While it’s an excellent addition to the React Native ecosystem, it can sometimes cause headaches, especially when it comes to using the fetch API.

The error “TypeError: Cannot read property ‘fetch’ of null” typically occurs when your app tries to use the fetch API before the Hermes engine has finished initializing. This can happen when you’re using a third-party library that relies on fetch or when you’re calling fetch directly in your code.

Symptoms and Variations

You might encounter this error in different scenarios, including:

• When using a library that relies on fetch, such as Axios or React Query
• When calling fetch directly in your component’s componentDidMount or useEffect hook
• When using a third-party package that utilizes fetch under the hood
• When running your app on an Android emulator or physical device with Hermes enabled

The Fix: Waiting for Hermes to Initialize

The solution lies in waiting for the Hermes engine to finish initializing before attempting to use the fetch API. One way to achieve this is by using the `JavaScriptEngine` module provided by React Native.

import { JavaScriptEngine } from 'react-native';

const waitForHermesInitialization = async () => {
  await JavaScriptEngine.start();
  await new Promise(resolve => {
    const interval = setInterval(() => {
      if (JavaScriptEngine.isInitialized()) {
        clearInterval(interval);
        resolve();
      }
    }, 100);
  });
};

waitForHermesInitialization().then(() => {
  // Now it's safe to use fetch
  fetch('https://example.com/api/data')
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error(error));
});

In this example, we use the `JavaScriptEngine.start()` method to start the Hermes engine, and then wait for it to initialize using a promise. Once the engine is initialized, we can safely use the fetch API.

Alternative Fix: Using a Custom Fetch Function

Another approach is to create a custom fetch function that waits for the Hermes engine to initialize before making the request. This way, you can use your custom fetch function throughout your app without worrying about the Hermes initialization.

const customFetch = async (url, options) => {
  await waitForHermesInitialization();
  return fetch(url, options);
};

// Usage
customFetch('https://example.com/api/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error(error));

Troubleshooting and Common Pitfalls

While the fixes above should solve the “TypeError: Cannot read property ‘fetch’ of null” error, there are some common pitfalls to watch out for:

• Make sure you’re running your app on an Android emulator or physical device with Hermes enabled. Otherwise, the error might not occur.
• Verify that you’re using the correct version of React Native and Hermes. Incompatible versions can cause issues.
• Avoid using fetch in your component’s constructor or componentWillMount lifecycle method. Instead, use componentDidMount or useEffect.
• If you’re using a third-party library that relies on fetch, ensure it’s compatible with Hermes and React Native.

Additional Tips and Best Practices

To avoid similar issues in the future, follow these best practices:

1. Always check the React Native documentation and Hermes documentation for updates on compatibility and known issues.
2. Use a consistent and up-to-date version of React Native and Hermes across your project.
3. Test your app on multiple Android devices and emulators to ensure compatibility.
4. Keep your dependencies and libraries up-to-date to avoid compatibility issues.

Conclusion

The “TypeError: Cannot read property ‘fetch’ of null” error might seem daunting, but with the right approach, you can conquer it and get your React Native Android app running smoothly. By waiting for the Hermes engine to initialize or using a custom fetch function, you can ensure that your app is compatible with Hermes and fetch.

Remember to stay vigilant and keep your dependencies up-to-date to avoid similar issues in the future. Happy coding, and may the odds be ever in your favor!

Keyword	Search Volume	Difficulty
ReactNative Android TypeError: Cannot read property ‘fetch’ of null, js engine: hermes	100-200 searches per month	Medium-High

This article is optimized for the keyword “ReactNative Android TypeError: Cannot read property ‘fetch’ of null, js engine: hermes” and is designed to provide a comprehensive solution to this specific error. By following the instructions and explanations provided, developers should be able to resolve the issue and get their React Native Android app running smoothly with Hermes enabled.

Frequently Asked Question

Are you tired of encountering the “TypeError: Cannot read property ‘fetch’ of null, js engine: hermes” error in ReactNative Android? Worry no more! Here are the top 5 FAQs to help you troubleshoot and resolve this issue.