How to Fix TypeError: undefined is not a function in React Native with Hermes Engine

Learn how to resolve the `TypeError: undefined is not a function` error in React Native applications using the Hermes JavaScript engine.
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How to Fix TypeError: undefined is not a function in React Native with Hermes Engine

React Native is a popular framework for building mobile applications using JavaScript and React. One of the powerful features it offers is the Hermes engine, a JavaScript engine optimized for React Native. Despite its many advantages, developers sometimes encounter the unsettling TypeError: undefined is not a function error while using Hermes.

Understanding the Error

The error message TypeError: undefined is not a function typically suggests that the code is attempting to invoke a function that is not defined or accessible in the current scope. This can happen for various reasons, especially when using a specialized JavaScript engine like Hermes.

Common Causes

Unsupported JavaScript Features
The Hermes engine does not support all JavaScript features. Ensure you are not using any JavaScript functionality that is not supported by Hermes. Refer to the official Hermes documentation to confirm feature support.

Incorrect Module Imports
Sometimes this error can be caused by improperly importing modules or using functions that are not part of the imported module.

Coding Errors
Simple mistakes in your code, such as typos or incorrect logic, can inadvertently trigger this error.

Metro Bundler Issues
Hermes heavily relies on the Metro bundler, and sometimes the bundler might not serve the modules correctly due to caching issues or other configuration problems.

Steps to Resolve the Issue

Step 1: Check Your Code for Mistakes
Review your code to ensure there are no typographical or logical errors. Verify that functions you intend to call are actually defined and within the correct scope.

Step 2: Ensure Compatibility with Hermes
Make sure that the JavaScript features you are using are supported by Hermes. If a feature is not supported, consider alternative approaches or polyfills.

Step 3: Verify Module Imports
Check that your imports are correct and complete. Verify the modules you are importing contain the functions you are trying to use.

Step 4: Clear Metro Bundler Cache
Sometimes, clearing the cache of the Metro bundler helps resolve the issue. You can do this by running:

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Step 5: Restart the Development Server
It's always a good idea to restart your React Native development server to ensure all changes take effect. You can stop the server, clear any lingering state, and start it again.

Step 6: Check Hermes Documentation
Refer to the official Hermes documentation for any updates or additional troubleshooting tips specific to the Hermes engine.

Conclusion

Encountering the TypeError: undefined is not a function error in React Native while using Hermes can be frustrating, but it is often rectifiable through careful code review and ensuring compatibility with the Hermes engine. By following the steps outlined above, you should be able to diagnose and fix the root cause of the error, allowing you to continue with your React Native development smoothly.