Solving the TypeError: Cannot read properties of null in Your Create React App with JSDOM

Discover how to troubleshoot the common TypeError in Create React App related to JSDOM and Firebase analytics. Learn how to efficiently mock functions for testing without complications.
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Troubleshooting the TypeError: Cannot read properties of null in Create React App Tests

If you've been working with Create React App (CRA) and Jest for testing, you might have encountered a frustrating issue: the TypeError: Cannot read properties of null (reading 'URL') error originating from JSDOM. This error can halt your testing process, especially when it appears unexpectedly during your test runs. In this guide, we will delve into the problem and walk you through the solution step-by-step, making your testing experience smoother and more efficient.

Understanding the Problem

When running tests in your Create React App project, you may see an error like the following in your console:

[[See Video to Reveal this Text or Code Snippet]]

This typically points to a problem with how JSDOM is managing its environment. Specifically, this error can occur when there are interactions with elements or functions that expect a fully-realized document, but find itself in a broken or incomplete state.

The Stack Trace Insights

From the stack trace you might find in your console, you can often identify dependencies that might be causing issues. In this case, you might notice the name of the firebase package appearing in the trace. This is not coincidental, and can typically provide clues as to what might need to be adjusted in your code.

The Solution: Mocking the getAnalytics Function

After hours of debugging and trial and error, the root cause of the issue was revealed: the getAnalytics function from the firebase/analytics package was the culprit. Since it's not necessary for your tests, mocking this function can eliminate the error and smooth out your testing process.

Steps to Mock the getAnalytics Function

[[See Video to Reveal this Text or Code Snippet]]

This simple mock tells Jest to replace the actual implementation of getAnalytics with a mock function that does nothing.

Run Your Tests: Now that you have mocked the offending function, you can go ahead and run your tests. The TypeError should no longer appear, allowing your test suite to function correctly.

Conclusion

In summary, unable to load or reference properties associated with null can often stem from issues related to third-party libraries like Firebase in conjunction with testing environments like JSDOM. By mocking these functions when they are not required for testing, you can effectively resolve your issues and get back to focusing on building wonderful applications with React and CRA.

If you encounter any other errors or have questions about enhancing your Jest testing strategies, feel free to leave your comments below. Happy coding!