Understanding the TypeError: Cannot read properties of undefined in Node.js Testing with Jest

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The Problem

You are attempting to test a handler function in your Express application, but running your Jest tests results in the error mentioned above. Let's take a look at the relevant code to understand where things may have gone wrong.

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Test Code

Here’s an excerpt from your test code where the error occurs:

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Understanding the Cause

Key Point to Remember

You should reference your exported functions directly, or wrap them correctly in a named object before exporting.

The Solution

To resolve this issue, you have two options.

Option 1: Access the Function Directly

Change your test code to access the exported function directly, like this:

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With this change, your test should find the homepageHandler function if it’s exported correctly.

Option 2: Properly Structure Module Exports

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In this case, your test code remains as it is because indexExports is now a defined property of the exported object.

Testing the Change

After applying either of the above changes, rerun your Jest tests. You should no longer see the TypeError and the test should execute successfully.

Here’s how your test might look with both approaches:

Using Direct Access

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Using the Object Structure

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Conclusion

If you found this post helpful, feel free to share it or ask any questions in the comments below!