Resolving TypeError: Cannot read property 'then' of undefined in Jest and NestJS Testing

Learn how to fix the common error `TypeError: Cannot read property 'then' of undefined` encountered in Jest and NestJS unit testing.
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Troubleshooting the TypeError: Cannot read property 'then' of undefined Error in Jest and NestJS

When writing unit tests for your NestJS API, you might come across several hurdles, one of which is the frustrating TypeError: Cannot read property 'then' of undefined. This error typically arises when a method you are trying to mock doesn't properly return a promise. In this guide, we’ll explore how to effectively identify and fix this issue in your unit tests.

Understanding the Problem

You might face this error in scenarios like when simulating a service class method in Jest testing. Let's consider the situation where you are trying to write a test for user creation logic within your UserService. If your test encounters the TypeError, it generally indicates that a mocked function didn't resolve or return the expected promise.

The Code Context

Let's take a look at the core parts of our service and testing code:

Your Service Code

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Your Test Code

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The Error Trace

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From the trace, it’s clear that the issue lies in the implementation of the save method within your test setup.

Solution: Fixing the Mock Implementation

The root cause of this error is that the save() method of your user repository is not properly returning a promise. To resolve this, you can adjust your mocking strategy to ensure it provides a resolved value correctly.

Here’s how to fix it:

Modify the mock implementation of the save method in the beforeEach block of your test as follows:

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Explanation of the Changes

With these adjustments in place, your tests should now run without throwing the TypeError, correctly simulating the behavior you expect from the repository methods.

Conclusion

In conclusion, encountering a TypeError: Cannot read property 'then' of undefined in Jest tests for NestJS applications often stems from improperly mocked methods that don't return promises. By ensuring that you return resolved promises from your mocked functions, you can set yourself up for success in your unit tests. Happy coding!