Скачать или смотреть Resolving the TypeError: Cannot read property 'subscribe' of undefined in Angular Testing

Discover how to troubleshoot the error message `Cannot read property 'subscribe' of undefined` in your Angular tests using Karma and Jasmine. Learn the right approach to mock HTTP calls effectively!
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This video is based on the question https://stackoverflow.com/q/68459795/ asked by the user 'Lucas Lima' ( https://stackoverflow.com/u/16200469/ ) and on the answer https://stackoverflow.com/a/68460230/ provided by the user 'AliF50' ( https://stackoverflow.com/u/7365461/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting TypeError: Cannot read property 'subscribe' of undefined in Angular Testing

During Angular testing, you may encounter a frustrating error: TypeError: Cannot read property 'subscribe' of undefined. This issue can happen when you're trying to test an API call in your service but encounter problems mocking HTTP calls correctly. In this guide, we’ll explore how to identify and resolve this error effectively.

Understanding the Problem

You are attempting to write a unit test for a service's method that makes an HTTP GET request. The intention is to create a mock response that allows you to verify the behavior of your code. However, due to incorrect mocking of the HTTP client, you receive an error that informs you that your observable is undefined, thus leading to the failure when trying to call subscribe().

Example Scenario

Suppose your Angular service has the following function:

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

You then write a unit test in your spec.ts file, but when running the test, you encounter the error mentioned before.

Analyzing the Spec File

Your test file contains code that is attempting to mock the HTTP request:

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

Solution Breakdown

Issue with Mocking

The main issue you’re facing stems from the use of both and.returnValue() and and.callThrough(). These two methods are mutually exclusive in mocks:

and.returnValue(value) will return a specific mocked value when the function is called.

and.callThrough() allows the actual function to be called, which is not suitable here since you want to mock the behavior of httpClientSpy.

Proposed Fix

To resolve the issue and eliminate the subscribe error, you should modify the mocking line in your test as follows:

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

Updated Spec Example

After making this change, your updated test should look like this:

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

Conclusion

By simplifying the mock setup in your Angular unit tests, you can avoid the TypeError: Cannot read property 'subscribe' of undefined error. The crucial takeaway is to ensure that when you mock HTTP calls, you use either and.returnValue for a mock return or and.callThrough for real function execution, but not both. Now you’re equipped to proceed with testing your Angular application smoothly!

For more insights and troubleshooting tips related to Angular testing, stay tuned to our blog!