Скачать или смотреть Resolving Jest Test Error: subscribe is not a function in Angular Apps

Learn how to resolve the Jest test error related to the `subscribe` function in Angular applications, following best practices for mocking Observables in tests.
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Troubleshooting the subscribe is not a function Error in Jest Tests

When working with Jest to test your Angular applications, you may encounter various error messages that throw a wrench into your testing process. One such issue is the confusing error: subscribe is not a function. This error typically arises when trying to mock a service that uses an observable. In this post, we'll clarify why this error occurs and how to resolve it effectively.

Understanding the Problem

In your Angular component, you're likely subscribing to an observable property of a service. The error occurs because the mocked service does not return a correctly structured observable, leading to the test system failing to recognize the subscribe method. Here's a simplified example of what the code might look like:

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In your test, you might have tried mocking the service in a way that doesn't provide the observable's functionality correctly, like so:

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When this test runs during initialization, the system throws: myDummyService.permissions$.subscribe is not a function.

Steps to Resolve the Issue

To properly resolve this error, you'll want to ensure the mocked observable behaves in a way that mimics the real service. Below you'll find methods to create an appropriate mock.

1. Provide an Observable

Instead of using jest.fn() for permissions$, you should provide an actual observable. The simplest way to simulate a non-emitting observable is by using EMPTY from rxjs, which provides a simple, empty observable.

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This will give permissions$ all the standard observable methods, including subscribe, pipe, and more, thus eliminating the error.

2. Using Subjects for Emission Control

If you need your mock to emit values, consider using Subject. A Subject is a type of observable that you can manually emit values to, giving you greater control over the test's behavior.

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You can then control the emissions by calling mockPermissionsSubject.next(value), allowing you to test various scenarios based on emitted permissions.

Conclusion

By following the steps outlined above, you can easily fix the subscribe is not a function error in your Jest tests for an Angular application. Always ensure that you’re mocking observables accurately, either through predefined options like EMPTY or through subjects if you need to emit values within your tests. These best practices will not only help you avoid common pitfalls in testing but also lead to more robust and reliable test setups.

By understanding how to mock observables correctly, you’ll be well on your way to writing cleaner, more effective tests for your Angular applications.