Скачать или смотреть Testing Value Changes from Subscriptions in Angular with rxjs

Learn how to effectively test value changes in Angular components when using `rxjs` subscriptions with our comprehensive guide.
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Testing Value Changes from Subscriptions in Angular

When working with Angular, particularly when using rxjs for managing asynchronous operations, you may encounter scenarios where you need to verify changes to component properties after an observable emits a value. A common use case involves testing changes triggered by API calls within your components. This guide addresses the challenge of writing tests to confirm that a variable properly reflects the outcome of an API call.

The Problem: Testing Subscription Value Changes

In a typical Angular component, you might have a method that performs an API call and consequently updates a component property based on the response. Below is a simplified code example:

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

The need arises to ensure that the postStuff() method not only calls the API but also updates this.variable correctly to reflect the success of the operation.

Imagine you are writing a test to validate this behavior. Your initial attempt might look something like this:

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However, you find that the test fails when checking if component.variable is true, and you're left wondering what went wrong.

The Solution: Adjusting the Test

1. Remove callThrough()

The first step to resolving this issue is to eliminate the .and.callThrough() method from your spy on the postStuff method. The callThrough() method allows the actual implementation to run, which can lead to unexpected results in your test scenario.

2. Use and.returnValue(of(true))

Instead of letting the actual method execute, return a controlled outcome. You can use and.returnValue() combined with of() from rxjs to simulate the observable's emission:

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

This adjustment allows the method to return an observable that immediately emits a value, ensuring your subscription logic can execute as intended.

3. Handle fakeAsync() and tick()

In your original test, the use of fakeAsync() and tick() might have seemed necessary. However, since you are emitting a value directly, they are not required in this case. The of() method handles the async nature of observables automatically.

Final Test Code

Putting it all together, your test case should look something like this:

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

Conclusion

By following these steps—removing callThrough(), using and.returnValue(of(true)), and simplifying your asynchronous handling—your test will appropriately verify that your component's variable changes in response to an API call.

Key Takeaways

Use and.returnValue(of(...)) to return a simulated value from your service.

Eliminate unnecessary async handling when dealing with observables that emit immediately.

Always verify proper method calls and their effects on component state.

Now you're equipped to write effective tests for your Angular components that rely on rxjs subscriptions. Happy coding!