Скачать или смотреть Resolving Expectation Fails in JavaScript Fetch with Jasmine Testing

How to fix expectation failures in JavaScript testing when using `fetch` with Jasmine and Angular. Learn best practices and solutions for effective asynchronous testing.
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Understanding the Expectation Fails Issue with JavaScript Fetch in Jasmine Testing

Testing asynchronous code can often be tricky, especially when utilizing promises and methods like fetch. One common issue developers face is when expectations fail, even when it seems like the method should have been called. In this post, we’ll break down a specific case involving window.open() in a then block of a fetch call and how you can fix it.

The Problem

You might find yourself in a situation where you want to verify that window.open() gets called after a successful fetch operation. Below is a minimal example of the code that demonstrates this problem:

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When running this test, you may receive the error:

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This occurs even though window.open() is actually being called. The main issue here lies with the handling of promises created by fetch, which are not controlled by tick().

The Solution: Modify How You Handle Fetch

Step 1: Spy on the fetch Method

To address this issue, you should spy on the fetch method and ensure that it returns a promise that resolves immediately. This allows the then block to execute as expected during the test. Here’s how to implement this solution:

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Key Changes Explained

Spying on window.fetch: By spying on the fetch method and having it return a resolved promise, you're simulating the fetch call without making an actual HTTP request. This is crucial for unit testing, where you often want to isolate the unit of work from external dependencies.

Using tick(): The tick() function is called after invoking the asynchronous operation. This allows the test to simulate the passage of time, helping the promise to resolve within the test environment.

Best Practices to Keep in Mind

Avoid Actual HTTP Calls in Unit Tests: It’s generally advised not to make real HTTP calls during unit tests. Spying on functions like fetch ensures that your tests remain fast, reliable, and do not depend on external factors.

Use Mocks for Enhanced Control: Mocking network requests helps maintain control over your tests and isolates them from external influences such as network latency or server errors.

Conclusion

By utilizing a spy on the fetch method and modifying how promises are handled, you can resolve the expectation failures you're encountering in your Jasmine tests. This approach not only helps in passing the tests but also adheres to good testing practices that keep your tests fast and reliable.

Implement this solution in your testing suite, and you should find that your expectations hold up even when dealing with asynchronous code!