How to Unit Test mergeMap in a Service Call Using Angular and RxJS

Learn how to effectively unit test a service call using `mergeMap` in Angular. This guide provides insights into setting up your tests to ensure both API responses are handled correctly.
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Understanding How to Unit Test mergeMap in Angular Service Calls

Unit testing can sometimes feel overwhelming, especially when dealing with complex service calls that involve multiple API requests. In this guide, we’ll unravel the challenges of unit testing a service method that utilizes mergeMap to call two APIs. We'll walk through a detailed example and explain how to ensure that both API calls are successfully tested.

The Problem

Imagine you have a service method that needs to fetch data from two different APIs in sequence using RxJS mergeMap. You may find that your unit tests are only validating the first API call, leaving you puzzled about how to incorporate the second API call effectively.

This scenario is common, especially when you are new to testing asynchronous code in Angular. To illustrate, let’s look at an example of an Angular service method:

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

Here, the getRequest() method successfully merges two API calls. Now, let’s explore how to test this method properly.

The Solution

Step 1: Update Your Test Framework

First, ensure that your test framework is set up correctly. Your test should wait for asynchronous operations to complete before it concludes. By changing the parameter of your test to include done, you can signal when your test has finished. Here’s how you can modify the existing test:

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

Step 2: Invoke the Second API Call

In addition to testing the first API call, you also need to simulate the second API response correctly. After the initial flush() call for the first API, add another flush implementation for the second API like so:

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

Complete Test Case

Here’s a complete example of how your test case might look:

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

Conclusion

By following the steps outlined above, you can effectively unit test service methods that utilize mergeMap for API calls in Angular. Always ensure you handle asynchronous operations in your tests correctly, and don't forget to provide specific expectations for the output after both API calls complete.

With these tips, you're better equipped to tackle unit testing service calls in Angular, ensuring you thoroughly test all aspects of your code.