How to Create an Unresolved Promise in Angular Unit Tests Using Karma

Learn how to effectively test Angular promises in your unit tests, ensuring methods are only called after promises resolve.
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Introduction

Writing unit tests for Angular applications can sometimes become tricky, especially when dealing with asynchronous code like promises. If you've ever wondered how to ensure a method gets called only after a promise has resolved, you're in the right place! In this post, we will explore how to create unresolved promises in your unit tests and confirm that certain actions are executed only after these promises are fulfilled.

The Problem

You have a promise function like waitUntilIsReady, which allows your application to execute specific code only when certain conditions are met. However, writing a test to ensure that a method is called only after this promise resolves can be complicated.

Consider the following scenario where you need to check if a method doSomething() in the OtherService is invoked only after waitUntilIsReady() has resolved:

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

To write an effective test, you need to create a promise that does not resolve immediately, allowing you to control when it should resolve during testing. Let's dig into how we can achieve this.

Solution: Structuring the Test

To create an unresolved promise and manually resolve it during your test, follow these organized steps:

Step 1: Setting Up the Test

First, you need to initialize a new promise along with a variable that will hold the function used to resolve that promise.

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

In the beforeEach block, we'll set up our promise and the resolve function:

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

This guarantees that every time you set up a new test, you will have a fresh promise and the ability to resolve it on demand.

Step 2: Writing the Test Case

Now, we can write our actual test case. We will check that doSomething() hasn’t been called yet, resolve the promise, and then ensure that the method was invoked.

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

This test verifies that:

doSomething() has not been called initially.

The promise is resolved, simulating the intended application behavior.

Finally, it asserts that doSomething() was indeed called following the promise resolution.

Conclusion

Testing promises in Angular can be straightforward if you break it down into manageable steps. This approach allows you to gain granular control over when promises resolve in your tests. By creating an unresolved promise and resolving it at your discretion, you can ensure methods are called strictly in their intended sequence.

Building tests is crucial for maintaining the integrity of your applications, and understanding how to work with promises in this way enhances the reliability of your unit tests.

Happy coding and testing!