Understanding Jest Unit Testing for Asynchronous Functions Using Promises

Learn how to effectively use Jest to unit test functions that return promises, ensuring assertions are made only after the asynchronous code completes.
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Unraveling Jest Unit Testing for Asynchronous Functions

When it comes to writing unit tests for JavaScript applications, dealing with asynchronous functions can be a bit tricky, especially when they return promises. In this guide, we’ll explore how to properly test a function that calls another function which returns a promise using Jest, and ensure that our assertions are made at the right time.

The Problem: Testing Asynchronous Behavior

In the scenario we'll discuss, we have a function called functionA that invokes another function, functionB, which returns a promise. This promise can either resolve or reject, leading to different code paths in functionA. We want to write unit tests that assert the appropriate behavior based on the outcome of functionB. Specifically, we need a way to confirm that the external function, External.functionC, is being called under correct conditions, and that our tests are accurately reflecting the asynchronous nature of the code.

Example Code

The following is a simplified version of the code we’re dealing with:

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

The Solution: Making Jest Wait for Promises

The crux of the issue is that Jest does not inherently wait for asynchronous code to process before making assertions. This can lead to unexpected results in our tests. To resolve this issue, we can create a helper function that makes Jest wait until all the promises have been resolved before proceeding. This will ensure our tests accurately reflect the expected behavior.

Using a Wait Function

To facilitate this, we can create a waitForPromises function that utilizes setImmediate, which effectively defers execution of the promises until the next event loop cycle:

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

Implementing the Wait Function in Our Tests

We can now use waitForPromises in our tests like so:

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

Conclusion

By implementing the waitForPromises function, we can effectively synchronize our tests with the asynchronous code being executed. This way, we ensure that our assertions accurately reflect the state of the application after all promises have been resolved. Adopting this practice will lead to more reliable unit tests that can correctly handle asynchronous behavior, making the testing process smoother and more effective.

Now you have the tools you need to confidently write Jest unit tests for functions that return promises, ensuring that your tests will perform as expected regardless of the asynchronous nature of the code.

Happy testing!

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