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Learn how to test a JavaScript function that returns a promise using Jest, especially when dealing with timeouts and rejections. Follow our simple guide for solving this common testing issue.
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Testing a Function with Promises in Jest: Handling Rejections Efficiently

When developing JavaScript applications, particularly those with asynchronous operations, testing can get tricky — especially when it comes to functions that return promises. A common scenario involves testing a function that may reject after a timeout, which can lead to unnecessary delays in our unit tests. In this post, we will explore how to efficiently test such functions using Jest, a popular testing framework. We'll break down a practical example of a function that waits for a stream to appear on a page and see how to correctly handle the rejection when it times out.

Understanding the Problem

Let’s look at the function we want to test:

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

In this function, we're attempting to wait for a selector (a stream indicator) on a page. If the selector does not appear within a set timeout, it throws an error.

The Test Challenge

Although writing a test for this function is straightforward, there is a catch: if the test takes time to execute the promise, we end up waiting unnecessarily long. In standard testing configurations, a timeout might delay execution by several seconds, leading to inefficiencies.

Initial Attempts at Testing

One attempt at testing this function, as shown below, leads to long execution times:

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

In this case, the test function would wait for its full duration as expected, making it inefficient. Fortunately, there’s a much better way to handle this using Jest's fake timers.

Solution: Using Jest's Fake Timers

By utilizing Jest's fake timers, we can fast-track the passing of time instead of waiting for actual seconds. Here’s how to set it up for our test:

Step-by-Step Implementation

Setup Fake Timers: Activate Jest's fake timers to control the passage of time.

Call the Asynchronous Function: Immediately call awaitFirstStreamForPage without waiting for it to complete.

Advance the Timers: Use jest.advanceTimersByTime to simulate the timeout period.

Handle the Promise Rejections: Utilize the await expect(promise).rejects.toThrow syntax to confirm that the promise rejects as expected.

Updated Test Code

Here’s the revised version of our test:

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

Key Points to Remember

Handling Uncaught Promise Rejections: Rejections should be caught after calling advanceTimersByTime and running any pending promises with flushPromises().

Effective Use of Fake Timers: By leveraging Jest’s fake timers, we avoid long wait times in our tests while effectively simulating how the function performs under normal conditions.

Conclusion

Testing functions that return promises can be complex, especially when dealing with timeouts and rejections. By following the structured approach discussed above, you can efficiently test asynchronous operations in Jest without unnecessary delays. This not only improves the speed of your tests but also provides clarity on handling expected outcomes during promise rejections, ensuring a smoother development workflow.

With these strategies, you can confidently tackle promise-based functions in your testing, allowing you to focus on building robust applications. Happy testing!