Solving the function did not throw Error in Jest When Testing Async Functions

Learn how to properly test async functions with Jest by ensuring your custom error classes extend the built-in Error class, preventing the 'function did not throw' issue.
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Troubleshooting Jest Tests for Asynchronous Functions

Testing asynchronous functions can sometimes feel like navigating a maze, especially when things don't go as expected. If you've recently encountered the frustrating error message "function did not throw" while using Jest to test an async function, you're not alone. In this post, we'll dissect the issue and provide a clear solution to get your tests passing once again.

The Problem: Testing for Errors in Async Functions

Imagine you have an async function that is supposed to throw an error when certain conditions aren't met. In your case, the function looks like this:

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

You've written a test to ensure that when foo(false) is called, it throws an UnexpectedRequestError. Your test code looks like this:

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

However, when you run this test, you receive the following error message:

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

You start wondering what went wrong. Let’s explore the potential causes and arrive at the solution.

Understanding the Root Cause

The key issue here is that your custom error class, UnexpectedRequestError, does not inherit from the built-in Error class. In JavaScript (and TypeScript), if you want to create custom errors that behave properly with the try/catch blocks or testing libraries like Jest, they need to extend the native Error class.

Why is Inheritance Important?

When a custom error class extends Error, it ensures that:

The error behaves as expected within expected environments such as async functions and try/catch blocks.

The stack trace provides useful debugging information since it will show the location where the error was thrown.

The Solution: Fix the Error Class

To resolve the issue, you need to modify your UnexpectedRequestError class as follows:

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

Steps to Implement the Solution:

Update Your Error Class: Make sure your custom error class extends the built-in Error class.

Update the Constructor: Be sure to call the super constructor with the error message. This will properly initialize the error.

Run Your Tests Again: After making this change, run your test suite again to see if the issue is resolved.

Conclusion: Successfully Testing Async Functions in Jest

By ensuring your custom error classes extend the native Error class, you can create robust tests for your asynchronous functions with Jest. This change not only resolves the "function did not throw" error but also improves the overall reliability of your error handling in the application.

Now you are equipped with the knowledge to avoid similar pitfalls in future testing scenarios. Happy coding and testing!