How to Effectively Test super in Your Custom Error Class in Typescript

Learn how to write comprehensive tests for your custom error classes in TypeScript, focusing on testing the `super` call and ensuring all properties are accurately returned.
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How to Effectively Test super in Your Custom Error Class in Typescript

When developing applications in TypeScript, the introduction of custom error classes can significantly streamline error handling. However, writing tests for these classes, particularly testing the super constructor, can often be a challenge. This guide will provide you with a step-by-step guide on how to effectively test your custom error classes, focusing specifically on how to cover the super call, and confirm that all properties are accurately returned when an instance is created.

Understanding Your Custom Error Class

Before we dive into testing, let’s break down the example of a simple custom error class you're working with:

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In this example:

BarcodeDecoderError extends the built-in Error class in TypeScript.

It takes three parameters: message, barcode, and errorType, with the former being passed to the super constructor to set the error message.

Now that we have a clear understanding of the code, let’s move on to how you can test this class effectively.

Writing the Test Cases

Writing tests for your custom error class should be straightforward. Your goal is to ensure that the constructor behaves as expected and covers all lines, specifically focusing on the super call. Here’s how you can do it using Jest, a popular testing framework.

Step 1: Test Instantiation

The first step is to create a test that verifies if your BarcodeDecoderError is instantiated correctly with all its properties. Here’s how you can implement this test:

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Explanation:

Expectations: This code checks if the values passed are correctly set in the instance. The first expectation confirms that the message handled by the super call is indeed the one provided, while the following checks verify that both barcode and errorType are established correctly.

Step 2: Ensuring Error Thrown

The second step is to ensure that your custom error is thrown correctly in real-world scenarios where you might expect an error. You can encapsulate this test case as follows:

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Explanation:

Testing for Throws: This test simulates a scenario where a decode operation would throw an error, confirming that when this happens, it throws an instance of BarcodeDecoderError with the expected properties.

Conclusion

Achieving full test coverage for your custom error classes in TypeScript doesn’t have to be daunting. By focusing on testing the super call and ensuring your properties are accurately returned, you can maintain confidence in your error handling mechanisms. Remember, the main goal of testing is not just to achieve 100% coverage but also to ensure your code behaves as expected in various scenarios.

With this guide, you should now feel more equipped to write thorough tests for your custom error classes in TypeScript!

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