Understanding Why Jest Runs Both TypeScript and Compiled JS Test Files

Discover the reasons why Jest runs your TypeScript test files alongside compiled JavaScript test files and learn how to configure Jest for optimal testing.
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Understanding Why Jest Runs Both TypeScript and Compiled JS Test Files

When working with Jest, a JavaScript testing framework, it's common to run into issues that can be perplexing, particularly when your tests are not behaving as expected. One frequent point of confusion is when Jest runs both your TypeScript test files and the compiled JavaScript test files. This can lead to inconsistent test results, as you might find tests failing or passing unexpectedly.

In this guide, we'll explore why this happens and how to resolve the issue effectively.

The Problem Explained

When you run Jest for testing, it automatically searches through your project to find any test files. This means that if you have both TypeScript (.ts) files and their compiled JavaScript (.js) counterparts in the same directory, Jest will pick up on both, potentially leading to:

Conflicting test results: You might have identical tests in both formats that could yield different outcomes.

An increased number of failing or passing tests: The reported test results can be misleading, making it difficult to identify which tests are valid and which are not.

From the scenario described, it seems there are 9 failing tests and 11 passing out of a total of 20, despite there only being 10 unique tests in two different test files. This discrepancy is often due to Jest's test file discovery process picking up multiple instances of the same tests.

Solution to the Problem

To mitigate this issue, you can take one of the following approaches:

1. Exclude Test Files from the Build Pipeline

The recommended approach is to modify your TypeScript configuration to exclude test files from being included in your build output. To do this:

Update this config to exclude test files.

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

Then, ensure your build command uses this configuration:

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

By doing this, you prevent Jest from picking up these test files during tests, thus simplifying your testing process.

2. Exclude Build Directories from Jest

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

Make sure to replace 'dist/' with the actual path of your compiled files. This configuration tells Jest to avoid executing tests from the specified directories.

Conclusion

By understanding why Jest runs both TypeScript and compiled JavaScript test files and implementing one of the solutions provided, you can streamline your testing process and gain clearer insights into your test results.

Key Takeaways:

Excluding test files from your build pipeline is the cleanest way to prevent Jest from running unwanted tests.

Alternatively, configure Jest to ignore build directories where compiled files reside, ensuring only relevant tests are executed.

Implementing these practices will help you maintain efficient testing workflows in your development projects.