How to test a standalone typescript file with Jest

A guide on how to test standalone TypeScript files using Jest by refactoring the file for better testability.
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Testing a Standalone TypeScript File Using Jest

When working with TypeScript, it’s common to write standalone scripts that execute specific tasks. However, testing these scripts can be troublesome if they lack a structured approach. In this guide, we will explore a common problem regarding testing a standalone TypeScript file and how to effectively address it with Jest.

The Problem

Imagine you have a TypeScript file that is intended to be run as a standalone script, performing a simple operation like sending a notification. Here’s an example of such a script:

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

You may find yourself wondering how to test this functionality using Jest, especially to assert that the notify() method is called correctly.

The Challenge of Testing Standalone Scripts

Testing functions and classes is relatively straightforward, but direct testing of a standalone script like the one above presents challenges. When code is structured in a way that makes it difficult to isolate functionality for testing, it’s a sign that you may need to refactor.

Key Points:

Isolated Functionality: Functions and classes should be designed to be tested independently.

Refactoring: If testing is cumbersome, consider restructuring your code for better testability.

A Better Way: Refactor for Testability

To make your code more testable, you can refactor the standalone script into a named and exported function. Here’s how you can do it:

Refactored Code:

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

Why Refactor?

Modularity: By exporting the function, you can import it into your Jest test files.

Example Usage:

Now, instead of running the script directly, you can invoke this function where needed:

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

Writing Tests for the Refactored Code

Now that we have refactored our code, let’s discuss how to set up Jest tests.

Setup Jest Mocks:

Example Test Case:
Here’s an example of how you might structure your Jest test:

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

Conclusion

By refactoring a standalone TypeScript script into a modular function, you pave the way for easier and more efficient testing using Jest. This approach not only enhances the testability of your code but also contributes to better code organization and reliability. If you’ve been struggling to test similar scripts, consider applying this strategy for improved results.

By transforming your code for testing, you’ll find that not only is your workflow streamlined, but you also gain confidence in the robustness of your application.

Happy Coding!