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Discover how to make Jest resolve `ESM local imports` in a TypeScript project by using the proper configuration. Fix import issues and streamline your testing process efficiently.
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This video is based on the question https://stackoverflow.com/q/76472340/ asked by the user 'Willem Mulder' ( https://stackoverflow.com/u/1037703/ ) and on the answer https://stackoverflow.com/a/76472341/ provided by the user 'Willem Mulder' ( https://stackoverflow.com/u/1037703/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solve Your Issues with Jest: ESM Local Imports in TypeScript

When developing a TypeScript project, you might encounter an issue that stops you in your tracks, especially when running tests with Jest. If you've tried importing an ES module file and received an error message stating, "Cannot find module './b.js' from 'a.ts'", you're not alone. This guide will guide you through the solution to this specific problem that many developers face while using Jest in a TypeScript environment.

Understanding the Problem

Picture this: you have two TypeScript files in your project, a.ts and b.ts. You want to import b.ts into a.ts for seamless functioning. The import statement looks like this:

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

What Happens Next?

While TypeScript understands and compiles this import statement without a hitch, Jest throws an error when you attempt to run your tests. The message you encounter is:

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

This happens because Jest, which under the hood uses ts-node, struggles to resolve .js imports in your TypeScript files. Fortunately, this can be resolved with a simple configuration tweak.

The Solution: Modify Your Jest Configuration

To address this issue, you will need to configure the Jest settings in your jest.config.ts file to ensure that Jest can correctly interpret .js imports as corresponding .ts files. Follow these steps:

Step 1: Update Your jest.config.ts

Add the following code snippet to your Jest configuration file:

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

Step 2: Understanding the Above Code

moduleNameMapper: This is a Jest configuration option that allows you to define how module names are mapped. It's particularly useful for resolving modules based on certain patterns.

'^(\.{1,2}/.*)\.js$': This regular expression matches any local relative file path that ends with .js.

'$1': This replacement keeps the matched part of the path but removes the .js extension, allowing Jest to locate the corresponding TypeScript file without confusion.

Step 3: Verify the Jest Preset

Make sure you also have the following line of code in your jest.config.ts, necessary for TypeScript support:

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

Your final configuration in jest.config.ts should look similar to this:

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

Conclusion

By tweaking the configuration in your Jest setup, you can effectively resolve ESM local imports in your TypeScript project. Remember, incorporating the moduleNameMapper in your jest.config.ts is essential for helping Jest understand that a .js import is, in fact, a TypeScript file. With this simple adjustment, you can streamline your testing process and avoid unnecessary headaches while working on your projects.

Now you can continue with your testing without stumbling over module import issues! Happy coding!