What This Repository Does

This is a bug report repository documenting a problem with running tests on JavaScript code that uses modern syntax features. When someone tries to test a React application using Jest (a testing tool), the code fails to run because the test runner doesn't understand the spread operator syntax (the ... in { ...base, extends: true }). The repository exists to help reproduce and track down why this incompatibility happens.

The Problem in Context

The issue shows up in two different ways depending on which testing command you run. When using react-scripts test, you get an error about something being undefined. When running Jest directly, it complains that it doesn't recognize the spread operator syntax at all. Both point to the same underlying problem: the tool that's supposed to translate modern JavaScript into a format the test runner understands isn't working correctly.

Who Would Care About This

This matters to anyone building React applications and trying to write tests for their code. The repository is essentially a minimal example — just enough code to demonstrate the bug — that makes it easier for the people maintaining Jest and Babel (the translation tool) to figure out what went wrong and fix it. If you're a developer hitting this error in your own project, a repository like this helps you confirm whether the problem is something you caused or a known issue with the tools themselves.

How to Use It

The README provides Docker commands to set up an identical environment and reproduce the exact error. This approach ensures that anyone — whether they're using Windows, Mac, or Linux — can run the same test and see the problem happen consistently, which is crucial for debugging.