jestjs/jest ? reverse-engineered prompt

Build me a JavaScript testing tool that feels really easy to use out of the box, like Jest.

I want to be able to install it in a project, write simple test files, and run them from the command line with a nice pass or fail output. It should support common matchers like checking exact values, work well with JavaScript and TypeScript projects, and have a fast watch mode that only reruns the tests related to changed files. I also want snapshot testing for big objects or UI output, so it can save and compare snapshots over time.

Make it so the setup is simple for most projects, but still flexible enough to work with things like Babel and modern frontend setups. Include a basic config flow, a helpful README, and a few example tests so I can see how it’s supposed to be used. If you need to, look up the current docs online and follow the expected Jest style.