I'm a core contributor on Excalibur.js, a TypeScript-based 2D game engine. There are 4 members of the core team and we've worked on this for the past 5-6 years now--we build games with it for fun, have a transparent roadmap, and do it on the side in our (increasingly) spare time. We spend a lot of time on code quality, API documentation, and automation to make it easy to contribute.
We've started to get a lot more external contributors in the past year, so if you're interested we have tagged first-timer issues you could jump in on. We welcome any contributions: typos, fixes, and even big enhancements (though talk to us on the issue first!).
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