The game client is built with PixiJS, and otherwise mostly uses vanilla JS. The backend is built with Node.js, and the game uses a custom WebSocket protocol cal

The game client is built with PixiJS, and otherwise mostly uses vanilla JS. The backend is built with Node.js, and the game uses a custom WebSocket protocol called Ribbon. In fact, there's a whole list of open source dependencies at https://tetr.io/about/acknowledgements/ https://tetr.io/about/acknowledgements/.