I make a game engine in Javascript for web browser games (mainly my own game). I use the library three.js for 3D rendering, but the rest I've written myself with some borrowed code from https://grelf.itch.io. I've written about it here https://kbrecordzz.itch.io/this-is-not-a-car-club-chapter-one/devlog/497670/buil... (now this may look like self-promotion, maybe it is, but the text describes my engine very clearly)