This is a list of open source projects I personally used to learn different aspects of Javascript development (backend and frontend).
It is very, very incomplete. I am trying to figure out an efficient way to discover and classify projects that others can learn by contributing. If you have any ideas, please help me!
Feel free to add your favorite project and what other can learn from it!
https://github.com/babel/babel/tree/master/packages/babel-parser - learn how works javascript parser that powers popular tools like babeljs and prettier
https://github.com/kentcdodds/babel-plugin-macros - learn how to write a babeljs plugin
https://github.com/webpack/tapable - learn a generic hooks-based architecture that powers webpack
https://github.com/ncthbrt/nact - learn about nano-services from this NodeJS implementation
https://github.com/GoogleChromeLabs/prerender-loader/blob/master/src/index.js - ???
https://github.com/tcoopman/image-webpack-loader -learn how to write a simple webpack plugin
https://github.com/willyelm/pug-html-loader - learn how to write a simple webpack loader
https://github.com/aholachek/animate-css-grid - learn how to animate complex layouts
https://github.com/nosir/cleave.js - learn how to format input text + how to create a library that works both for react and for angular
https://github.com/streamich/react-use - an excellent collection of ReactJS hooks
https://github.com/jaredpalmer/formik - to learn all the aspect of effecient forms validation
https://github.com/mui-org/material-ui - you will learn how to create a large-scale, themable, high-quality components library
https://github.com/Semantic-Org/Semantic-UI - another example of large-scale and themable compontents library
https://github.com/jamiebuilds/unstated - you will learn how to create simple and effective state management frameworks for ReactJS applications
https://github.com/GantMan/ReactStateMuseum - a compilation of different examples to managing state for ReactJS applications