JQuery.
I'm serious. Every time I see modern JS I look back at the JQuery ecosystem and it seems like we were all expelled from paradise. Elegant, modular, flexible, simple. Easy to use and extend. Even had a GUI library. No need for cross-compilation. No need for a package manager with a centralized namespace and a brittle and easily sabotaged dependency tree, because the web itself is the package manager (as God intended.)
I remember jquery being an early classic with some annotated versions being available when I had an interest in learning js. No idea how much it has dated now.
Although it's not my stack of choice, it's generally a good starting point:
https://github.com/ixartz/SaaS-Boilerplate