Typescript

Even though it’s a fairly complex transpiler, the authors did a good job modularizing and leaving lots of contextual comments on what each part does.

Also typescript baseline tests are a simple but very effective way to get lots of coverage on the compiler.

I’ve read source code for Babel, typescript, coffeescript and flow. Typescript architecture stands out.

Typescript not only does fascinating things like magical code completion abilities and great tooling for IDEs but their codebase has been an inspiration for me to build better front end code.

I may be a bit biased since I’ve worked at Microsoft before.