I recently made a JavaScript library for fundamental statistics (fun-stats).
It was fun! I learned how to use rollup.js for bundling code, and how to publish packages to npm. I also learned a lot about the mathematics behind the statistics I have been applying for years, which is a hole in my knowledge I have been wanting to fill for a while.
my package on npm
You can see the documentation for this package here. It has functions for calculating a plotting a few probability distributions. If you use it, that would be cool.