Have a look at underscore js source:
https://github.com/jashkenas/underscore/blob/master/undersco...
I really loved it! Especially those _ functions are small, concise and easy to understand as well. Very good codebase. (very good example of functional programming concepts)
There's an annotated version available here, which might be a little easier on the eyes: http://underscorejs.org/docs/underscore.html
Underscore's codebase has grown some warts and wrinkles over the years, as optimizations and browser edge cases find their way in ... but the guiding principle is still to keep it as svelte as we can manage.