Understand the main purpose of what you're looking at and then add the code to a beautifier. That's a good start. Could try and find an open source version of it somewhere.

Also, what you're asking about is called Reverse Engineering.

In addition to the other suggestions, using an editor like Visual Studio Code (code.visualstudio.com) allows you to CTRL-click on a symbol to display or jump to it's definition.

It's like an interactive "How-To..." book and saves the time most of us would spend browsing through the MDN web docs.

Some techniques I use:

• Use an IDE like VS Code or WebStorm that is able to index the symbols (functions, classes, variables, etc) between files and lets you navigate to the declaration of a symbol or quickly locate everything using it.
• I also draw out my own understanding of components or starting writing pseudocode for a function on paper (maybe it's something about the slow process of writing it out that forces you to think through it more).
• See if the library has Typescript or Flow annotations - these can help a bit too in understanding the interfaces
• Start from an entry point and mentally trace through the execution. Running some example code with your own console.log statements in the library helps too.

If you're not looking for something in particular, but rather looking to understand the library in-and-out, then this tweet thread by Dan Abramov is really good advice. 😄