This project uses pnpm to manage dependencies. All the dependencies are development-time only.
The testing framework is Vitest.
To gain more insight on performance, it is often useful to run the code in a browser instead of in Node, as the developer tools in browsers do a better job of visualizing performance traces than command line tools. Vite is included in devDependencies because of this, and it is required by Vitest anyway.
The Implementation Details page in docs for sortedcontainers for Python explains the data structure better than I can. The talks listed on its homepage are also worth watching if you prefer videos. Even though they are in Python, the general principle stays the same for JavaScript. I will not repeat what has been presented there.
The API design considerations are thoroughly documented in Design Notes.