> instead of just writing a JS file with your code and then loading it

If your js does no I/O at all, you can write pure js. But if you want to show some results, there is no way that you can do it without learning the relevant API to do that.

Every library is overhead that you need to first understand. There's no "just writing a JS file" or just writing a C++/C#/Java/Python/etc. file.