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ClojureScript ends up somewhere around the same size. Maybe it's some magic number that a full-featured environment converges to.

Since Scala.js (like CLJS) is backed by Google Closure Compiler, I bet you'll also reach for the Closure Library for a lot of stuff where one might reach for JQuery or equivalent otherwise. The stuff in there is of course optimized for Closure Compiler, and slims down quite well.

There is a significant difference between CLJS and Scala.js regarding their use of GCC (Google Closure Compiler).

In ClojureScript, GCC is a user-visible feature of the toolchain, so users are encouraged to use GCC-compatible libraries and the Closure Library in particular.

In Scala.js, GCC is really an implementation detail of the toolchain, and its constraints are completely abstracted away from the user. This does not really encourage using GCC-oriented libraries like the Closure Library.

The two approaches each have pros and cons:

* CLJS' approach allows tree shaking across the CLJS code and the GCC-compatible libraries that are used. However, it makes it more difficult to use libraries that do not comply with GCC's Advanced Optimizations requirements, as one needs to declare the proper `externs` files.

* Scala.js' approach means that you can use any JavaScript library out of the box, whether or not it was designed for GCC, and without having to declare any `externs` file. However, it means that GCC's tree shaking will not be applied across the Scala.js code and the JS libraries. It only applies to Scala.js' own code.

I think the externs file maneuver might be outdated.

Admittedly, I'm using shadow-cljs as my go-to compiler (so I'm not sure what the state of vanilla is), but all I do is yarn add <package>, and then require it in the file where I want to use it, and interact with it as if it were any other library.

Though it is true of shadow-cljs as well that it doesn't try to tree-shake external (non-CLJS/Closure) libs by default, though it will do it for project code and Clojure libraries.

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