[amgreg]

Yes and yes; in the past, prior to ECMAScript providing first-class inheritance, module ex/imports etc, the Library supplied methods to achieve these in development, and the Compiler would identify these cases and perform the appropriate prototype chaining, bundling, etc. See, eg, goog.provide

For the most part, I would guess people still use the Closure Compiler because of its aggressive minification or for legacy reasons. I think both are probably true for ClojureScript, as well as the fact that the Compiler is Java-based so it has a Java API that (I am guessing here) made it easier to bootstrap on top of the JVM Clojure tooling / prior art.

[yladiz]

I've been doing frontend development for over 10 years, and obviously it's anecdotal but I never heard anyone use the Closure Compiler outside of ClojureScript, and I imagine that in practice most people doing frontend development are using Webpack, Vite, Parcel, etc. The idea of really small bundles sounds nice, but in practice because the advanced optimizations require manual tweaking in many cases to get it to work (externs) it's something few people would want to deal with and the small bundle size improvement isn't worth it compared to the standard tools like UglifyJS/Terser.

There may be other reasons, but I assume the main reason that the Closure Compiler was chosen for ClojureScript was because it's Java based, so it was straightforward to get working. Moving away from it now would be a huge breaking change, so it's unlikely to happen in the official compiler anytime soon or ever. I think the only way it would actually happen is if an alternative like Cherry got enough traction and people moved to using mainly the alternative.

[amgreg]

Yeah nowadays I think non-ClojureScript people use it mostly for legacy reasons or the aggressive minification. Back in the day, aside from the pre-ES5 conveniences I mentioned surrounding inheritance and module bundling, it was also a way for developers to do some basic type enforcement (via JSDoc annotations that the Compiler would check). TypeScript essentially rendered that obsolete.

See: https://effectivetypescript.com/2023/09/27/closure-compiler/