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.
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.