[amgreg]

I think you are conflating the Closure Library with the Closure Compiler. They are related but not identical. The Compiler, I think, is what makes it difficult to use externs; its “advanced optimizations” can and often does break libraries that weren’t written with the Compiler’s quirks in mind. But advanced optimizations is an option; if you don’t need aggressive minification, function body inlining, etc. you can opt out.

Shadow CLJS has made working with external libraries quite easy and IIRC it lets you set the compilation options for your libraries declaratively.

[john2x]

Ahh right. Yes I am in fact conflating the two.

But can the compiler be used without the library? Or can the library be used without the compiler/would it still be beneficial?

[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.