For the same reason I don't call my c++ libraries (foo.asm) or dart libs (foobar.js). Fundamentally it is a different language with different syntax and semanti

For the same reason I don't call my c++ libraries (foo.asm) or dart libs (foobar.js). Fundamentally it is a different language with different syntax and semantics. Just because something can compile to js does not make your library a "js" library. Note that this is not an argument against coffeescript as a language. Use it if you want. Just realize that calling things ".js" when they are not is both confusing to newcomers and those expecting to read a lib written in js.