Clojurians Log v2

The CLJ one was implemented first, CLJS significantly later. I have heard (but have not studied in detail myself) that CLJS tends to implement much more using Clojure's defprotocol, where in CLJ similar things were implemented via Java interfaces and methods.

yeah, the CLJS came already with protocols, so their DS were started with then already. what is not making sense to me here, is that CLJS does have the IAssociative protocol that contains the -contains-key? method, so why the CLJ contains? does check and use this protocol, but CLJS doesn't?