Mastery comes when you know most of the 1st order stuff and can even anticipate the stuff you don't know because you also know the 2nd order stuff. In Java this is easy because after you get proficiency you can really dig in and read the JVM Spec and the JLS(1), and you have lots of great, powerful tools available to examine the running JVM, bytecode, etc. In other languages, this is a lot harder. JavaScript (ES6) kinda sorta has a specification(2) but it didn't start out that way and the jsvm doesn't have one (AFAIK). The implementation and the tooling for JS is widely varying, and typically includes lots of non-JS stuff like the DOM and networking - which is (really!) great but lacks focus. Somehow lots of popular languages exist without specifications, like Python, Ruby, PHP, Markdown. This is, IMHO, a very serious drawback because to become a master of these languages requires that you become a master of the concrete interpreter implementation, which is easily an order-of-magnitude harder and less direct than reading a specification.

(1) https://docs.oracle.com/javase/specs/

(2) http://www.ecma-international.org/ecma-262/6.0/