Depends on what you mean by "know". I was able program in Python without ever learning it (not by choice), I had to look up a few things, but my code worked in the end.

To actually fully know any language and all its quirks is tough. Look at Ecmascript specification, it's quite long. And that doesn't cover all the undefined behaviors.

https://www.ecma-international.org/ecma-262/8.0/index.html