So, when you see functions referred to that you don’t know, don’t default to the hints or answers. You have other tools at your disposal. A “true developer” will pop on over to the Mozilla Developer’s Network (MDN), and research each of these higher-order functions there. There is a WEALTH of information, and code samples, to be found on that site.
map (or, if you want to get there quicker,
The sign of a solid developer isn’t what she KNOWS. It’s does she know how to RESEARCH.