> understand the implementation of every function Rather: remember three things that make up the majority of Array iterators' callback functions' signatures:

> understand the implementation of every function Rather: remember three things that make up the majority of Array iterators' callback functions' signatures: Element, Index, Array. Shared by: .map, .every, .forEach, .filter, and probably some that I am forgetting. The exception I think is just .reduce[Right], which by definition requires its previous return value, so you have (retVal, elem, i, arr). Quite literally, if you remember .map callback, you remember .every callback :) Javascript deserves shtick for its truly bad parts (with, arguments, ...) and some missing parts, but .map and its friends aren't it.