It's implicit in all of these that the concept is explained to you. Nobody would ask you to "implement a X" expecting you to know what X was (be it Pascals triangle, fizzbuzz, map, etc) unless X happened to be a fundamental concept in the field you are working in (not the case for any of the examples so far)

> Therefore you would have to write up your own Array.prototype.map and Array.prototype.reduce to make them work. Is that true?

Yes, that's sort of the point here. Javascript has these functions as methods of arrays (in some implementations), but the idea of the test is to write your own. They're both pretty simple, basically a for-loop and some logic.

On another note I have a hard time believing you work in JavaScript on a day to day basis and are not working with arrays. If you use jQuery or a similar library, you are working with arrays all the time and also using map-like functionality even if you don't know it by that name.

$(".foo").click(function(){})

Takes an array of elements and maps over them.