Map, reduce

Question

can someone explain me that

Answer 1

I think this is a too deep topic to be covered here ^^

However, a quick google "js map filter" and I find at least ths link, which seems to be a good starting point ( I don't have yet read it completly, but I will do also, even I can find some in my language -- that's often easier to understand for me ;P )...

Answer 2

Very good explanation in the link I though have previously posted:
http://elijahmanor.com/reducing-filter-and-map-down-to-reduce/

( sorry for the unintentional announcement effect ^^ )