A more functional way to implement map, reduce, and filter:

Recursive Map

let map = (fn, list) => 
  !list.length ? [] : [fn(list[0])].concat(map(fn, list.slice(1)));

map(x => x + 1, [1, 2, 3, 4]); // [2, 3, 4, 5]

## Recursive Reduce
javascript
let reduce = (fn, value, list) => 
  !list.length ? value : reduce(fn, fn(value, list[0]), list.slice(1));

reduce((x, y) => x + y, 0, [1, 2, 3, 4]); // 10

## Recursive Filter
javascript
let filter = (predicate, list) => 
  !list.length ? [] : 
    (predicate(list[0]) ? [list[0]] : [])
      .concat(filter(predicate, list.slice(1)));

filter(x => x > 2, [1, 2, 3, 4, 5]); // [3, 4, 5]