todo: write tests. little recursive function fun.

map.js

function map(fn) {
  let newArr = []
  let idx = 0
  return function mapper(arr) {
    if (arr.length === idx) {
      return newArr
    }
    const item = fn(arr[idx])
    newArr.push(item)
    idx++
    return mapper(arr)
  }
}

// todo: write tests
var timesTwo = a => a * 2
var mapTimesTwo = map(timesTwo)
mapTimesTwo([1, 2])