map.js

var boringArray = [1,2,3,4];

const rReduce = (arr, func, acc) => {
    acc ? acc : arr[0];
  
    if(arr.length === 0) {
        return acc;
    }

  return rReduce(arr.slice(1), func, func(acc, arr[0]))
};


const doubler = (i) => {
  return i * 2;
}

const rMap = (arr, func) => {
  const accumulate = (acc, e) => {
    acc.push(func(e));
    
    return acc;
  }; 
  
  return rReduce(arr, accumulate, [])
};

console.log(rMap(boringArray, doubler));