Implementation of JavaScript's Array map function without the use of a loop (for, while etc). Recursion is used instead.

customMap.js

Array.prototype.customMap = function(fn) {
  if (typeof fn !== 'function') throw Error('Am I a joke to you?');

  const arr = this;
  const arrLen = arr.length;

  const getNewArr = (index, newArr) => {
    if (index < arrLen) {
      const newItem = fn(arr[index], index, arr);
      return getNewArr(index + 1, [...newArr, newItem]);
    } else {
      return newArr;
    }
  };

  return getNewArr(0, []);
}

// test
const arr = [1, 2, 3, 5, 6];
const newArr = arr.customMap((item, i) => {
  return item + i;
});
console.log({ newArr });