🧑‍💻 Custom Reduce Function

Array Reduce

function arrayReduce(arr, func, initial) {
  let result = initial;
  arr.forEach((val) => {
    result = (result === undefined) ? val : func(result, val);
  });
  return result;
}

const arr = [1, 2, 3, 4, 5];
const sum = arrayReduce(arr, (result, val) => result + val);
const prod = arrayReduce(arr, (result, val) => result * val, 2);
console.log(sum, prod);

15 240

Object Reduce

function objectReduce(obj, func, initial, ownPropertiesOnly = true) {
  let result = initial;
  for (const key in obj) {
    if (!ownPropertiesOnly || obj.hasOwnProperty(key)) {
      result = func(result, key, obj[key]);
    }
  }
  return result;
}

const result = objectReduce({ 'a': 1, 'b': 2, 'c': 1 }, function(result, key, value) {
  (result[value] || (result[value] = [])).push(key);
  return result;
}, {});
console.log(result);

{ '1': [ 'a', 'c' ], '2': [ 'b' ] }