reduce.js

Array.prototype.myReduce = function(func, initialValue) {
  const arr = arguments.length > 1
    ? [initialValue, ...this]
    : this

  let accumulator = arr[0]

  for (let i = 1; i < arr.length; i++) {
    accumulator = func(accumulator, arr[i], i, this) 
  }

  return accumulator
}