Array.prototype.scan = function (callback, initialValue) {
  const appendAggregate = (acc, item) => {
    const aggregate = acc[acc.length-1] //get last item
    const newAggregate = callback(aggregate, item)
    return [...acc, newAggregate]
  }

  const accumulator = [initialValue]

  return this.reduce(appendAggregate, accumulator)
}