gildedRose.js

const update = (items) => {
  items.map(update_item)
}

const update_item = (item) => {
  switch (item.name) {
    case "Sulfuras, Hand of Ragnaros":
      return item
      break
    case "Aged Brie":
      return constrain_quality(update_brie(update_age(item)))
      break
    case "Backstage passes to a TAFKAL80ETC concert":
      return constrain_quality(update_ticket(item))
      break
    default:
      return constrain_quality(update_regular_item(update_age(item)))
  }
}

const update_brie = (item) => {
  if (item.sell_in < 0) {
    item.quality += 2
  } else {
    item.quality += 1
  }
  return item
}

const update_ticket = (item) => {
  if (item.sell_in < 0) {
    item.quality = 0
  } else if (item.sell_in < 5) {
    item.quality += 3
  } else if (item.sell_in < 10) {
    item.quality += 2
  } else {
    item.quality += 1
  }
  return item
}

const update_regular_item = (item) => {
  if (item.sell_in < 0) {
    item.quality -= 2
  } else {
    item.quality -= 1
  }
  return item
}

const update_age = (item) => {
  item.sell_in--
  return item
}

const constrain_quality = (item) => {
  if (item.quality < 0) {
    item.quality = 0
  } else if (item.quality > 50) {
    item.quality = 50
  }
  return item
}