CodeWars: Pagination Helper Solution

1// The constructor takes in an array of items and a integer indicating how many
2// items fit within a single page
3function PaginationHelper(collection, itemsPerPage) {
4  this.collection = collection
5  this.itemsPerPage = itemsPerPage
6}
7
8// returns the number of items within the entire collection
9PaginationHelper.prototype.itemCount = function () {
10  return this.collection.length
11}
12
13// returns the number of pages
14PaginationHelper.prototype.pageCount = function () {
15  return Math.ceil(this.itemCount() / this.itemsPerPage)
16}
17
18// returns the number of items on the current page. page_index is zero based.
19// this method should return -1 for pageIndex values that are out of range
20PaginationHelper.prototype.pageItemCount = function (pageIndex) {
21  if (pageIndex + 1 < this.pageCount()) {
22    return this.itemsPerPage
23  } else if (pageIndex + 1 === this.pageCount()) {
24    return this.itemCount() % this.itemsPerPage
25  }
26  return -1
27}
28
29// determines what page an item is on. Zero based indexes
30// this method should return -1 for itemIndex values that are out of range
31PaginationHelper.prototype.pageIndex = function (itemIndex) {
32  const res = Math.floor(itemIndex / this.itemsPerPage)
33  return 0 <= res && res < this.pageCount() ? res : -1
34}