Pagination Helper Kata

pagination.js

// http://www.codewars.com/kata/515bb423de843ea99400000a/train/javascript

// TODO: complete this object/class

// The constructor takes in an array of items and a integer indicating how many
// items fit within a single page
function PaginationHelper(collection, itemsPerPage){
    this.collection = collection;
    this.itemsPerPage = itemsPerPage;
}

// returns the number of items within the entire collection
PaginationHelper.prototype.itemCount = function() {
    return this.collection.length;
}

// returns the number of pages
PaginationHelper.prototype.pageCount = function() {
  return Math.ceil(this.itemCount() / this.itemsPerPage);
}

// returns the number of items on the current page. page_index is zero based.
// this method should return -1 for pageIndex values that are out of range
PaginationHelper.prototype.pageItemCount = function(pageIndex) {
    var itemsPerPage = this.itemsPerPage,
        itemCount = this.itemCount(),
        pageCount = this.pageCount();
      
  if (pageIndex >= pageCount || pageIndex < 0) return -1;
  return this.items.slice(pageIndex * itemsPerPage, itemCount).splice(0, itemsPerPage).length;
}

// determines what page an item is on. Zero based indexes
// this method should return -1 for itemIndex values that are out of range
PaginationHelper.prototype.pageIndex = function(itemIndex) {
  if (this.itemCount() === 0 || itemIndex < 0 || itemIndex > this.itemCount()) return -1;
  if (itemIndex === 0 || itemIndex / this.itemsPerPage === 1) return 0;
  return Math.floor((itemIndex / this.itemsPerPage)); 
}

tests.js

// Create your own tests here. These are some of the methods available:
//  Test.expect(boolean, [optional] message) 
//  Test.assertEquals(actual, expected, [optional] message)
//  Test.assertSimilar(actual, expected, [optional] message)
//  Test.assertNotEquals(actual, expected, [optional] message)
var helper = new PaginationHelper(['a','b','c','d','e','f'], 4);
Test.assertEquals(helper.itemCount(), 6);
Test.assertEquals(helper.pageCount(), 2);
Test.assertEquals(helper.pageItemCount(0), 4);
Test.assertEquals(helper.pageItemCount(1), 2);
Test.assertEquals(helper.pageItemCount(2), -1);
Test.assertEquals(helper.pageItemCount(-10), -1);
Test.assertEquals(helper.pageIndex(5), 1);
Test.assertEquals(helper.pageIndex(6), 1);
Test.assertEquals(helper.pageIndex(2), 0);
Test.assertEquals(helper.pageIndex(4), 0);
Test.assertEquals(helper.pageIndex(6), 1);
Test.assertEquals(helper.pageIndex(-10), -1);
Test.assertEquals(helper.pageIndex(7), -1);