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.items = collection;
  this.itemsPerPage = itemsPerPage;
}

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