JavaScript趣题：前端分页

//第一个参数是要分页的JSON对象 
//第二个参数是每一页的最大项数 
var helper = new PaginationHelper(['a','b','c','d','e','f'], 4); 
//总共多少页 => Math.ceil(6 / 4) 
helper.pageCount(); // 2 
//总共多少项 => array.length 
helper.itemCount(); // 6 
//求当前页的项数，这个页的索引是从0开始的 
helper.pageItemCount(0); // 4 
//6 - 4 = 2 
helper.pageItemCount(1); // 2 
//总共才2页，所以当前页无效，返回-1 
helper.pageItemCount(2); // -1 
 
//当前索引是属于第几页？ 
helper.pageIndex(5); // 1 
helper.pageIndex(2); // 0 
//总共都才6条记录，所以20无效 
helper.pageIndex(20); // -1 
//索引小于0，无效返回-1 
helper.pageIndex(-10); // -1

function PaginationHelper(collection, itemsPerPage){ 
 this.collection = collection; 
 this.itemsPerPage = itemsPerPage; 
} 
 
PaginationHelper.prototype.itemCount = function() { 
 return this.collection.length; 
} 
 
PaginationHelper.prototype.pageCount = function() { 
 return Math.ceil(this.itemCount() / this.itemsPerPage); 
} 
 
PaginationHelper.prototype.pageItemCount = function(pageIndex) { 
 if(pageIndex < this.pageCount() - 1){ 
 return this.itemsPerPage; 
 } 
 else if(pageIndex == this.pageCount() - 1){ 
 return this.itemCount() - pageIndex * this.itemsPerPage; 
 } 
 else{ 
 return -1; 
 } 
} 
 
PaginationHelper.prototype.pageIndex = function(itemIndex) { 
 if(itemIndex >=0 && itemIndex < this.itemCount()){ 
 return Math.floor(itemIndex / this.itemsPerPage); 
 } 
 return -1; 
}