Interesting JavaScript questions: Front-end paging

//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;  
}