MongoDB pagination

helper.js

// Terrible code formatting but it works for me

// totalItems = total number of items (across all pages)
// perPage = number of items to show per page
// maxPages = maximum number of pages to show at once
// selectedPage = page currently on
// link = start of link, eg. /search (and then ?page is appended although this code doesn't fully do this)
// reverseSort = if the sortation should be backwards
module.exports.paginate = function(totalItems, perPage, maxPages, selectedPage, link, reverseSort) {
    return new Promise(function(resolve, reject) {
        var totalPages = Math.ceil(totalItems / perPage);
        var hasNext = (selectedPage + 1) <= totalPages;
        var hasPrev = (selectedPage - 1) > 0 && (selectedPage - 1) <= totalPages;

        if(selectedPage > totalPages) {
            return reject();
        }

        if(totalPages > 1) {
            var pagination = {
                current: selectedPage,
                total: totalPages,
                pages: [],
                prev: hasPrev ? (selectedPage - 1) : false,
                next: hasNext ? (selectedPage + 1) : false
            };

            var dots = true;
            for (var i = 0; i < pagination.total; i++) {
                var j = i + 1;

                // Bits of this use ajax on frontend to work out link so this is kind of a placeholder
                if(j == 1 || j == pagination.total || (j >= pagination.current - maxPages && j <= pagination.current + maxPages)) {
                    dots = true;
                    pagination.pages.push({number: j, active: (j == pagination.current), link: link + '?page=' + j});
                } else if(dots) {
                    dots = false;
                    pagination.pages.push({dots: true});
                }
            }

            var gt = (perPage * (selectedPage - 1));
            var lte = gt + perPage;
            if(reverseSort) {
                gt = (totalItems - gt);
                lte = (totalItems - lte);
            }

            // Returns with a query that can be used in mongo
            // Using .skip and .limit is inefficient this is an example
            // of range based pagination  --  https://docs.mongodb.com/manual/reference/method/cursor.skip/#behavior
            // In this case, id is an incrementing id which is used in my case
            // Purely for visual reasons for end users
            return resolve([pagination, {id: {'$gt' : (reverseSort ? lte : gt), '$lte' : (reverseSort ? gt : lte)}} ]);
        } else {
            // Pagination disabled
            return resolve([false, {}]);
        }
    });
}