Example of endless scrolling via ajax. Scroll event with underscore.js debounce

endless_scroll.js

/*
 * Use ajax to live-refresh issues/issues/ when the filters are updated
 */
$(function () {
    if (window.currentPage && window.currentPage == 'issue_list') {
        /*
         * sends ajax request via GET for a given query at the current url.
         * callbacks: fail, done
         */
        var get_page_num_from_query = function () {
            var match = /page=(\d+)(?:(?!page=\d+).)*$/;
            var results = match.exec(window.location.search);
            return results ? parseInt(results[1], 10) : 1;
        }
        var issues_ajax_GET = function (query, callbacks) {
            $('.issues-table').addClass('loading');
            $.ajax({
                url: window.location.pathname,
                data: query,
                dataType: 'html',
                method: 'GET',
                cache: false
            }).done(function(data) {
                // if browser supports it, update browser's url with query string
                if (window.history) {
                    window.history.replaceState(null, "",
                        window.location.pathname + '?' + query);
                }

                callbacks.done && callbacks.done(data);
                $('.issues-table').removeClass('loading');
            }).fail(function (err) {
                console.error('Error loading query:', err);
                callbacks.fail && callbacks.fail(err);
                $('.issues-table').removeClass('loading');
                $('.issues-table').addClass('error');
            });
        }

        var form_data = $('form.filter-form').serialize();
        var curr_page = get_page_num_from_query();
        console.log('curr page is ', curr_page);

        $('.filter-form input, .filter-form select').change(function (e) {
            form_data = $('form.filter-form').serialize();
            curr_page = 1;
            issues_ajax_GET(form_data, {
                done: function (data) {
                    $('.issues-table tbody').html(data);
                }
            });
        });
        $(window).scroll(_.debounce(function (event) {
            var element = document.body;
            if (element.scrollHeight - element.scrollTop < element.clientHeight + 10) {
                issues_ajax_GET(form_data + "&page=" + (++curr_page), {
                    done: function (data) {
                        $('.issues-table tbody').append(data);
                    }
                });
            } 
        }, 150, false));
    }
});


/*
 * Methods taken from underscore.js
 */
var _ = {}
_.debounce = function(func, wait, immediate) {
    var timeout, args, context, timestamp, result;

    var later = function() {
        var last = _.now() - timestamp;

        if (last < wait && last >= 0) {
            timeout = setTimeout(later, wait - last);
        } else {
            timeout = null;
            if (!immediate) {
                result = func.apply(context, args);
                if (!timeout) context = args = null;
            }
        }
    };

    return function() {
        context = this;
        args = arguments;
        timestamp = _.now();
        var callNow = immediate && !timeout;
        if (!timeout) timeout = setTimeout(later, wait);
        if (callNow) {
            result = func.apply(context, args);
            context = args = null;
        }

        return result;
    };
}
_.now = Date.now || function() {
    return new Date().getTime();
};