链接下拉列表选择列表代码 -- 连接数据

我正在努力将我在网上找到的链式下拉功能改编到我的网站上。我在网上找到的一些代码如下。一个问题是我无法理解读取var connection = selected.data('connection');连接属性是 jquery 共有的行吗？还是在代码中的某个位置设置？如果是这样，这组在代码中的什么位置？

$(function(){
    var questions = $('#questions');
    function refreshSelects(){
        var selects = questions.find('select');
        // Improve the selects with the Chose plugin
        selects.chosen();
        // Listen for changes
        selects.unbind('change').bind('change',function(){
            // The selected option
            var selected = $(this).find('option').eq(this.selectedIndex);
            // Look up the data-connection attribute
            var connection = selected.data('connection');
            // Removing the li containers that follow (if any)
            selected.closest('#questions li').nextAll().remove();
            if(connection){
                fetchSelect(connection);
            }
        });
    }
    var working = false;
    function fetchSelect(val){
        if(working){
            return false;
        }
        working = true;
        $.getJSON('ajax.php',{key:val},function(r){
            var connection, options = '';
            $.each(r.items,function(k,v){
                connection = '';
                if(v){
                    connection = 'data-connection="'+v+'"';
                }
                options+= '<option value="'+k+'" '+connection+'>'+k+'</option>';
            });
            if(r.defaultText){
                // The chose plugin requires that we add an empty option
                // element if we want to display a "Please choose" text
                options = '<option></option>'+options;
            }
            // Building the markup for the select section
            $('<li>'
                <p>'+r.title+'</p>'
                <select data-placeholder="'+r.defaultText+'">'
                    '+ options +''
                </select>'
                <span class="divider"></span>'
            </li>').appendTo(questions);
            refreshSelects();
            working = false;
        });
    }
    $('#preloader').ajaxStart(function(){
        $(this).show();
    }).ajaxStop(function(){
        $(this).hide();
    });
    // Initially load the product select
    fetchSelect('productSelect');
});