不適用所有瀏覽器，經測試支援Chrome，IOS上的SAFARI

("#觸發點").click(function(e) {
            var element = $("#下拉框")[0], worked = false;
            if (document.createEvent) { // all browsers
                var e = document.createEvent("MouseEvents");
                e.initMouseEvent("mousedown", true, true, window, 0
 
, 0, 0, 0, 0, false, false, false, false, 0, null);
                worked = element.dispatchEvent(e);
            } else if (element.fireEvent) { // ie
                worked = element.fireEvent("onmousedown");
            }
            if (!worked) { // unknown browser / error
                alert("It didn't worked in your browser."
 
);
            }
        });