编程方式触发默认事件

function open(elem) {
if (document.createEvent) {
var e = document.createEvent("MouseEvents");
e.initMouseEvent("mousedown", true, true, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
//e.initMouseEvent("mousedown") //据说不要参数也可以
elem[0].dispatchEvent(e);
} else if (element.fireEvent) {
elem[0].fireEvent("onmousedown");
}
}