触发 select

<select>
<option>a</option>
<option>a</option>
<option>a</option>
<option>a</option>
<option>a</option>
</select>

<button>a</button>
<script>

$(document).ready(function() {
$('button').click(function() {
focusSel($('select'));
});
});
function focusSel(elem) {
if (document.createEvent) {
var e = document.createEvent("MouseEvents");
e.initMouseEvent("mousedown");
elem[0].dispatchEvent(e);
} else if (element.fireEvent) {
elem[0].fireEvent("onmousedown");
}
}