testing select mulitple
HTML
<select id="mySelect" multiple>
<option value="a">a</option>
<option value="b">b</option>
<option value="c">c</option>
<option value="d">d</option>
</select>
JavaScript
var selectedArr = ["a", "c"];
var myselect = document.getElementById('mySelect');//.value = selectedArr;
for(var i = 0; i < myselect.options.length; ++i){
if(selectedArr.indexOf(myselect.options[i].value) >= 0){
myselect.options[i].selected= true;
}
}