code：
//html
//The first option must be empty in order to get placeholder working!
<select id="belongingUserId" placeholder="请选择归属人">
<option></option>
<option>Option value</option>
</select>
//js
$(function () {
$("#belongingUserId").select2({ allowClear: true});
});
//未选择时，取值时为空，取值时要判断
var belongingUserId = $('#belongingUserId').val();
if (belongingUserId == '') belongingUserId = '-1';//如果是空，赋默认值