JQuery EasyUI combobox动态添加option

<input id="select_city" name="select_city" class="easyui-combobox" style="width:435px" data-options="valueField:'id', textField:'text' >

var data, json;
json='[{"id":1 , "text":"兰州" },{"id":14 ,"text":"敦煌" ,"selected":true},{"id":15 , "text":"临夏"}]';
data = $.parseJSON(json);
$("#select_city").combobox("loadData", data);

var data,json;
data = [];
data.push({ "text": "测试", "id": 100 });
$("#select_city").combobox("loadData", data);