Ajax异步处理二级联动

$.ajax({
url : url,
type : "post",

dataType : "json",

data:data,

success : function(msg) {
var arr = eval(list);
$("#XXX").empty()
var str = "<option value=''>请选择</option>";
$.each(arr, function(n, value) {
if (value.XXX==XXX) {
str += "<option value='" + value.XXX + "'  selected >" + value.name
+ " </option>";
}else{
str += "<option value='" + value.XXX + "' >" + value.name
+ " </option>";
}
});
$("#XXX:first").append(str);
//$("#XXX").find("option[value="+XXX+"]").attr("selected",true);
}
});