jquery用Ajax中的回调函数时注意事项

<script language="javascript" type="text/javascript" src="<?php echo base_url();?>/js/jquery-1.7.2.min.js"></script>
<script>
function add_ip(bip,eip,area_id) {
var action = "<?php echo site_url('test/add_test'); ?>";
var yn = window.confirm("测试");
if (!yn) {
return;
}
$.ajax({
type:"post",
dataType:"json",
url:action + "?rr=" + Math.random(),
data:{
"bip": bip,
"eip": eip,
"area": area_id
},
success:function(retObj) {
if (retObj.code == 0) {
alert("设置成功");
} else {
alert("设置失败:"+retObj.msg);
}
},
fail:function(retObj) {
alert("发送请求失败code="+retObj.code);
}
});
}

</script>

function add_test() {
$bip = $this->input->post('bip', true);
$eip = $this->input->post('eip', true);
$area_id = intval($this->input->post('area_id',true));
if ($this->test_mdl->add($bip, $eip, $area_id)) {
//      echo "add OK";
} else {
//      echo "add fail";
}
echo '{"code":0, "msg":""}';
}