jquery - 为什么alert(json[0].subject);给未定义？

Question

我有这个代码:

 <script>
    $(document).ready(function()
    {
        refresh();
    });

    function refresh()
    {      
        $.get('getMessageDetails.php', function (json) {
            alert(json);
            alert(json[0].subject);
        },"json");
        window.setTimeout(refresh,30000);
    }   
</script>

然后我有 getMessageDetails.php:

<?php
//header('Content-Type: application/json; charset=utf-8');
include('header_application.php');
$lastNewMessageCnt = $obj_clean->getUnopenedMessagesCount($_SESSION['user_id']) + 1;
$last_unopened_message_row = $obj_clean->getLastUnopenedMessage($_SESSION['user_id'],$lastNewMessageCnt); 
echo json_encode($last_unopened_message_row); 
?>

然后我有警报(json)，其中显示:

[{"subject":"Freechat ddd","id":"19","created_at":"2011-08-29 14:58:27","unique_code":"ALLEYCA000RC","opened_once":"0"}] 

正确的是

但是alert(json[0].subject);给出了未定义？？？

请帮忙？谢谢

Answer 1

您必须将 json 变量转换为正确的 json 格式变量。

目前它是一个字符串变量。

您必须按以下方式使用它:

<script>
    $(document).ready(function()
    {
        refresh();
    });

    function refresh()
    {      
        $.get('getMessageDetails.php', function (json) {
            alert(json);  // this is a string variable.
            json = $.parseJSON(json);  //now json varible is in correct json format.
            alert(json.subject); //you can call it dirctly like a associative array. No need to include '[0]'.
        },"json");
        window.setTimeout(refresh,30000);
    }   
</script>