Question

此脚本每半秒抛出此错误：

Uncaught InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable. signals.js:9
req.onreadystatechange signals.js:9
update_table signals.js:26
req.onreadystatechange

这是signals.js - 我希望它每5秒重新加载一次，如果有新内容，则触发“声音”警报。

    function update_table()
{

    var old_table = document.getElementById('signals').innerHTML;

    var req = new XMLHttpRequest();

    req.onreadystatechange = function(){
    if(req.status == 200)
    {
        if(req.readyState == 4)
        {
            var new_table = req.responseText;
            alert(old_table);
            alert(new_table);
            if(old_table != new_table)
            {
                //play sound
                alert("Sound!");
            }
            alert("Refresh!");
            setTimeout(update_table, 5000);
        }
        }
    }
    var link = "table.php?refresh=true";

    req.open("GET", link, false);
    req.send();

}

Answer 1

首先检查req.readyState是否等于4，然后检查req.status是否等于200。在处理请求之前未设置HTTP状态代码，因此在readyState等于4之前不能使用它。

您可以查看this link以获取有关onreadystatechange事件的更多信息。

Answer 2

您需要先检查req.readyState是否等于4（表示DONE），然后才检查req.status：

function update_table() {
    var old_table = document.getElementById('signals').innerHTML;

    var req = new XMLHttpRequest();

    req.onreadystatechange = function() {

        // Here: first check for readyState
        if(req.readyState === 4 && req.status == 200) {
            var new_table = req.responseText;
            alert(old_table);
            alert(new_table);
            if(old_table != new_table)
            {
                //play sound
                alert("Sound!");
            }
            alert("Refresh!");
            setTimeout(update_table, 5000);
        }
    }
    var link = "table.php?refresh=true";    

    req.open("GET", link, false);
    req.send();

}

有关详细信息，请参阅XMLHttpRequest doc。

JavaScript settimeout不起作用

2 个答案: