javascript - xml getElementsByTagName 返回 null

Question

我正在尝试创建网页的一部分，通过将评论存储在 xml 文件中，使用 javascript 提取数据并设置循环参数，然后设置 html 来显示它，从而循环浏览评论。我的代码是：

xml布局：

<reviewlist>
<review>
<author>Author Name Here</author>
<comment>Author Comment Here</comment>
</review> <!-- i have 4 reviews in total -->
</reviewlist>

javascript和html：

function reviews() {

    if (window.XMLHttpRequest) {                                  
        xmlhttp = new XMLHttpRequest();
    } else {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");       
    }
    xmlhttp.open("GET", "../xml/Reviews.xml");
    xmlhttp.send();
    xmlDoc = xmlhttp.responseXML;

    x = xmlDoc.getElementsByTagName("reviewlist")[0];   //pull all 'review' elements into a string
    var tot = x.childNodes.length[0];                   //pull the number of 'review' elements as an Int. This shows the total number of reviews to cycle through
    var i = 0;                                          //a counter which will be used to identify which review number to display

    var content = "current review no." + i;
    var author = "total reviews: " + tot;
    document.getElementById('review').innerHTML = content;
    document.getElementById('reviewer').innerHTML = author;

    var id = setInterval(revolve, 10000); //each time the function repeats, check that the counter is a lower number than the total number of reviews. If true, show the current review no and how many reviews there are in total.
    function revolve() {
        if (i << tot) {
            content = "current review no. " + i;
            author = "total reviews: " + tot;
            i++;
        } else {                         //when the counter and the total reviews variables are equal, reset counter to 0 and loop through the reviews again
            content = "current review no. " + i;
            author = "total reviews: " + tot;
            i = 0;
        }
        document.getElementById('review').innerHTML = content;
        document.getElementById('reviewer').innerHTML = author;
    }
}

<blockquote class="review" id="review" style="top:0;"></blockquote>
<p class="reviewer" id="reviewer" style="bottom:0;right:0;"></p>

在我的网络开发人员中，我收到错误“TypeError：无法读取 null 的属性 'getElementsByTagName'”，但我看不出哪里出错了。我可以做些什么来解决这个错误并最终让 tot 变量返回 4（我的 XML 文件中的评论数）？

标签： javascripthtmlajaxxml