执行XQuery语句遇DOMException错误，如何统计XML唯一title数量？

统计XML文件中所有唯一<title>的数量时，触发以下错误：

Uncaught DOMException: Document.evaluate: The expression is not a legal expression

示例XML结构：

<channel>
  <item>
    <title>Trans Researchers Want Google Scholar to Stop Deadnaming Them</title>
    <link>https://www.wired.com/story/trans-researchers-want-google-scholar-to-stop-deadnaming-them/</link>
    <description>The academic search engine’s policy on name changes is out of step with other search tools and publishers.</description>
    <category>Business</category>
    <thumbnail>https://media.wired.com/photos/630fe159567860ab6c66c667/master/pass/business-deadnames-google.jpg</thumbnail>
  </item>
  <item>
    <title>Babylon Disrupted the UK’s Health System. Then It Left</title>
    <link>https://www.wired.com/story/babylon-disrupted-uk-health-system-then-left/</link>
    <description>The AI-powered online doctor app is ditching its controversial NHS contracts as it focuses on the US market.</description>
    <category>Business</category>
    <thumbnail>https://media.wired.com/photos/63042ca2f06d7dee6f53bbc6/master/pass/Babylon-Leaves-NHS-Business-1253255825.jpg</thumbnail>
  </item>
</channel>

使用的JavaScript代码：

function displayCount() {
        // get the number of distinct titles using xquery
        var xquery = "count(distinct-values(/channel/item/title))";
        var count = xmlFile.evaluate(xquery, xmlFile, null, XPathResult.ANY_TYPE, null);
        //document.getElementById("count").innerHTML = count.textContent;
        console.log(count.numberValue);
}

浏览器原生的Document.evaluate()仅支持XPath 1.0，而distinct-values()是XPath 2.0及以上版本才新增的函数，因此该表达式会被判定为非法。

方法一：原生JavaScript手动去重计数 #

通过XPath获取所有<title>节点后，用Set数据结构自动去重，再统计数量，无需依赖高阶XPath特性：

function displayCount() {
  // 用XPath 1.0语法获取所有title节点
  var xpathExpr = "/channel/item/title";
  var nodeResult = xmlFile.evaluate(xpathExpr, xmlFile, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
  
  // 提取title文本并去重
  var uniqueTitles = new Set();
  for (let i = 0; i < nodeResult.snapshotLength; i++) {
    const titleText = nodeResult.snapshotItem(i).textContent.trim();
    uniqueTitles.add(titleText);
  }
  
  // 输出唯一title的数量
  console.log(uniqueTitles.size);
}

方法二：引入XPath 2.0支持库（可选） #

若坚持使用distinct-values()这类XPath 2.0函数，可引入第三方库提供高阶XPath支持，但需要额外加载库并调整代码逻辑，不如原生方案轻便。

内容的提问来源于stack exchange，提问作者S. Tiss