在XPath中使用xml.evaluate

http://www.w3schools.com/xml/tryit.asp?filename=try_xpath_select_cdnodes 嗨，這是原始示例的鏈接。我也複製粘貼在這裏的代碼。在XPath中使用xml.evaluate

<!DOCTYPE html> 
<html> 
<body> 

<p id="demo"></p> 

<script> 
var xhttp = new XMLHttpRequest(); 
xhttp.onreadystatechange = function() { 
    if (this.readyState == 4 && this.status == 200) { 
     showResult(xhttp.responseXML); 
    } 
}; 
xhttp.open("GET", "books.xml", true); 
xhttp.send(); 

function showResult(xml) { 
    var txt = ""; 
    path = "/bookstore/book/title" 
    if (xml.evaluate) {     // what is this line of code for? 
     var nodes = xml.evaluate(path, xml, null, XPathResult.ANY_TYPE, null); 
     var result = nodes.iterateNext(); 
     while (result) { 
      txt += result.childNodes[0].nodeValue + "<br>"; 
      result = nodes.iterateNext(); 
     } 
    // Code For Internet Explorer 
    } else if (window.ActiveXObject || xhttp.responseType == "msxml-document") { 
     xml.setProperty("SelectionLanguage", "XPath"); 
     nodes = xml.selectNodes(path); 
     for (i = 0; i < nodes.length; i++) { 
      txt += nodes[i].childNodes[0].nodeValue + "<br>"; 
     } 
    } 
    document.getElementById("demo").innerHTML = txt; 
} 
</script> 

</body> 
</html>

我不明白if(xml.evaluate)聲明的目的。它看起來像是在測試瀏覽器對XPath的支持，但它看起來好像在測試xml.evaluate是否返回true或false。

感謝

來源

2017-02-14 John Smith Sr.