Libxmljs - Finding XML elements with namespaces using xpath

Today I spent a stupid time trying to parse a XML input to JS. Even using a great module libxmljs I was having problems to read any element that was not inside of client element (xpath: //client).

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <soap:Body>
    <Response xmlns="http://tempuri.org/">
      <Result>
        <client xmlns="">
          <msg>SEARCH OK.</msg>
          <code>0</code>
        </client>
      </Result>
    </Response>
  </soap:Body>
</soap:Envelope>

Why I could not get any of these elements?

• soap:Envelope - xpath: //soap:Envelope
• soap:Body - xpath: //soap:Body
• Response - xpath: //Response
• Result - xpath: //Result

This question forced me study a bit more about xml structure. In a first moment I didn't want to study it because XML is too boring. But I was spending too much time just guessing and searching a solved solution for my problem.

Reading XML Namespaces and How They Affect XPath and XSLT I could finally understand why I could get only elements inside of client.

Summarizing that article there are three types of namespaces:

Prefix Namespaces🔗

A namespace is composed of a name and a URI specification. The scope of the prefix-namespace mapping is that of the element that the namespace declaration occurs on as well as all its children.

<bk:bookstore xmlns:bk="urn:xmlns:25hoursaday-com:bookstore">
 <bk:book>
    <bk:title>Lord of the Rings</bk:title>
    <bk:author>J.R.R. Tolkien</bk:author>
    <inv:inventory status="in-stock" isbn="0345340426"
        xmlns:inv="urn:xmlns:25hoursaday-com:inventory-tracking" />
 </bk:book>
</bk:bookstore>

Default Namespaces🔗

Since use prefix namespace for all element is too verbose. Any element without a prefix namespaces is associated to the default namespace.

<bookstore xmlns="urn:xmlns:25hoursaday-com:bookstore">
 <book>
    <title>Lord of the Rings</title>
    <author>J.R.R. Tolkien</author>
    <inv:inventory status="in-stock" isbn="0345340426"
        xmlns:inv="urn:xmlns:25hoursaday-com:inventory-tracking" />
 </book>
</bookstore>

Undeclared Namespaces🔗

If an element has blank namespace (xmlns="") then it is a undeclared namespace and any children element without a prefix namespace is associated to that undeclared namespace.

<bookstore xmlns="urn:xmlns:25hoursaday-com:bookstore">
 <book xmlns="">
    <title>Lord of the Rings</title>
    <author>J.R.R. Tolkien</author>
    <inv:inventory status="in-stock" isbn="0345340426"
        xmlns:inv="urn:xmlns:25hoursaday-com:inventory-tracking" />
 </book>
</bookstore>

Understanding these namespaces helped me finally solve my problem. The reason I could only get the client element and its children was because it was using an undeclared namespace. Using libxmljs is necessary specify the namespace on reading an element with namespace. Which means I was not passing the namespace for those elements with namespace and their children.

This is simple example to clarify the final solution:

var libxmljs = require("libxmljs");
var xml = [
  '<?xml version="1.0" encoding="utf-8"?>',
  '<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema">',
  "  <soap:Body>",
  '    <Response xmlns="http://tempuri.org/">',
  "      <Result>",
  '        <client xmlns="">',
  "          <msg>SEARCH OK.</msg>",
  "          <code>0</code>",
  "        </client>",
  "      </Result>",
  "    </Response>",
  "  </soap:Body>",
  "</soap:Envelope>",
].join("");

var doc = libxmljs.parseXml(xml);

console.log("-->soap:Envelope", doc.get("/soap:Envelope"));
// does not find element

console.log("-->soap:Envelope", doc.get("/soap:Envelope", { soap: "http://schemas.xmlsoap.org/soap/envelope/" }));
// finds element

console.log("-->soap:Body", doc.get("//soap:Body", { soap: "http://schemas.xmlsoap.org/soap/envelope/" }));
// finds element

console.log("-->Response", doc.get("//xmlns:Response", "http://tempuri.org/"));
// finds element

console.log("-->Response", doc.get("//Response"));
// does not find element

console.log("-->Result", doc.get("//Result"));
// does not find element

console.log("-->client", doc.get("//client"));
// finds element

console.log("-->msg", doc.get("//msg"));
// finds element

console.log("-->xmlns:Result", doc.get("//xmlns:Result", "http://tempuri.org/").get("//msg"));
// finds element