Slightly cleaned up version of xml2json by Stefan Goessner/2006 http://goessner.net

xml2json.js

/*  This work is licensed under Creative Commons GNU LGPL License.
    License: http://creativecommons.org/licenses/LGPL/2.1/
    Version: 0.9
    Author:  Stefan Goessner/2006
    Web:     http://goessner.net/

    Original file: https://goessner.net/download/prj/jsonxml/
*/

function xml2json(xml) {
   const X = {
      toObj: function (xml) {
         let o = {};
         if (xml.nodeType == Node.ELEMENT_NODE) {
            if (xml.attributes.length)
               for (let i = 0; i < xml.attributes.length; i++)
                  o["@" + xml.attributes[i].nodeName] = (xml.attributes[i].nodeValue || "").toString();
            if (xml.firstChild) { // element has child nodes ..
               let textChild = 0, cdataChild = 0, hasElementChild = false;
               for (let n = xml.firstChild; n; n = n.nextSibling) {
                  if (n.nodeType == Node.ELEMENT_NODE) hasElementChild = true;
                  else if (n.nodeType == Node.TEXT_NODE && n.nodeValue.trim()) textChild++; // non-whitespace text
                  else if (n.nodeType == Node.CDATA_SECTION_NODE) cdataChild++; // cdata section node
               }
               if (hasElementChild) {
                  if (textChild < 2 && cdataChild < 2) { // structured element with evtl. a single text or/and cdata node ..
                     for (let n = xml.firstChild; n; n = n.nextSibling) {
                        if (n.nodeType == Node.TEXT_NODE) { // text node
                           if (n.nodeValue.trim())
                              o["#text"] = X.escape(n.nodeValue);
                        }
                        else if (n.nodeType == Node.CDATA_SECTION_NODE)  // cdata node
                           o["#cdata"] = X.escape(n.nodeValue);
                        else if (o[n.nodeName]) {  // multiple occurence of element ..
                           if (o[n.nodeName] instanceof Array)
                              o[n.nodeName][o[n.nodeName].length] = X.toObj(n);
                           else
                              o[n.nodeName] = [o[n.nodeName], X.toObj(n)];
                        }
                        else  // first occurence of element..
                           o[n.nodeName] = X.toObj(n);
                     }
                  }
                  else { // mixed content
                     if (!xml.attributes.length)
                        o = X.escape(X.innerXml(xml));
                     else
                        o["#text"] = X.escape(X.innerXml(xml));
                  }
               }
               else if (textChild) { // pure text
                  if (!xml.attributes.length)
                     o = X.escape(X.innerXml(xml));
                  else
                     o["#text"] = X.escape(X.innerXml(xml));
               }
               else if (cdataChild) { // cdata
                  if (cdataChild > 1)
                     o = X.escape(X.innerXml(xml));
                  else
                     for (let n = xml.firstChild; n; n = n.nextSibling)
                        o["#cdata"] = X.escape(n.nodeValue);
               }
            }
            if (!xml.attributes.length && !xml.firstChild) o = null;
         }
         else if (xml.nodeType == Node.DOCUMENT_NODE) { // document.node
            o = X.toObj(xml.documentElement);
         }
         else
            console.error("unhandled node type: " + xml.nodeType);
         return o;
      },
      toJson: function (o, name) {
         let json = name ? ("\"" + name + "\"") : "";
         if (o instanceof Array) {
            o = o.map(m => X.toJson(m, ""));
            json += (name ? ":[" : "[") + (o.length > 1 ? o.join(",") : o.join("")) + "]";
         }
         else if (o == null)
            json += (name && ":") + "null";
         else if (typeof (o) == "object") {
            let arr = [];
            for (let m in o)
               arr[arr.length] = X.toJson(o[m], m);
            json += (name ? ":" : "") + "{" + (arr.length > 1 ? arr.join(",") : arr.join("")) + "}";
         }
         else if (typeof (o) == "string")
            json += (name && ":") + "\"" + o.toString() + "\"";
         else
            json += (name && ":") + o.toString();
         return json;
      },
      innerXml: function (node) {
         let s = ""
         if ("innerHTML" in node)
            s = node.innerHTML;
         else {
            const asXml = function (n) {
               let s = "";
               if (n.nodeType == Node.TEXT_NODE)
                  s += n.nodeValue;
               else if (n.nodeType == Node.CDATA_SECTION_NODE)
                  s += "<![CDATA[" + n.nodeValue + "]]>";
               else if (n.nodeType == Node.ELEMENT_NODE) {
                  s += "<" + n.nodeName;
                  n.attributes.map(attr => { s += " " + attr.nodeName + "=\"" + (attr.nodeName || "").toString() + "\""; })
                  if (n.firstChild) {
                     s += ">";
                     for (let c = n.firstChild; c; c = c.nextSibling)
                        s += asXml(c);
                     s += "</" + n.nodeName + ">";
                  }
                  else
                     s += "/>";
               }
               return s;
            };
            for (let c = node.firstChild; c; c = c.nextSibling)
               s += asXml(c);
         }
         return s;
      },
      escape: function (txt) {
         return txt.replace(/[\\]/g, "\\\\")
            .replace(/[\"]/g, '\\"')
            .replace(/[\n]/g, '\\n')
            .replace(/[\r]/g, '\\r');
      }
   };
   if (xml.nodeType == Node.DOCUMENT_NODE) {
      xml = xml.documentElement;
   }
   return JSON.parse('{' + X.toJson(X.toObj(xml), xml.nodeName) + '}');
}