iD/js/lib/jxon.js

const JXON = new (function () {
  const
    sValueProp = "keyValue", sAttributesProp = "keyAttributes", sAttrPref = "@", /* you can customize these values */
    aCache = [], rIsNull = /^\s*$/, rIsBool = /^(?:true|false)$/i;

  function parseText (sValue) {
    if (rIsNull.test(sValue)) { return null; }
    if (rIsBool.test(sValue)) { return sValue.toLowerCase() === "true"; }
    if (isFinite(sValue)) { return parseFloat(sValue); }
    if (isFinite(Date.parse(sValue))) { return new Date(sValue); }
    return sValue;
  }

  function EmptyTree () { }
  EmptyTree.prototype.toString = function () { return "null"; };
  EmptyTree.prototype.valueOf = function () { return null; };

  function objectify (vValue) {
    return vValue === null ? new EmptyTree() : vValue instanceof Object ? vValue : new vValue.constructor(vValue);
  }

  function createObjTree (oParentNode, nVerb, bFreeze, bNesteAttr) {
    const
      nLevelStart = aCache.length, bChildren = oParentNode.hasChildNodes(),
      bAttributes = oParentNode.hasAttributes(), bHighVerb = Boolean(nVerb & 2);

    var
      sProp, vContent, nLength = 0, sCollectedTxt = "",
      vResult = bHighVerb ? {} : /* put here the default value for empty nodes: */ true;

    if (bChildren) {
      for (var oNode, nItem = 0; nItem < oParentNode.childNodes.length; nItem++) {
        oNode = oParentNode.childNodes.item(nItem);
        if (oNode.nodeType === 4) { sCollectedTxt += oNode.nodeValue; } /* nodeType is "CDATASection" (4) */
        else if (oNode.nodeType === 3) { sCollectedTxt += oNode.nodeValue.trim(); } /* nodeType is "Text" (3) */
        else if (oNode.nodeType === 1 && !oNode.prefix) { aCache.push(oNode); } /* nodeType is "Element" (1) */
      }
    }

    const nLevelEnd = aCache.length, vBuiltVal = parseText(sCollectedTxt);

    if (!bHighVerb && (bChildren || bAttributes)) { vResult = nVerb === 0 ? objectify(vBuiltVal) : {}; }

    for (var nElId = nLevelStart; nElId < nLevelEnd; nElId++) {
      sProp = aCache[nElId].nodeName.toLowerCase();
      vContent = createObjTree(aCache[nElId], nVerb, bFreeze, bNesteAttr);
      if (vResult.hasOwnProperty(sProp)) {
        if (vResult[sProp].constructor !== Array) { vResult[sProp] = [vResult[sProp]]; }
        vResult[sProp].push(vContent);
      } else {
        vResult[sProp] = vContent;
        nLength++;
      }
    }

    if (bAttributes) {
      const
        nAttrLen = oParentNode.attributes.length,
        sAPrefix = bNesteAttr ? "" : sAttrPref, oAttrParent = bNesteAttr ? {} : vResult;

      for (var oAttrib, nAttrib = 0; nAttrib < nAttrLen; nLength++, nAttrib++) {
        oAttrib = oParentNode.attributes.item(nAttrib);
        oAttrParent[sAPrefix + oAttrib.name.toLowerCase()] = parseText(oAttrib.value.trim());
      }

      if (bNesteAttr) {
        if (bFreeze) { Object.freeze(oAttrParent); }
        vResult[sAttributesProp] = oAttrParent;
        nLength -= nAttrLen - 1;
      }
    }

    if (nVerb === 3 || (nVerb === 2 || nVerb === 1 && nLength > 0) && sCollectedTxt) {
      vResult[sValueProp] = vBuiltVal;
    } else if (!bHighVerb && nLength === 0 && sCollectedTxt) {
      vResult = vBuiltVal;
    }

    if (bFreeze && (bHighVerb || nLength > 0)) { Object.freeze(vResult); }

    aCache.length = nLevelStart;

    return vResult;
  }

  function loadObjTree (oXMLDoc, oParentEl, oParentObj) {
    var vValue, oChild;

    if (oParentObj instanceof String || oParentObj instanceof Number || oParentObj instanceof Boolean) {
      oParentEl.appendChild(oXMLDoc.createTextNode(oParentObj.toString())); /* verbosity level is 0 */
    } else if (oParentObj.constructor === Date) {
      oParentEl.appendChild(oXMLDoc.createTextNode(oParentObj.toGMTString()));    
    }

    for (var sName in oParentObj) {
      vValue = oParentObj[sName];
      if (isFinite(sName) || vValue instanceof Function) { continue; } /* verbosity level is 0 */
      if (sName === sValueProp) {
        if (vValue !== null && vValue !== true) { oParentEl.appendChild(oXMLDoc.createTextNode(vValue.constructor === Date ? vValue.toGMTString() : String(vValue))); }
      } else if (sName === sAttributesProp) { /* verbosity level is 3 */
        for (var sAttrib in vValue) { oParentEl.setAttribute(sAttrib, vValue[sAttrib]); }
      } else if (sName.charAt(0) === sAttrPref) {
        oParentEl.setAttribute(sName.slice(1), vValue);
      } else if (vValue.constructor === Array) {
        for (var nItem = 0; nItem < vValue.length; nItem++) {
          oChild = oXMLDoc.createElement(sName);
          loadObjTree(oXMLDoc, oChild, vValue[nItem]);
          oParentEl.appendChild(oChild);
        }
      } else {
        oChild = oXMLDoc.createElement(sName);
        if (vValue instanceof Object) {
          loadObjTree(oXMLDoc, oChild, vValue);
        } else if (vValue !== null && vValue !== true) {
          oChild.appendChild(oXMLDoc.createTextNode(vValue.toString()));
        }
        oParentEl.appendChild(oChild);
     }
   }
  }

  this.build = function (oXMLParent, nVerbosity /* optional */, bFreeze /* optional */, bNesteAttributes /* optional */) {
    const _nVerb = arguments.length > 1 && typeof nVerbosity === "number" ? nVerbosity & 3 : /* put here the default verbosity level: */ 1;
    return createObjTree(oXMLParent, _nVerb, bFreeze || false, arguments.length > 3 ? bNesteAttributes : _nVerb === 3);    
  };

  this.unbuild = function (oObjTree) {    
    const oNewDoc = document.implementation.createDocument("", "", null);
    loadObjTree(oNewDoc, oNewDoc, oObjTree);
    return oNewDoc;
  };

  this.stringify = function (oObjTree) {
    return (new XMLSerializer()).serializeToString(JXON.unbuild(oObjTree));
  };
})();
// var myObject = JXON.build(doc);
// we got our javascript object! try: alert(JSON.stringify(myObject));

// var newDoc = JXON.unbuild(myObject);
// we got our Document instance! try: alert((new XMLSerializer()).serializeToString(newDoc));