Convert XML to JSON using the browser's DOMParser without external dependencies.

xml2json.ts

export interface IObject {
  [key: string]: any;
}

/*
  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,
  Conversion: Erik Vullings/2021 converted to TypeScript
  Web:        https://goessner.net/download/prj/jsonxml/
*/

/**
 * Convert an XML string to JSON
 * @param xmlStr: the XML input as string
 * @param excludeKeys: an optional set of keys to exclude when parsing string values to numbers.
 * @param jsonifyKeys: If true, convert the keys to start with a lowercase letter
 */
export const xml2json = <T extends IObject>(
  xmlStr: string,
  excludeKeys: Set<string> = new Set(),
  jsonifyKeys = false
): T => {
  let xml = new DOMParser().parseFromString(xmlStr, 'text/xml') as Document | HTMLElement;
  var X = {
    toObj: function (xml: Document | HTMLElement | ChildNode): IObject | string | null {
      var o = {} as IObject | string | null;
      if (xml.nodeType == 1) {
        // element node ..
        if ((xml as HTMLElement).attributes.length)
          // element with attributes  ..
          for (var i = 0; i < (xml as HTMLElement).attributes.length; i++)
            (o as IObject)['@' + (xml as HTMLElement).attributes[i].nodeName] = (
              (xml as HTMLElement).attributes[i].nodeValue || ''
            ).toString();
        if (xml.firstChild) {
          // element has child nodes ..
          var textChild = 0,
            cdataChild = 0,
            hasElementChild = false;
          for (let n = xml.firstChild as HTMLElement | ChildNode | null; n; n = n.nextSibling) {
            if (n.nodeType == 1) hasElementChild = true;
            else if (n.nodeType == 3 && n.nodeValue && n.nodeValue.match(/[^ \f\n\r\t\v]/))
              textChild++;
            // non-whitespace text
            else if (n.nodeType == 4) cdataChild++; // cdata section node
          }
          if (hasElementChild) {
            if (textChild < 2 && cdataChild < 2) {
              // structured element with evtl. a single text or/and cdata node ..
              X.removeWhite(xml as HTMLElement);
              for (var n = xml.firstChild as HTMLElement | ChildNode | null; n; n = n.nextSibling) {
                if (n.nodeType == 3 && n.nodeValue)
                  // text node
                  (o as IObject)['#text'] = X.escape(n.nodeValue);
                else if (n.nodeType == 4 && n.nodeValue)
                  // cdata node
                  (o as IObject)['#cdata'] = X.escape(n.nodeValue);
                else if ((o as IObject)[n.nodeName]) {
                  // multiple occurence of element ..
                  if ((o as IObject)[n.nodeName] instanceof Array)
                    (o as IObject)[n.nodeName][(o as IObject)[n.nodeName].length] = X.toObj(n);
                  else (o as IObject)[n.nodeName] = [(o as IObject)[n.nodeName], X.toObj(n)];
                } // first occurence of element..
                else (o as IObject)[n.nodeName] = X.toObj(n);
              }
            } else {
              // mixed content
              if (!(xml as HTMLElement).attributes.length)
                o = X.escape(X.innerXml(xml as HTMLElement));
              else (o as IObject)['#text'] = X.escape(X.innerXml(xml as HTMLElement));
            }
          } else if (textChild) {
            // pure text
            if (!(xml as HTMLElement).attributes.length)
              o = X.escape(X.innerXml(xml as HTMLElement));
            else (o as IObject)['#text'] = X.escape(X.innerXml(xml as HTMLElement));
          } else if (cdataChild) {
            // cdata
            if (cdataChild > 1) o = X.escape(X.innerXml(xml as HTMLElement));
            else
              for (let n = xml.firstChild as ChildNode | null; n; n = n.nextSibling)
                (o as IObject)['#cdata'] = n.nodeValue ? X.escape(n.nodeValue) : '';
          }
        }
        if (!(xml as HTMLElement).attributes.length && !xml.firstChild) o = null;
      } else if (xml.nodeType == 9) {
        // document.node
        o = X.toObj((xml as Document).documentElement);
      } else alert('unhandled node type: ' + xml.nodeType);
      return o;
    },
    toJson: function (
      o: { [key: string]: any } | string | number | boolean | Array<any> | null,
      name: string,
      ind: string
    ) {
      var json = name ? '"' + name + '"' : '';
      if (o instanceof Array) {
        for (var i = 0, n = o.length; i < n; i++) o[i] = X.toJson(o[i], '', ind + '\t');
        json +=
          (name ? ':[' : '[') +
          (o.length > 1
            ? '\n' + ind + '\t' + o.join(',\n' + ind + '\t') + '\n' + ind
            : o.join('')) +
          ']';
      } else if (o == null) json += (name && ':') + 'null';
      else if (typeof o == 'object') {
        var arr = [];
        for (var m in o) arr[arr.length] = X.toJson(o[m], m, ind + '\t');
        json +=
          (name ? ':{' : '{') +
          (arr.length > 1
            ? '\n' + ind + '\t' + arr.join(',\n' + ind + '\t') + '\n' + ind
            : arr.join('')) +
          '}';
      } else if (typeof o == 'string') json += (name && ':') + '"' + o.toString() + '"';
      else json += (name && ':') + o.toString();
      return json;
    },
    innerXml: function (node: Document | HTMLElement) {
      var s = '';
      if ('innerHTML' in node) s = node.innerHTML;
      else {
        var asXml = function (n: HTMLElement) {
          var s = '';
          if (n.nodeType == 1) {
            s += '<' + n.nodeName;
            for (var i = 0; i < n.attributes.length; i++)
              s +=
                ' ' +
                n.attributes[i].nodeName +
                '="' +
                (n.attributes[i].nodeValue || '').toString() +
                '"';
            if (n.firstChild) {
              s += '>';
              for (let c = n.firstChild as HTMLElement | ChildNode | null; c; c = c.nextSibling)
                s += asXml(c as HTMLElement);
              s += '</' + n.nodeName + '>';
            } else s += '/>';
          } else if (n.nodeType == 3) s += n.nodeValue;
          else if (n.nodeType == 4) s += '<![CDATA[' + n.nodeValue + ']]>';
          return s;
        };
        for (var c = node.firstChild; c; c = c.nextSibling) s += asXml(c as HTMLElement);
      }
      return s;
    },
    escape: function (txt: string) {
      return txt
        .replace(/[\\]/g, '\\\\')
        .replace(/[\"]/g, '\\"')
        .replace(/[\n]/g, '\\n')
        .replace(/[\r]/g, '\\r');
    },
    removeWhite: function (e: ChildNode) {
      e.normalize();
      for (var n = e.firstChild; n; ) {
        if (n.nodeType == 3) {
          // text node
          if (!n.nodeValue!.match(/[^ \f\n\r\t\v]/)) {
            // pure whitespace text node
            var nxt = n.nextSibling;
            e.removeChild(n);
            n = nxt;
          } else n = n.nextSibling;
        } else if (n.nodeType == 1) {
          // element node
          X.removeWhite(n);
          n = n.nextSibling;
        } // any other node
        else n = n.nextSibling;
      }
      return e;
    },
  };
  if (xml.nodeType == 9)
    // document node
    xml = (xml as Document).documentElement;
  const jsonStr = `{${X.toJson(X.toObj(X.removeWhite(xml as HTMLElement)), xml.nodeName, '\t')}}`;
  const toNumber = (key: string, value: any) =>
    !excludeKeys.has(key) && typeof value === 'string' && !isNaN(+value) ? +value : value;

  const multipleUpperCaseLettersRegex = /([A-Z]+)([A-Z])/;
  const reviver = jsonifyKeys
    ? function (_key: string, value: any) {
        if (value && typeof value === 'object' && !Array.isArray(value)) {
          // It's a non-null, non-array object, create a replacement with the keys initially-capped
          const newValue = {} as IObject;
          for (const key in value) {
            const newKey = multipleUpperCaseLettersRegex.test(key)
              ? key.replace(
                  multipleUpperCaseLettersRegex,
                  (_full, a, b) => `${a.toLowerCase()}${b}`
                )
              : key.charAt(0).toLowerCase() + key.slice(1);
            newValue[newKey] = toNumber(key, value[key]);
          }
          return newValue;
        }
        return value;
      }
    : toNumber;
  return JSON.parse(jsonStr, reviver);
};

This is lovely and worked for most of my xml files, but then I hit case with a windows style path where I wasn't able to get it running using your code. I see you're trying to escape backslashes on line 153, but I wasn't able to pinpoint to why it's happening.

Here's a snippet for testing if you'd like to have a look at it.

<?xml version="1.0"?>
<Project name="C:\Path\To\File.xml"></Project>

I can't guarantee it's valid xml, however it's a snippet of exported file from Autocad Civil 3D, so I'd assume it's valid.