Convert XML to JSON (and back) using Javascript

I've created a recursive function based on regex, in case you don't want to install library and understand the logic behind what's happening:

const xmlSample = '<tag>tag content</tag><tag2>another content</tag2><tag3><insideTag>inside content</insideTag><emptyTag /></tag3>';
console.log(parseXmlToJson(xmlSample));

function parseXmlToJson(xml) {
    const json = {};
    for (const res of xml.matchAll(/(?:<(\w*)(?:\s[^>]*)*>)((?:(?!<\1).)*)(?:<\/\1>)|<(\w*)(?:\s*)*\/>/gm)) {
        const key = res[1] || res[3];
        const value = res[2] && parseXmlToJson(res[2]);
        json[key] = ((value && Object.keys(value).length) ? value : res[2]) || null;

    }
    return json;
}

Regex explanation for each loop:

• res[0] - return the xml (as is)
• res[1] - return the xml tag name
• res[2] - return the xml content
• res[3] - return the xml tag name in case the tag closes itself. In example: <tag />

You can check how the regex works here: https://regex101.com/r/ZJpCAL/1

Note: In case json has a key with an undefined value, it is being removed. That's why I've inserted null at the end of line 9.

You can also use txml. It can parse into a DOM made of simple objects and stringify. In the result, the content will be trimmed. So formating of the original with whitespaces will be lost. But this could be used very good to minify HTML.

const xml = require('txml');
const data = `
<tag>tag content</tag>
<tag2>another content</tag2>
<tag3>
  <insideTag>inside content</insideTag>
  <emptyTag />
</tag3>`;

const dom = xml(data); // the dom can be JSON.stringified

xml.stringify(dom); // this will return the dom into an xml-string

Disclaimer: I am the author of txml, the fastest xml parser in javascript.

I was using xmlToJson just to get a single value of the xml.
I found doing the following is much easier (if the xml only occurs once..)

let xml =
'<person>' +
  ' <id>762384324</id>' +
  ' <firstname>Hank</firstname> ' +
  ' <lastname>Stone</lastname>' +
'</person>';

let getXmlValue = function(str, key) {
  return str.substring(
    str.lastIndexOf('<' + key + '>') + ('<' + key + '>').length,
    str.lastIndexOf('</' + key + '>')
  );
}


alert(getXmlValue(xml, 'firstname')); // gives back Hank

Here' a good tool from a documented and very famous npm library that does the xml <-> js conversions very well: differently from some (maybe all) of the above proposed solutions, it converts xml comments also.

var obj = {name: "Super", Surname: "Man", age: 23};

var builder = new xml2js.Builder();
var xml = builder.buildObject(obj);

In 6 simple ES6 lines:

xml2json = xml => {                                                                                                                                                     
  var el = xml.nodeType === 9 ? xml.documentElement : xml                                                                                                               
  var h  = {name: el.nodeName}                                                                                                                                          
  h.content    = Array.from(el.childNodes || []).filter(e => e.nodeType === 3).map(e => e.textContent).join('').trim()                                                  
  h.attributes = Array.from(el.attributes || []).filter(a => a).reduce((h, a) => { h[a.name] = a.value; return h }, {})                                                 
  h.children   = Array.from(el.childNodes || []).filter(e => e.nodeType === 1).map(c => h[c.nodeName] = xml2json(c))                                                    
  return h                                                                                                                                                              
}  

Test with echo "xml2json_example()" | node -r xml2json.es6 with source at https://github.com/brauliobo/biochemical-db/blob/master/lib/xml2json.es6