Generate file with nodejs

Question

I have the following problem, I have an xml to json converter in javascript and this converter saves the converted object in a variable. I need to adapt this code to make it possible to write the file from the nodejs, but so far I have not been able to solve the problem. Converter code:

function XMLtoJSON() {
    var me = this; //
 
    me.fromFile = function(xml, rstr) {
        var xhttp = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
        // sets and sends the request for calling "xml"
        xhttp.open("GET", xml, false);
        xhttp.send(null);

        // gets the JSON string
        var json_str = jsontoStr(setJsonObj(xhttp.responseXML));

        return (typeof(rstr) == 'undefined') ? JSON.parse(json_str) : json_str;
    }

    me.fromStr = function(xml, rstr) {
        if (window.DOMParser) {
            var getxml = new DOMParser();
            var xmlDoc = getxml.parseFromString(xml, "text/xml");
        } else {
            var xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
            xmlDoc.async = "false";
        }

        var json_str = jsontoStr(setJsonObj(xmlDoc));
        return (typeof(rstr) == 'undefined') ? JSON.parse(json_str) : json_str;
    }

    var setJsonObj = function(xml) {
        var js_obj = {};
        if (xml.nodeType == 1) {
            if (xml.attributes.length > 0) {
                js_obj["@attributes"] = {};
                for (var j = 0; j < xml.attributes.length; j++) {
                    var attribute = xml.attributes.item(j);
                    js_obj["@attributes"][attribute.nodeName] = attribute.value;

                }
            }
        } else if (xml.nodeType == 3) {
            js_obj = xml.nodeValue;
        }
        if (xml.hasChildNodes()) {
            for (var i = 0; i < xml.childNodes.length; i++) {
                var item = xml.childNodes.item(i);
                var nodeName = item.nodeName;
                if (typeof(js_obj[nodeName]) == "undefined") {
                    js_obj[nodeName] = setJsonObj(item);
                    if (nodeName == "child") {
                        //js_obj[nodeName] = setJsonObj(item);
                        js_obj[nodeName] = [];
                        js_obj[nodeName].push(old);
                        js_obj[nodeName].push(setJsonObj(item));
                    }

                } else {
                    if (typeof(js_obj[nodeName].push) == "undefined") {

                        var old = js_obj[nodeName];
                        js_obj[nodeName] = [];
                        js_obj[nodeName].push(old);
                    }
                    js_obj[nodeName].push(setJsonObj(item));
                }
            }
        }
        return js_obj;
    }


    var jsontoStr = function(js_obj) {
        var rejsn = JSON.stringify(js_obj, undefined, 2).replace(/(\\t|\\r|\\n)/g, '').replace(/"",[\n\t\r\s]+""[,]*/g, '').replace(/(\n[\t\s\r]*\n)/g, '').replace(/[\s\t]{2,}""[,]{0,1}/g, '').replace(/"[\s\t]{1,}"[,]{0,1}/g, '').replace(/\[[\t\s]*\]/g, '""').replace(/null,/g, '').replace(/[\\]/g, ' - ');
        return (rejsn.indexOf('"parsererror": {') == -1) ? rejsn : 'Invalid XML format';
    }
};

var xml2json = new XMLtoJSON();

I have tried to add the following code inside this js:

var fs = require('fs');
fs.writeFile("/tmp/test", "Hey there!", function(err) {
    if(err) {
        return console.log(err);
    }

    console.log("The file was saved!");
}); 

But I haven’t been successful yet.
Right on the first line informs me that require is not set. Does anyone have any light for this case? I intend to create an html with a button and at the click of the button will generate the file.