File System Nodejs: "Uncaught TypeError: fs.writeFileSync is not a function"

No need for serverside code. Solution:

onGo(e){

    e.preventDefault();


    var json = {
        foo: 'bar',
        qux: 'moo',
        poo: 123
    }

    JSONToCSVConvertor(json, 'some-data', true);

 }

function JSONToCSVConvertor(JSONData, ReportTitle, ShowLabel) {
    //If JSONData is not an object then JSON.parse will parse the JSON string in an Object
    var arrData = typeof JSONData != 'object' ? JSON.parse(JSONData) : JSONData;

    var CSV = '';    
    //Set Report title in first row or line

    CSV += ReportTitle + '\r\n\n';

    //This condition will generate the Label/Header
    if (ShowLabel) {
        var row = "";

        //This loop will extract the label from 1st index of on array
        for (var index in arrData[0]) {

            //Now convert each value to string and comma-seprated
            row += index + ',';
        }

        row = row.slice(0, -1);

        //append Label row with line break
        CSV += row + '\r\n';
    }

    //1st loop is to extract each row
    for (var i = 0; i < arrData.length; i++) {
        var row = "";

        //2nd loop will extract each column and convert it in string comma-seprated
        for (var index in arrData[i]) {
            row += '"' + arrData[i][index] + '",';
        }

        row.slice(0, row.length - 1);

        //add a line break after each row
        CSV += row + '\r\n';
    }

    if (CSV == '') {        
        alert("Invalid data");
        return;
    }   

    //Generate a file name
    var fileName = "MyReport_";
    //this will remove the blank-spaces from the title and replace it with an underscore
    fileName += ReportTitle.replace(/ /g,"_");   

    //Initialize file format you want csv or xls
    var uri = 'data:text/csv;charset=utf-8,' + escape(CSV);

    // Now the little tricky part.
    // you can use either>> window.open(uri);
    // but this will not work in some browsers
    // or you will not get the correct file extension    

    //this trick will generate a temp <a /> tag
    var link = document.createElement("a");    
    link.href = uri;

    //set the visibility hidden so it will not effect on your web-layout
    link.style = "visibility:hidden";
    link.download = fileName + ".csv";

    //this part will append the anchor tag and remove it after automatic click
    document.body.appendChild(link);
    link.click();
    document.body.removeChild(link);
}