Can't add node to xml with javascript

I have a little form that i'm gonna use to add node to xml file:

and this is the xml I try to add node to:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<data>
    <games>
        <game id="1">
            <name>Bobs Bygg</name>
            <difficulty>5</difficulty>
            <numberOfRisks>4</numberOfRisks>
            <budget>1400000</budget>
            <targetGameTime>75</targetGameTime>
            <targetDays>85</targetDays>
            <numberOfWorkers>5</numberOfWorkers>
            <overtimeWorkers>2</overtimeWorkers>
        </game>

        <game id="2">
            <name>Bassams Bygg</name>
            <difficulty>2</difficulty>
            <numberOfRisks>4</numberOfRisks>
            <budget>14000000</budget>
            <targetGameTime>75</targetGameTime>
            <targetDays>85</targetDays>
            <numberOfWorkers>6</numberOfWorkers>
            <overtimeWorkers>1</overtimeWorkers>
        </game>
    </games>
</data>

So i want to make a new "game" node. I have not finished my function to get data from the inputboxes, but my function won't insert anything.

function addGame(){

        var xhttp = new XMLHttpRequest();
        xhttp.overrideMimeType('text/xml');
        xhttp.open("GET", "Data/gameData.xml", false);
        xhttp.send(null);
        var xmlDoc = xhttp.responseXML;
        var newgame = xmlDoc.createElement("gameTest");
        xmlDoc.getElementsByTagName("games")[0].appendChild(newgame);
        alert("hopefully success");
}

When I debug the code, the xml file gets loaded, and the xmldoc will get the new node inserted, but this won't get saved to the file. I have checked the file on server.