Leaflet Draw - toGeoJSON - html

Question

I am still new to Leaflet and have been teaching myself. I have made a map using the Leaflet Draw libraries which works well, and I would like to add a function that allows the user to save the drawn features to a GeoJSON file. I have tried using this guide but I am still not having any luck.
I am using a button for the export function:
<input type='button' id='export' value='Export Feature (.kml)' class='btn' />
With the following body:
var osmUrl = 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png',
osmAttrib = '© OpenStreetMap ',
osm = L.tileLayer(osmUrl, { maxZoom: 18, attribution: osmAttrib }),
map = new L.Map('map', { center: new L.LatLng(-29.5, 24.85), zoom: 6 }),
drawnItems = L.featureGroup().addTo(map);
L.control.layers({
'Street': osm.addTo(map),
"Satellite": L.tileLayer('https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}', {
attribution: 'Tiles © Esri</a>'
})
}, {'Survey Area': drawnItems }, { position: 'topleft', collapsed: false }).addTo(map);
map.addControl(new L.Control.Draw({
edit: {
featureGroup: drawnItems,
poly: {
allowIntersection: false
}
},
draw: {
polygon: {
allowIntersection: false,
showArea: true
}
}
}));
map.on(L.Draw.Event.CREATED, function (event) {
var layer = event.layer;
drawnItems.addLayer(layer);
});
document.getElementById('export').onclick = function(e) {
// Extract GeoJson from featureGroup
var data = featureGroup.toGeoJSON();
// Stringify the GeoJson
var convertedData = 'text/json;charset=utf-8,' + encodeURIComponent(JSON.stringify(data));
// Create export
document.getElementById('export').setAttribute('href', 'data:' + convertedData);
document.getElementById('export').setAttribute('download','data.geojson');
}
I think the issue is with featureGroup or drawnItems but I just can't figure it out.