I'm trying to update the viewer's position in a Leaflet map by using the watch: true option like this:

var redIcon = new L.Icon({
  iconUrl: 'https://raw.githubusercontent.com/pointhi/leaflet-color-markers/master/img/marker-icon-2x-red.png',
  shadowUrl: 'https://cdnjs.cloudflare.com/ajax/libs/leaflet/0.7.7/images/marker-shadow.png',
  iconSize: [25, 41],
  iconAnchor: [12, 41],
  popupAnchor: [1, -34],
  shadowSize: [41, 41]
});
map.locate({setView: true, watch: true, maxZoom: 18});
function onLocationFound(e) {
    L.marker(e.latlng, {icon: redIcon}).addTo(map)
}

map.on('locationfound', onLocationFound);

However the marker does not update its position. Do I have to wrap that code inside another function and make another call? Any hint will be appreciated, since my JS (programming knowledge) is very basic.