Geolocation, permission and Google Maps

I'm programming a site to get the user's location in real time, however I have 3 problems:

• accuracy
• permission to use the location (I do not know what event it takes when the user allows you to use your location or deny it, I can not work on it)
• Markers in Google Maps, if I leave a loop to pick up the user's location, I need to leave the marker in there and simply multiply the markers too.

setTimeout(function(){initMap();}, 500);

if('geolocation' in navigator){
  var mapa;
  function initMap() {
    window.myLatLng;
    function getPosition(){
      markers = []; 
        navigator.geolocation.getCurrentPosition(function(position, accuracy){
            var lat = position.coords.latitude;
            var lng = position.coords.longitude;
            var accuracy = position.coords.accuracy;
            myLatLng = {lat: lat, lng: lng};
            mapa.setCenter({lat:window.myLatLng.lat, lng:window.myLatLng.lng});

// marker.setMap(mapa);
            });
    }

    setInterval(function(){
        getPosition();

    }, 500);


    x = 35;
    y = 139;
    setTimeout(function(){

        mapa = new google.maps.Map(document.getElementById('mapa'), {
            center: {lat:window.myLatLng.lat, lng:window.myLatLng.lng},
            minZoom:16,
          maxZoom:18,
          zoom:17,
          zoomControl: false,
          scaleControl: false,
          fullscreenControl: false,
          streetViewControl: false,
          panControl: false,
          signInControl: false,
          mapTypeControl: false,
          noClear: true,
          disableDefaultUI: false

        });
    }, 1000);
  }
}else{
    alert('Seu navegador não possui suporte');
}