function addMarker(latLng) {
  let marker = new google.maps.Marker({
      map: map,
      position: latLng,
      draggable: true
  });

  // add listener to redraw the polyline when markers position change
  marker.addListener('position_changed', function() {
    drawPolyline();
  });

  //store the marker object drawn in global array
  markersArray.push(marker);
}