polyline.js

let map;
let markersArray = [];
// define global variable to store polyline
let polyline = null;

function initMap() {
  ...
  map.addListener('click', function(e) {
    //console.log(e);
    addMarker(e.latLng);
    
    drawPolyline();
  });
}

function addMarker(latLng) {
  ...
}
  
function drawPolyline() {
  let markersPositionArray = [];
  // obtain latlng of all markers added on map
  markersArray.forEach(function(e) {
    markersPositionArray.push(e.getPosition());
  });
  
  // check if there is already polyline drawn on map
  // remove the polyline from map before we draw new one
  if (polyline !== null) {
    polyline.setMap(null);
  }
  
  // draw new polyline at markers' position
  polyline = new google.maps.Polyline({
    map: map,
    path: markersPositionArray,
    strokeOpacity: 0.4
  });
}