Waypoints in directions

HTML

<div id="map"></div>
<div id="right-panel">
<div>
<b>Start:</b>
<select id="start">
  <option value=""></option>
  <option value="Halifax, NS">Halifax, NS</option>
  <option value="Boston, MA">Boston, MA</option>
  <option value="New York, NY">New York, NY</option>
  <option value="Miami, FL">Miami, FL</option>
</select>
<br>
<b>Waypoints:</b> <br>
<i>(Ctrl+Click or Cmd+Click for multiple selection)</i> <br>
<select multiple id="waypoints">
  <option value="montreal, quebec">Montreal, QBC</option>
  <option value="toronto, ont">Toronto, ONT</option>
  <option value="chicago, il">Chicago</option>
  <option value="winnipeg, mb">Winnipeg</option>
  <option value="fargo, nd">Fargo</option>
  <option value="calgary, ab">Calgary</option>
  <option value="spokane, wa">Spokane</option>
</select>
<br>
<b>End:</b>
<select id="end">
  <option value="Vancouver, BC">Vancouver, BC</option>
  <option value="Seattle, WA">Seattle, WA</option>
  <option value="San Francisco, CA">San Francisco, CA</option>
  <option value="Los Angeles, CA">Los Angeles, CA</option>
</select>
<br>
  <input type="submit" id="submit">
</div>
<div id="directions-panel"></div>
</div>
<!-- Replace the value of the key parameter with your own API key. -->
<script async defer
src="https://maps.googleapis.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk&callback=initMap">
</script>

CSS

#right-panel {
  font-family: 'Roboto','sans-serif';
  line-height: 30px;
  padding-left: 10px;
}

#right-panel select, #right-panel input {
  font-size: 15px;
}

#right-panel select {
  width: 100%;
}

#right-panel i {
  font-size: 12px;
}
html, body {
  height: 100%;
  margin: 0;
  padding: 0;
}
#map {
  height: 100%;
  float: left;
  width: 70%;
  height: 100%;
}
#right-panel {
  margin: 20px;
  border-width: 2px;
  width: 20%;
  height: 400px;
  float: left;
  text-align: left;
  padding-top: 0;
}
#directions-panel {
  margin-top: 10px;
  background-color: #FFEE77;
  padding: 10px;
  overflow: scroll;
  height: 174px;
}

JavaScript

var map;
var markers = [];
function initMap() {
  var directionsService = new google.maps.DirectionsService;
  var directionsDisplay = new google.maps.DirectionsRenderer({
    polylineOptions: {
      strokeColor: "red"
    }
  });
  map = new google.maps.Map(document.getElementById('map'), {
    zoom: 6,
    center: {lat: 41.85, lng: -87.65}
  });
  directionsDisplay.setMap(map);
    directionsDisplay.setOptions({ suppressMarkers: true });

  document.getElementById('submit').addEventListener('click', function() {
    calculateAndDisplayRoute(directionsService, directionsDisplay);
  });
}

function calculateAndDisplayRoute(directionsService, directionsDisplay) {
  var waypts = [];
  var checkboxArray = document.getElementById('waypoints');
  for (var i = 0; i < checkboxArray.length; i++) {
    if (checkboxArray.options[i].selected) {
      waypts.push({
        location: checkboxArray[i].value,
        stopover: true
      });
    }
  }

  directionsService.route({
    origin: document.getElementById('start').value,
    destination: document.getElementById('end').value,
    waypoints: waypts,
    optimizeWaypoints: true,
    travelMode: 'DRIVING'
  }, function(response, status) {
    if (status === 'OK') {
      directionsDisplay.setDirections(response);
       var leg = response.routes[0].legs[0];
        for (var i = 0; i < markers.length; i++) {
          markers[i].setMap(null);
        }
       markers = [];
      var markerA = new google.maps.Marker({
          position: leg.start_location,
          label: 'A',
          map: map
        });
      var markerB = new google.maps.Marker({
          position: leg.end_location,
          label: 'B',
          map: map
        });
        markers.push(markerA);
        markers.push(markerB);
      }
    else {
      window.alert('Directions request failed due to ' + status);
    }
  });
}