JSFiddle - React, Tailwind, and code Playground

HTML

<script type="text/javascript" src="http://maps.googleapis.com/maps/api/js?sensor=false"></script><div>
<strong>Start: </strong>
<select id="start" onchange="calcRoute();">
  <option value="chicago, il">Chicago</option>
  <option value="st louis, mo">St Louis</option>
  <option value="joplin, mo">Joplin, MO</option>
  <option value="oklahoma city, ok">Oklahoma City</option>
  <option value="amarillo, tx">Amarillo</option>
  <option value="gallup, nm">Gallup, NM</option>
  <option value="flagstaff, az">Flagstaff, AZ</option>
  <option value="winona, az">Winona</option>
  <option value="kingman, az">Kingman</option>
  <option value="barstow, ca">Barstow</option>
  <option value="san bernardino, ca">San Bernardino</option>
  <option value="los angeles, ca">Los Angeles</option>
</select>
<strong>End: </strong>
<select id="end" onchange="calcRoute();">
  <option value="chicago, il">Chicago</option>
  <option value="st louis, mo">St Louis</option>
  <option value="joplin, mo">Joplin, MO</option>
  <option value="oklahoma city, ok">Oklahoma City</option>
  <option value="amarillo, tx">Amarillo</option>
  <option value="gallup, nm">Gallup, NM</option>
  <option value="flagstaff, az">Flagstaff, AZ</option>
  <option value="winona, az">Winona</option>
  <option value="kingman, az">Kingman</option>
  <option value="barstow, ca">Barstow</option>
  <option value="san bernardino, ca">San Bernardino</option>
  <option value="los angeles, ca">Los Angeles</option>
</select>
</div>
<div id="map_canvas"></div>

CSS

html, body, #map_canvas { margin: 0; padding: 0; height: 100% }

JavaScript

var directionsDisplay;
var directionsService = new google.maps.DirectionsService();
var map;

function initialize() {
  directionsDisplay = new google.maps.DirectionsRenderer();
  var chicago = new google.maps.LatLng(41.850033, -87.6500523);
  var myOptions = {
    zoom:7,
    mapTypeId: google.maps.MapTypeId.ROADMAP,
    center: chicago
  }
  map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);
  directionsDisplay.setMap(map);
}

function calcRoute() {
  var start = document.getElementById("start").value;
  var end = document.getElementById("end").value;
  var request = {
    origin:start,
    destination:end,
    travelMode: google.maps.TravelMode.DRIVING
  };

  directionsService.route(request, function(result, status) {
    if (status == google.maps.DirectionsStatus.OK) {
        
      //I believe this is the async. function
      directionsDisplay.setDirections(result);
        
      // removing setTimeout removes desired functionality
      setTimeout(function() {
        map.setZoom(12);
        map.panTo(result.routes[0].overview_path[0]);
      }, 3000);
    }
  });
}
      google.maps.event.addDomListener(window, 'load', initialize);