Google Maps API throws “Uncaught ReferenceError: google is not defined” only when using AJAX

The API can’t be loaded after the document has finished loading by default, you’ll need to load it asynchronous.

modify the page with the map:

<div id="map_canvas" style="height: 354px; width:713px;"></div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false&callback=initialize"></script>
<script>
var directionsDisplay,
    directionsService,
    map;

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

</script>

For more details take a look at: https://stackoverflow.com/questions/14184956/async-google-maps-api-v3-undefined-is-not-a-function/14185834#14185834

Example: http://jsfiddle.net/doktormolle/zJ5em/