Error when using google maps api directions-Collection of common programming errors

I get the following error trying to declare

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

The error in my console

 TypeError: Result of expression 'google.maps.DirectionsService' [undefined] is not a       constructor.

Any ideas?

1. You must add script reference to the of your page:

Originally posted 2013-11-09 23:05:42.