Do you know what the co-ordinates are for the point? If so, the API for elevation will return the elevation, in both JSON and XML formats. I use the following URL (albeit in PHP, it should work the same for Javascript):
http://maps.googleapis.com/maps/api/elevation/xml?sensor=false&locations=1.111,1.111.
You'll need to parse the results in JS. Switch XML out for JSON in the URL to change the result type.
Edit: If you use this service, you need to use it in conjunction with a Google Map as per their terms and conditions.