What version of Exchange are you using ? i would suggestion having a read of the this first http://social.technet.microsoft.com/Forums/en-US/exchangesvrdevelopment/thread/77c07b69-eea6-43ab-9225-4b384cecfc9d and try posting more detail do you need to use Javascript ? its going to be a lot easier using other languages.
Cheers
Glen
From a API perspective you can use WebDAV on Exchange 2003 http://www.infinitec.de/post/2004/12/31/Retrieving-a-users-availability-%28freebusy-data%29.aspx
On 2010 you will need to use Exchange Web Services over SOAP using GetUserAvailibility
http://msdn.microsoft.com/en-us/library/aa564001(v=exchg.140).aspx.
Cheers
Glen