REST Service, Json and XML

REST support 2 native data formats: Json and XML. XML is default and is browser friendly with Atom feed. Json is programming friendly, particularly Ajax friendly because of its small dataload and easy deseralization. Either way, REST is very web friendly as it uses http like syntax which is easy for script to make call or for user to browse.

A common misconcept is Json is required for Ajax. Ajax can work with any format, but Json has significant advantage over XML. When making an Ajax call to a REST service, the request header can be specified with an "Accept:" format, but it is just a "wish", and will be granted only when the service has a capability for the requested format. So the following code will return error when the service doesn't response with Json data:

var data = response.get_responseData();
data = eval("(" + data + ")")

Also keep in mind, Visual Studio (2010 RC) doesn't provide java script intellisense for REST response data. In contrast, Ajax-enabled WCF service can emit a script proxy (via asp:serviceRefernce), so it has full intellisense support, and has built-in serialization/deserialization support with default Json format.