So far, the catalog service supports only the JSON format, and thus works only with the media type application/json. Let's assume our service has to offer data in different formats, for example, both JSON and XML. Then, the consumer needs to explicitly define the data format they need. The best way to carry out content negotiation in REST has been a very debatable subject for a long time.

In his famous discussion on implementing content negotiation correctly, Roy Fielding states the following:

However, that leaves a gap on how to expose the same resource in a different data format, so Roy continues with the following:

While one can still choose to stick with a URI-driven negotiation by providing the desired format with custom GET parameters, the REST community has chosen to stick to Roy's suggestion for agent-driven negotiation. Now that it has been almost a decade since this argument was initiated, it has been proven that they took the right decision. Agent-driven negotiation makes use of the Accept HTTP header.

The Accept HTTP header specifies the media type of the resource that the consumer is willing to process. In addition to the Accept header, the consumer may also make use of the Accept-Language and Accept-Encoding headers to specify what language and encoding the results should be provided in. If the server fails to provide the results in the expected format, it can either return a default value or make use of HTTP 406 Not acceptable in order not to cause data confusion errors on the client side.

The Node.js HTTP response object contains a method, format, that performs content negotiation based on the Accept HTTP header if set in the request object. It uses the built-in request.accepts() to select an appropriate handler for the request. If that is not found, the server invokes the default handler, which responds with HTTP 406 Not acceptable. Let's create a demo on how to use the format method within one of our routes. For that purpose, let's assume we have implemented a function within our catalog module, named list_groups_in_xml, that provides the group data in XML format:

app.get('/catalog', function(request, response) { 
    response.format( { 
      'text/xml' : function() { 
         response.send(catalog.findCategoiesXml()); 
      }, 
      'application/json' : function() { 
         response.json(catalog.findCategoriesJson()); 
      }, 
      'default' : function() {. 
         response.status(406).send('Not Acceptable'); 
      }    
    }); 
}); 

This is how you can implement content negotiation in a clear and straightforward way.