We spent quite some time addressing how RESTful services should represent each state, including erroneous ones, gracefully. A well-defined API should demand from its consumers to handle all its errors gracefully and to provide as much information per state as required, rather than just stating "An error has occurred". That is why it should look up the returned status code and clearly distinguish between client requests such as 400 Bad Request or 415 Unsupported media types caused by faulty payload, caused by wrong media types, or authentication-related errors, such as 401 Unauthorized.

The status code of an erroneous response is available in the error callback of the jQuery callback function and should be used to provide detailed information back to the request:

 $.ajax({
        url: "http://localhost:3000/catalog/v2/",
        type: "POST",
        dataType: "json",
        data: JSON.stringify(newItem),
        success: function (item, status, jqXHR) {
            alert(status);
        },
        error: function(jqXHR, statusText, error) {
            switch(jqXHR.status) {
               case 400: alert('Bad request'); break;
               case 401: alert('Unauthroizaed'); break;
               case 404: alert('Not found'); break;
               //handle any other client errors below
               case 500: alert('Internal server error); break;
               //handle any other server errors below
            }
        }
      });

Unsuccessful requests are handled by the error callback function. It provides jqXHR - the XmlHttpRequest JavaScript—object as its first argument. It carries across all the request/response related information, such as status code and headers. Use it to determine what the requested server has returned so that your application can handle different errors more granularly.