Why is jqXHR.responseText returning a string instead of a JSON object?

I had the same problem. I returns a string because it formulated from an exception. E.g. I use a kernel listener with serialization to json on my Symfony2 project. Which is correct for proper REST headers.

Anyway, just parse it; this works for me:

$.ajaxSetup({
    "error": function(jqXHR, status, thrownError) {
        alert('error');
        var responseText = jQuery.parseJSON(jqXHR.responseText);
        console.log(responseText);
    }
});