What exactly triggers jQuery ajax success?

I am building some ajax in a Perl web framework Dancer I am not sure it is responding with proper http headers as I cannot trigger jQuery's ajax success handlers from what appear to be otherwise successful requests. Using the ajax snippet below I get the following output in a browser console. The complete callback gets called successfully and gives what looks like successful output. Status:200 StatusText:"OK" However the success handlers never get called.

$.ajax({type: "GET", url: "/learn/faq",
 success: function(data){console.log('omg got it');},
 complete: function(data){console.log("complete", data);}
}).success(function(data){console.log('defered');});


Object
XHR finished loading: "https://www.localhost:4443/learn/faq". assets-d36e1bb9fd59ba3dbd0f8a0cbb37ed8e.js:1
complete 
Object {readyState: 4, responseText: "↵↵<!DOCTYPE html>↵<html xmlns="http://www.w3.org/1…ead/conversion.js"></script>↵↵↵↵</body>↵</html>↵↵", status: 200, statusText: "OK"

I should be seeing the omg got it and defered messages but am not. Looking at this I feel there is more to the jQuery success handler than the status and Dancer http implementation is not responding correctly.

Further more I have since added an error handler to the snippet and the error handler is getting triggered with what looks like a successful request.

$.ajax({type: "GET", url: "/learn/faq",
 success: function(data){console.log('omg got it');},
 complete: function(data){console.log("complete", data);},
error: function(data){console.log("error!", data);}
}).success(function(data){console.log('defered');});
Object
XHR finished loading: "https://www.localhost:4443/learn/faq". assets-8cd028b93e0db9dd9455125dc98d5ae1.js:1
error! 
Object {readyState: 4, responseText: "↵↵↵↵↵↵↵↵↵↵↵↵↵↵↵↵<!DOCTYPE html>↵<html xmlns="http:…></script>↵↵↵↵</body>↵</html>↵↵↵↵</body>↵</html>↵", status: 200, statusText: "OK"}
complete 
Object {readyState: 4, responseText: "↵↵↵↵↵↵↵↵↵↵↵↵↵↵↵↵<!DOCTYPE html>↵<html xmlns="http:…></script>↵↵↵↵</body>↵</html>↵↵↵↵</body>↵</html>↵", status: 200, statusText: "OK"}

Here are the response headers from jQuery getAllResponseHeaders()

complete Date: Tue, 01 Jan 2013 22:43:52 GMT
Content-Encoding: gzip
X-Powered-By: Perl Dancer 1.3095.1
Transfer-Encoding: chunked
Connection: keep-alive
Server: nginx/1.2.4
Strict-Transport-Security: max-age=2592000
Content-Type: text/xml; charset=utf-8

Solution

The success handler will be triggered if

The response has HTTP status code 200 (yours does)
jQuery is able to deserialize the response according to the Content-Type header in the response (or the dataType option, if you provide it, which overrides the Content-Type header)

So for instance, if your response had a Content-Type header of application/json or application/xml, the response you've quoted won't trigger the success handler because it cannot be successfully deserialized as either JSON or XML.

Your latest edit (as of this writing) reveals the problem:

Here are the response headers from jQuery getAllResponseHeaders()

...

Content-Type: text/xml; charset=utf-8

I suspect your response isn't valid XML, so jQuery can't deserialize it as XML, so it's failing.