Hi! I've found in the spec of XHR Level 2 that if a malformed JSON's received from a server, the response property would be set to null. But null is a valid JSON, so, if I understand correctly, there is no way to distinct a malformed JSON response from a response containing only 'null', which is, again, a valid JSON:
$ node -p -e 'JSON.parse("null")' null $ -- Kirill Dmitrenko Yandex Maps Team