onComplete and JSON in Xhr

if your code looks like that:
var xhr = new Xhr('/some/url/', {

    onComplete: function(request) {
        if (request.successful()) {
            var json = request.responseJSON;
            // json is undefined
        }
    }
,).send();

you will never get JSON content because in stateChanged "complete" fired before "success"
(2.2.3 - line 4568 in right-src.js) and tryScripts will be called after your code