xmlhttp.onreadystatechange = function()

in code/client/static/js/main.js [233:245]


    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == XMLHttpRequest.DONE) {   // XMLHttpRequest.DONE == 4
           if (xmlhttp.status == httpStatusOk) {
               renderListTrainer(xmlhttp.response);
           }
           else if (xmlhttp.status == httpStatusBadRequest) {
              alert('There was an error httpStatusBadRequest');
           }
           else {
               alert('something else other than httpStatusOk was returned');
           }
        }
    };