试试将xhr.send(serialize(form))
改为
var formData = new FormData(form);
xhr.send(formData);
formData是HTML5用于异步提交表单的,应该可以满足楼主的需求。
var fd = new FormData;
fd.append("username",document.querySelector("#username").value);
fd.append("password",document.querySelector("#password").value);
...
xhr.send(fd);