function createXMLHttpRequest() {

            try {

                return new XMLHttpRequest();

            } catch (e) {

                try {

                    return new ActiveXObject("Msxml2.XMLHTTP");

                } catch (e) {

                    return new ActiveXObject("Microsoft.XMLHTTP");

                }

            }

        }

	var xmlHttp = createXMLHttpRequest();

            xmlHttp.onreadystatechange = function() {

                if(xmlHttp.readyState == 4 && xmlHttp.status == 200) {

                    if(xmlHttp.responseText == "true") {

                        document.getElementById("error").innerText = "用户名已被注册！";

                        document.getElementById("error").textContent = "用户名已被注册！";

                    } else {

                        document.getElementById("error").innerText = "";

                        document.getElementById("error").textContent = "";

                    }

                }

            };

            xmlHttp.open("POST", "/ajax_check/", true, "json");

            xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

            var username = document.getElementById("username").value;

            xmlHttp.send("username=" + username);