Ajax 用户名验证是否存在

function callback(){
var xmlDoc = this.responseXML;
var root = xmlDoc.getElementsByTagName('root')[0];
var nodes = root.getElementsByTagName("username");
var currentNode = null;
var username = document.getElementById('username').value;
for(var i = 0; i nodes.length; i++) {
currentNode = nodes[i];
if(username == currentNode.childNodes[0].nodeValue){
document.getElementById('test').innerHTML = '对不起!'+username+'已经被注册。';
return;
}
}
document.getElementById('test').innerHTML = '用户名' + username +'可以使用!';
}