Ajax异步调用函数，同步调用函数举例

var xmlhttp;
//ajax 异步调用
function loadXMLDocAsyn(url, cfunc) {
	if (window.XMLHttpRequest) {
		// code for IE7+, Firefox, Chrome, Opera, Safari
		xmlhttp = new XMLHttpRequest();
	} else {
		// code for IE6, IE5
		xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
	}
	xmlhttp.onreadystatechange = cfunc;
	xmlhttp.open("GET", url, true);
	xmlhttp.send();
}

//ajax 同步调用
function loadXMLDocSyn(url) {
	if (window.XMLHttpRequest) {
		// code for IE7+, Firefox, Chrome, Opera, Safari
		xmlhttp = new XMLHttpRequest();
	} else {
		// code for IE6, IE5
		xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
	}
	xmlhttp.open("GET", url, false);
	xmlhttp.send();
	return xmlhttp.status;
}

 //检查用户名是否合法，包括客户端检查和服务端ajax调用

function checkName(){
	var username = document.getElementById("username").value;
	document.getElementById("tip").innerHTML = " ";
	if(username.length==0){	//用户名格式检查
		document.getElementById("tip").innerHTML = "<font color='#FF0000'>用户名不能为空</font>";
		document.getElementById("username").focus();
		document.getElementById("flag").value = "false";
	}else{//调用ajax检查用户名是否重复
		loadXMLDocAsyn("action/checkNameAction.php?username="+username, getResult);
//**************************************************************/
//以下是同步调用实现方式		
//		var status = loadXMLDocSyn("action/checkNameAction.php?username="+username);
//		if(status==200){
//			if(xmlhttp.responseText.length!=0){
//				document.getElementById("tip").innerHTML = "<font color='#FF0000'>用户名已被占用</font>";
//				document.getElementById("username").focus();
//				document.getElementById("flag").value = "false";
//			}else{
//				document.getElementById("flag").value = "true";
//			}
//		}
//******************************************************************/		
	}
}


//用户名查重checkName()中ajax的回调函数
function getResult() {
	if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
		if(xmlhttp.responseText.length!=0){
			document.getElementById("tip").innerHTML = "<font color='#FF0000'>用户名已被占用</font>";
			document.getElementById("username").focus();
			document.getElementById("flag").value = "false";
		}else{
			document.getElementById("flag").value = "true";
		}
	}
}