Ajax中将success返回值return给上一级函数

以邮箱验证功能为例

function ckEmail(){
    var email = $('#txtEmail').val();	
    if(/^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+(\.[a-zA-Z0-9-]+)*\.[a-zA-Z0-9]{2,6}$/.test(email)){
        $("#emailValidMsg").html("<img src='../images/load.gif' width='12' height='12' align='top'/><span style='color:#666666'>正在检测邮件可用性</span>");		
        var result = null;//定义全局变量，用来接收ajax返回参数
	$.ajax({
	    url:"${ pageContext.request.contextPath }/user/ckEmailServer",
	    type:"post",
	    data:"user.email="+email,
	    dataType:"json",
	    async:false,//将同步标志改为false，代表执行完后续代码才返回结果
	    success:function(json){
		 if("ok" == json.message) {
		  	$("#emailValidMsg").html("<span style='color:#666666'><img src='../images/right.gif'  width='12' height='12' align='top'/> 电子邮件地址可用</span>");		  				
		  	result = true;
		  } else {
		  	$("#emailValidMsg").html("<img src='../images/wrong.gif'  width='12' height='12' align='top'/> 抱歉，邮件地址已经被使用");		  				
		  	result = false;
		  	}
		}
	});
	return result;
    } else if(email.length==0){
	$("#emailValidMsg").html("<img src='../images/wrong.gif'  width='12' height='12' align='top'/> 电子邮件不能为空");
	    return false;
    } else{
	$("#emailValidMsg").html("<img src='../images/wrong.gif'  width='12' height='12' align='top'/> 电子邮件地址格式错误");
	    return false;
    }
}