function GetUserInfo(tp) {  
    var username;  
    $.ajax({  
        type: "POST",  
        cache: false,  
        data: "type=exlogin&tp=" + tp,  
        url: "Handle/OpeartionHandler.ashx",  
        success: function(userinfo) {  
            username = userinfo;  
        }, error: function(data) {  
            username = "";  
        }  
    });  
    return username;  
} 

对于此方法调用之后会一直返回undefined，原因是Jquery的ajax是异步的，所以大多时候没执行完AJAX就return htmlcontent了，所以会一直返回undefined, 解决方法：添加async: false,即修改此方法为同步

function GetUserInfo(tp) {  
    var username;  
    $.ajax({  
        type: "POST",  
        cache: false,  
        data: "type=exlogin&tp=" + tp,  
        async: false,  
        url: "Handle/OpeartionHandler.ashx",  
        success: function(userinfo) {  
            username = userinfo;  
        }, error: function(data) {  
            username = "";  
        }  
    });  
    return username;  
}