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;
}
 

解決方法：新增async: false,即修改此方法為同步

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;
}