如何在js中等待if语句内部的结果

我正在为web应用程序构建node.js后端，当我提交表单时，我会进行各种验证。其中之一是检查是否已经存在具有相同电子邮件地址的邀请。（isUserAlreadyInvited函数）

我为此创建了一个函数，但当我调用它时，我想响应并没有那么快，即使检查返回true，它也会移动到下一个语句。如何

    //Loop through emails one by one
    var emails_to_invite = ["test@test.com","invalid.com"];
    var response_items = [];
    async.each(emails_to_invite,
        function(email, callback){
            //Response item
            var response_item = {}
            response_item.email = email;                
            //Validate
            if(Joi.validate({ email: email }, apiSchema).error) {
                response_item.error = "not valid email";
                response_items.push(response_item); 
                callback();
            } else if(email == user.email) {
                response_item.error = "Sending an invitation to your own email is kinda dumb";
                response_items.push(response_item); 
                callback();
            } else if(isUserAlreadyInvited(email,user_id)) {
                response_item.error = "already invited";
                response_items.push(response_item); 
                callback();
            } else {
                sendInvitationEmail(email,user_id,null,null,function(invitation_code){
                    if(invitation_code) {
                        response_item.invitationCode = invitation_code;
                    } else {
                        response_item.error = "server error";
                    }
                    response_items.push(response_item);     
                    callback();     
                });
            };
        },
        function(err){
            //Return response
            res.status(200).send(response_items);
        }
    );  
    function isUserAlreadyInvited(email,invited_by) {
        User.findOne({
            email: email,
            invited_by: invited_by
        }, function(err, user) {
            if(err) {
                return false;
            } else {
                return true;
            }
        });
    }