该场景是当玩家在应用程序中点击“加入游戏”时，调用云函数将玩家添加到firebase数据库，并将结果作为响应传递。
数据库结构：

/games
|--/game_id
|----/players => this is a json array of uids and their status([{"uid1": true}, {"uid2":true},...])
|----/max_players = 10

云函数应该检查/players的大小是否小于/max_players，只有这样，它才应该用播放器的uid更新/players并返回响应。
以下是我的云函数：

export const addUserToGame = functions.https.onCall((data, context) => {

    // Expected inputs - game_id(from data) and UID(from context)

    if (context.auth == null) {
        return {
            "status": 403,
            "message": "You are not authorized to access this feature"
        };
    }

    const uid = context.auth.uid;
    const game_id = data.game_id;

    let gameIDRef = gamesRef.child(game_id);

    return gameIDRef.transaction(function (t) {

        if (t === null) {
            // don't do anything, can i just do return;
        } else {

            let playersNode;
            let isAlreadyPlayer = false;
            if (t.players) { 
                console.log("players is not empty");

                playersNode = t.players;
                let players_count = playersNode.length;
                let max_players: Number = t.max_players;

                if (players_count >= max_players) {
                    // how can i return an error from here?
                }

                for (var i = 0; i < playersNode.length; i++) {
                    if (playersNode[i].uid === uid) {
                        isAlreadyPlayer = true;
                    }
                    break;
                }

                if (!isAlreadyPlayer) {
                    playersNode.push({
                        [uid]: "active"
                    });
                }
              
             t.players = playersNode;
             return t;

            } else {
                playersNode = [];
                playersNode.push({
                    [uid]: "active"
                });

                t.players = playersNode;
                return t;
            }

        }

    },function(error, committed, snapshot) {

        if(committed) {
            return {
                "status": 200,
                "message": "Player added successfully"
            };
        } else {
            return {
                "status": 403,
                "message": "Error adding player to the game"
            };
        }

    });

});

请看上面代码中的注解，当一个条件失败时，我如何发送回响应？
感谢@Doug和@Renaud的快速回复，我还有几个问题：
Q1.所以我必须使用

return gameIDRef.transaction(t => {

    }).then(result => {

    }).catch(error => {

    });

但这是什么时候用的？这是@Renaud所指的回调版本吗？

function (error, committed, snapshot) {
});

Q2.第一种方式，当gameIDRef.transaction（t => {}）被调用时，当game_id没有值和game_id没有值时会发生什么，我想告诉用户没有指定id的游戏。我怎么能做到呢？

return gameIDRef.transaction(t => {
      if(t === null)
        return; 
//Where does the control go from here? To then? 

// Also i have a conditional check in my logic, which is to make sure the numbers of players doesn't exceed the max_players before adding the player

t.players = where the existing players are stored
t.max_players = maximum number of players a game can have

if(t.players.length >= t.max_players)
// How do i send back a message to client saying that the game is full?
// Throw an error like this?
throw new functions.https.HttpsError('failed-condition', 'The game is already full');

    }).then(result => {

    }).catch(error => {
// and handle it here to send message to client?
    });