当我加载页面时，Firebase .on（“ value”）方法给我一个空错误。裁判不存在

Question

我正在使用Firebase，并且在名为Shout的引用上有一个on（“ value”）事件。首次加载页面时，此shoutRef不存在。当我登陆Firebase时，在shoutRef内部的快照为空。如何避免这种情况？为什么要查看不存在的裁判？

    at main.js:127
    at EventRegistration.ts:133
    at qr (util.ts:586)
    at t.raise (EventQueue.ts:171)
    at t.raiseQueuedEventsMatchingPredicate_ (EventQueue.ts:123)
    at t.raiseEventsForChangedPath (EventQueue.ts:103)
    at t.onDataUpdate_ (Repo.ts:277)
    at h.onDataPush_ (PersistentConnection.ts:604)
    at h.onDataMessage_ (PersistentConnection.ts:597)
    at t.onDataMessage_ (Connection.ts:317)


var firebaseData = firebase.database();
    var shoutRef = firebaseData.ref("/shout ref");

 shoutRef.on("value", (snapshot) => {
        var snap = snapshot.val();
        //set global variable TODO:May not need in future
        console.log(snapshot.val());
});