AngularFire中没有任何内置功能可用于检测节点存在。但由于AngularFire是基于Firebase JavaScript SDK构建的，因此您可以使用JavaScript API执行此操作：

There is nothing built into AngularFire for detecting if a node exists. But since AngularFire is built on top of the Firebase JavaScript SDK, you can do this with the JavaScript API:

ref.child(uid).once('value', function(snapshot) {
    console.log(snapshot.exists());
});

要实现的重要一点是此代码段使用值 event，如果当前位置没有数据，将触发 null 。

The important thing to realize is that this snippet uses a value event, which will fire null if there is no data at the current location.

A $ firebaseArray（）使用Firebase的 child _ * 事件，这些事件不能用于检测是否存在集合中的特定孩子。

A $firebaseArray() from AngularFire on the other hand uses Firebase's child_* events, which cannot be used to detect existence of a specific child in the collection.