How to get the key from a Firebase data snapshot?

You could do something like this:

var key = Object.keys(snapshot.val())[0];

Ref: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/keys

The Object.keys() method returns an array of a given object's own enumerable properties, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

Realtime database:

For this you can simple use: snapshot.key

snapshot = firebase.database.DataSnapshot

this.app.database()
        .ref('/data/')
        .on('value', function(snapshot) {
            const id = snapshot.key;

            //----------OR----------//
            const data = snapshot.val() || null;
            if (data) {
              const id = Object.keys(data)[0];
            }
        });

Firestore:

snapshot.id

snapshot = firebase.firestore.DocumentSnapshot

this.app.firestore()
        .collection('collection')
        .doc('document')
        .onSnapshot(function(snapshot) {
            const id = snapshot.id;

            //----------OR----------//
            const data = snapshot.data() || null;
            if (data) {
              const id = Object.keys(data)[0];
            }
        });

users.orderByChild('email').equalTo(authData.user.email) is a Query (doc) that you have built by "chaining together one or more of the filter methods". What is a bit specific with your query is that it returns a dataSnapshot with only one child, since you query with equalTo(authData.user.email).

As explained here, in this exact case, you should loop over the returned dataSnapshot with forEach():

Attaching a value observer to a list of data will return the entire list of data as a single snapshot which you can then loop over to access individual children.

Even when there is only a single match for the query, the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result, as follows:

ref.once('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var childKey = childSnapshot.key;
    var childData = childSnapshot.val();
    // ...
  });
});