如何对firebase的文档进行排序？

firebase.database().ref('users').orderByChild('age').once('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var childKey = childSnapshot.key;
    var childData = childSnapshot.val();
    console.log(childKey, childData);
  });
});

firebase.database().ref('users').orderByKey().once('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var childKey = childSnapshot.key;
    var childData = childSnapshot.val();
    console.log(childKey, childData);
  });
});

firebase.database().ref('users').orderByValue().once('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var childKey = childSnapshot.key;
    var childData = childSnapshot.val();
    console.log(childKey, childData);
  });
});