Firebase - Event Types

Firebase offers several different event types for reading data. Some of the most commonly used ones are described below.

value

The first event type is value. We showed you how to use value in our last chapter. This event type will be triggered every time the data changes and it will retrieve all the data including children.

child_added

This event type will be triggered once for every player and every time a new player is added to our data. It is useful for reading list data because we get access of the added player and previous player from the list.

Example

Let us consider the following example.

var playersRef = firebase.database().ref("players/");

playersRef.on("child_added", function(data, prevChildKey) {
   var newPlayer = data.val();
   console.log("name: " + newPlayer.name);
   console.log("age: " + newPlayer.age);
   console.log("number: " + newPlayer.number);
   console.log("Previous Player: " + prevChildKey);
});
				

We will get the following result.

If we add a new player named Bob, we will get the updated data.

child_changed

This event type is triggered when the data has changed.

Example

Let us consider the following example.

var playersRef = firebase.database().ref("players/");

playersRef.on("child_changed", function(data) {
   var player = data.val();
   console.log("The updated player name is " + player.name);
});
				

We can change Bob to Maria in Firebase to get the update.

child_removed

If we want to get access of deleted data, we can use child_removed event type.

Example

Let us consider the following example.

var playersRef = firebase.database().ref("players/");

playersRef.on("child_removed", function(data) {
   var deletedPlayer = data.val();
   console.log(deletedPlayer.name + " has been deleted");
});
				

Now, we can delete Maria from Firebase to get notifications.