Add new item to Realtime Database array

How I could append an element to an array like that ?.

Answer

Adding an item to an array structure like that, requires three steps:

1. Read the existing data.
2. Determine the key of the new item.
3. Write the new item.

In code that’d be something like:

const ref = admin.database().ref("javascript");
ref.once("value").then((snapshot) => {
  let numChildren = parseInt(snapshot.numChildren());
  ref.child(""+(numChildren+1)).set(4);
});

Note that this type of data structure is fairly non-idiomatic when it comes to Firebase, and I recommend reading:

• Best Practices: Arrays in Firebase.