after deleting in firebase on data change dont works code example

Example 1: add to firebase database

db.collection("cities").add({
    name: "Tokyo",
    country: "Japan"
})
.then(function(docRef) {
    console.log("Document written with ID: ", docRef.id);
})
.catch(function(error) {
    console.error("Error adding document: ", error);
});

Example 2: firebase update data

await firebase.database().ref().update(updates);