Cloud functions for Firebase child_added

I am trying to use Cloud functions for Firebase by creating a function that will add an alias when a new user is added to the database (not auth). From Firebase Documention I found that I need to use:

ref.on("child_added", function(snapshot, prevChildKey) { 
    //Something
})

      

        
        
        
      

    

but i cant even run this function. My code:

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

var ref = admin.database().ref("/users");

ref.on("child_added", function(snapshot, prevChildKey) {
  console.log(snapshot)
})

      

        
        
        
      

    

Can anyone help me?

Update:

After working on it, this is now my code:

const functions = require('firebase-functions');
// Import Admin SDK
var admin = require("firebase-admin");
admin.initializeApp(functions.config().firebase);

var db = admin.database();
var ref = db.ref('users');
var childs = [];
var nicknames = [];

exports.newUser = functions.database.ref('users').onWrite(event => {
    const data = event.data.val();
    ref.on("child_added", function(snapshot, childKey) {
        if (!childs.includes(childKey)) {
            childs.push(childKey);
        };
        for (i in childs) {
            if (childs[i] == null) {
                childs.splice(i, 1);
            };
        };
    });

    ref.once("value", function(snapshot) {
        for (i in childs) {
            var child = data[childs[i]];
            if (!child.hasOwnProperty("nickname")) {
                console.log("Child does not have nickname: " + childs[i]);
                ref.child(childs[i]).update({
                    nickname: "user"+childs[i]
                });
            };
        };
    });
});

      

        
        
        
      

    

Based on what Doug Stevenson answered, I still need to figure out how to do this only if a new child is added (new user)