JavaScript: alerting a specific value on my real-time database

Question

I've been trying to achieve the following, but I couldn't

and here is my snapshot data:

If I wanted to alert ada's first name, what code should i use?

I used this, it returns undefined:

    if (user) {
    // User is signed in.
      alert('user signed')

      // Get a reference to the database service
var database = firebase.database();

      var query = firebase.database().ref("users");
query.once("value")
  .then(function(snapshot) {


    snapshot.forEach(function(childSnapshot) {
      // key will be "ada" the first time and "alan" the second time
      var key = childSnapshot.key.val;
      // childData will be the actual contents of the child
      var childData = childSnapshot.val();

        alert(key);
  });

but it didn't work, any suggestions?

error image:

note: completely new to JavaScript

Answer 1

in the docs , it is

// key will be "ada" the first time and "alan" the second time
var key = childSnapshot.key;

not var key = childSnapshot.key.val;