Retrieving data from Firebase for web not working

I'm trying to read and show the data from my Firebase Database using many ways but could not get it work. I've the following structure in my firebase : Firebase database

And this is my code:

var usersRef = firebase.database().ref('users');
     usersRef.on('value', function(snapshot)  {
     snapshot.forEach(function(childSnapshot) {
      var childData = childSnapshot.val();
      console.log(childData);
     });
    });

The console.log don't display anything.

Note: I already tried using "child_added" instead of "value" and it does not work.