var config = {

    apiKey: "AIzaSyCvpYQu44zmRXA0XQkcxEuakyytRsnWXSc",

    authDomain: "stackoverflow.firebaseapp.com",

    databaseURL: "https://stackoverflow.firebaseio.com",

    storageBucket: "project-8080059325282098184.appspot.com",

  };

  firebase.initializeApp(config);

​

var ref = firebase.database().ref("/39195191");

var userId = 1;

​

​

function listenTo(name, query) {

  query.once('value', function(snapshot) {

    console.log(name+': resulted in '+snapshot.numChildren()+' results');

    snapshot.forEach(function(child) {

      console.log(child.key+': '+JSON.stringify(child.val(), null,'  '));

    });

  });

}

​

listenTo(

  'absence of child prop', 

  ref.orderByChild('child').equalTo(null)

);

listenTo(

  'id=1', 

  ref.orderByChild('user/id').equalTo(1)

);

listenTo(

  'absence of a nested prop', 

  ref.orderByChild('user/name').equalTo(null)

);

​