var config = {

    apiKey: "AIzaSyCvpYQu44zmRXA0XQkcxEuakyytRsnWXSc",

    authDomain: "stackoverflow.firebaseapp.com",

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

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

};

firebase.initializeApp(config);

​

console.clear();

var ref = firebase.database().ref("/42115037");

var auth = firebase.auth();

​

var offset = null;

​

ref.child('articles')

    .orderByChild('topic')

    .startAt('a')

    .endAt('a', '-KcNAv5Dd33KE2qfWVFa')

    //.startAt(offset)

    .limitToLast(2)

    .once('value', snapshot => {

      snapshot.forEach(s => console.log(s.key));

    })