Untitled

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Firebase project</title>
    <style>
        #game {
            background: black;
            width: 20%;

            text-align: center;
        }

        #txt {
            color: #fff;
        }
    </style>
    <script src="https://www.gstatic.com/firebasejs/5.2.0/firebase.js"></script>
</head>


<div id="game">
    <h1 id="txt"></h1>
</div>
<button id="btn">Click</button><br><br>


<body>


    <script>
        let txt = document.querySelector('#txt'),
            btn = document.querySelector('#btn'),
            score = 0,
            database;


        var config = {
            apiKey: "AIzaSyCF70ZhC-lNHxITMLP_DPhC7N-jx58GLM4",
            authDomain: "first-firebase-project-bd0ae.firebaseapp.com",
            databaseURL: "https://first-firebase-project-bd0ae.firebaseio.com",
            projectId: "first-firebase-project-bd0ae",
            storageBucket: "first-firebase-project-bd0ae.appspot.com",
            messagingSenderId: "50887066191"
        };
        firebase.initializeApp(config);
        database = firebase.database();
        let ref = database.ref('scores');
        ref.on('value', gotData, errData);

        function gotData(data) {
            console.log(data.val());
            let scores = data.val();
            let keys = Object.keys(scores);
            console.log(keys);
            for (let i = 0; i < keys.length; i++) {
                let k = keys[i],
                    score = scores[k].score;
                console.log(score);

            }

        }

        function errData(err) {
            console.log("error");
            console.log(err);

        }


        btn.onclick = function() {
            let data = {
                score: score - 1
            }

            console.log(data);
            let ref = database.ref('scores');

            ref.push(data);

            txt.innerHTML = score++;
        }
    </script>
</body>

</html>