// cdn
<script src="https://www.gstatic.com/firebasejs/4.9.0/firebase.js"></script>
<script src="https://www.gstatic.com/firebasejs/4.9.0/firebase-firestore.js"></script>

// js
var db = firebase.firestore();
firebase.initializeApp({
  apiKey: 'AIzaSyAaJ-xa21TAN8f8PbEubfMTh1234567890',
  authDomain: 'helloworld-11594.firebaseapp.com',
  projectId: 'helloworld-11594'
});

// read data
db.collection("device").get().then((querySnapshot) => {
    querySnapshot.forEach((doc) => {
        console.log(doc.data());
    });
});

// write data
db.collection("user").add({
    first: "Ada",
    last: "Lovelace",
    born: 1815
})
.then(function(docRef) {
    console.log("Document written with ID: ", docRef.id);
})
.catch(function(error) {
    console.error("Error adding document: ", error);
});