server.js

const { Firestore, FieldValue } = require("@google-cloud/firestore");
const credentials = JSON.parse(process.env.FIREBASE_SERVICE_ACCOUNT);
const db = new Firestore({ projectId: credentials.project_id, credentials });

const docRef = db.collection("counters").doc("visitors");

docRef.set({}, { merge: true })

const incrementAndGetVisitorCount = async () =>
  docRef
    .update({ count: FieldValue.increment(1) })
    .then(() => docRef.get())
    .then(doc => doc.data().count);

app.get("/count", async (request, response) => {
  incrementAndGetVisitorCount().then(count => {
    response.json(count);
  });
});