Polling data from the server and show notification if there a new data

index.html

<button onclick="sendMessage('start')">Start Listen</button>
<button onclick="sendMessage('stop')">Stop Listen</button>
<script>
  let se;
  navigator.serviceWorker.register('/sw.js').then(data => {
    se = data.active;
    console.log('se ready!');
  });
  function sendMessage(message) {
    if (!se) {
      console.log('sw is not ready yet');
      return;
    }
    se.postMessage(message);
  }
  Notification.requestPermission().then(registration => {
    console.log(registration);
  });
</script>

server.js

// content of index.js
const http = require('http')
const port = 3000

const requestHandler = (request, response) => {
  response.writeHead(200, {
    "Content-Type": "application/json",
    "Access-Control-Allow-Origin": "*",
  });
  response.end(JSON.stringify(new Array(3).fill(0).map((u, i) => ({
    _id: i
  }))));
}

const server = http.createServer(requestHandler)

server.listen(port, (err) => {
  if (err) {
    return console.log('something bad happened', err)
  }

  console.log(`server is listening on ${port}`)
})

sw.js

let interval;
let cache = [];

self.addEventListener('notificationclick', function(event) {
  event.notification.close();

  event.waitUntil(clients.matchAll({
    type: 'window'
  }).then(function(clientList) {
    if (clientList.length) {
      const [client] = clientList;

      if ('focus' in client) {
        return client.focus();
      } else if (clients.openWindow) {
        return clients.openWindow('/#notification');
      }
    }
  }));
});

function startListen() {
  interval = setInterval(() => {
    fetch('http://localhost:3000')
      .then(data => data.json())
      .then(data => {
        if (data.some(i => !cache.includes(i._id))) {
          sendNotification();
          cache = data.map(u => u._id);
        }
      });
  }, 1000);
}

function stopListen() {
  clearInterval(interval);
}

function sendNotification() {
  self.registration.showNotification(`New user!`, {
    body: 'Click to go to approval panel',
    icon: '',
    tag: '',
  });
}

self.addEventListener('message', ({data}) => {
  switch (data) {
    case 'start':
      startListen();
      break;
    case 'stop':
      stopListen();
      break;
  }
});