Apollo GraphQL Subscription issues

index.js

import { createServer } from 'http';
import { execute, subscribe } from 'graphql';
import { SubscriptionServer } from 'subscriptions-transport-ws';
import { makeExecutableSchema } from '@graphql-tools/schema';
import express from 'express';
import { ApolloServer } from 'apollo-server-express';

import { gql } from 'apollo-server-core';
import { PubSub } from 'graphql-subscriptions';

let currentNumber = 0;

const typeDefs = gql`
  type Query {
    currentNumber: Int
  }
  type Subscription {
    incremented: Int
  }
`;

const resolvers = {
  Query: {
    currentNumber() {
      return currentNumber;
    },
  },
  Subscription: {
    incremented: {
      subscribe: () => pubsub.asyncIterator('NUMINCREMENTED'),
    },
  },
};

const pubsub = new PubSub();

(async function () {
  const app = express();
  const httpServer = createServer(app);
  const schema = makeExecutableSchema({
    typeDefs,
    resolvers,
  });

  const subscriptionServer = SubscriptionServer.create(
    {
      schema,
      execute,
      subscribe,
      onConnect(connectionParams, webSocket, context) {
        console.log(connectionParams);
        console.log('Connected!');
        return context;
      },
      onDisconnect(webSocket, context) {
        console.log('Disconnected!');
      },
    },
    { server: httpServer }
  );

  const server = new ApolloServer({
    schema,
    plugins: [
      {
        async serverWillStart() {
          return {
            async drainServer() {
              subscriptionServer.close();
            },
          };
        },
      },
    ],
  });
  await server.start();
  server.applyMiddleware({ app });

  const PORT = 4000;
  httpServer.listen(PORT, () => {
    console.log(`Server is now running on http://localhost:${PORT}/graphql`);

    function incrementNumber() {
      currentNumber++;
      pubsub.publish('NUMBER_INCREMENTED', {
        numberIncremented: currentNumber,
      });
      setTimeout(incrementNumber, 1000);
    }

    incrementNumber();
  });
})();