send a websocket message in a route, Express.js

The Goal:

Let's say we have a marketplace full of shops. I'm creating a specific page localhost:3000/livePurchases/:storeId for a shop owner to monitor live notifications of when they got a new purchase.

alert('you received a new purchase') should be triggered by a WebSocket when an item is purchased.

The Problem:

I need to set up WebSockets on my express server so that the websocket can be triggered somewhere in my code different from where the websocket server was set up. But I don't understand how to do this.

The route /newPurchase/:storeId would be requested by the browser of a customer after they successfully purchase an item. The websocket should send a message within the code of the route "/newPurchase/:storeId" (backend) to the websocket on "/livePurchases/:storeId" (frontend) where the shop owner can monitor live purchases.

app.js

const express = require("express");

module.exports = (config) => {
  const app = express();

  app.post("/newPurchase/:storeId", (req, res, next) => {
    const { storeId } = req.params;
    // trigger websocket message to `localhost:3000/livePurchases/:storeId`
    // when client requests this route

  });

  return app;
};

But app.js is exported and run from another script, www.js. In real scenarios this is to connect a database before running the app.:

www.js

const app = require("../server/app")();

const port = process.env.PORT || "4000";
app.set("port", port);

app
  .listen(port)
  .on("listening", () =>
    console.log("info", `HTTP server listening on port ${port}`)
  );

module.exports = app;

So that means that the web socket server needs to be set up in www.js.

Below is a notifier service I got it from this tutorial, which seemed like it was trying to solve the problem I have, but it didn't explain how to implement it. It is a class that handles the websocket.

NotifierService.js

const url = require("url");
const { Server } = require("ws");

class NotifierService {
  constructor() {
    this.connections = new Map();
  }

  connect(server) {
    this.server = new Server({ noServer: true });
    this.interval = setInterval(this.checkAll.bind(this), 10000);
    this.server.on("close", this.close.bind(this));
    this.server.on("connection", this.add.bind(this));
    server.on("upgrade", (request, socket, head) => {
      console.log("ws upgrade");
      const id = url.parse(request.url, true).query.storeId;

      if (id) {
        this.server.handleUpgrade(request, socket, head, (ws) =>
          this.server.emit("connection", id, ws)
        );
      } else {
        socket.destroy();
      }
    });
  }

  add(id, socket) {
    console.log("ws add");
    socket.isAlive = true;
    socket.on("pong", () => (socket.isAlive = true));
    socket.on("close", this.remove.bind(this, id));
    this.connections.set(id, socket);
  }

  send(id, message) {
    console.log("ws sending message");

    const connection = this.connections.get(id);

    connection.send(JSON.stringify(message));
  }

  broadcast(message) {
    console.log("ws broadcast");
    this.connections.forEach((connection) =>
      connection.send(JSON.stringify(message))
    );
  }

  isAlive(id) {
    return !!this.connections.get(id);
  }

  checkAll() {
    this.connections.forEach((connection) => {
      if (!connection.isAlive) {
        return connection.terminate();
      }

      connection.isAlive = false;
      connection.ping("");
    });
  }

  remove(id) {
    this.connections.delete(id);
  }

  close() {
    clearInterval(this.interval);
  }
}

module.exports = NotifierService;

Where I left off implementing the `NotifierService`

I added the websocket server with the NotifierService in www.js

www.js with websockets added

const app = require("../server/app")();
const NotifierService = require("../server/NotifierService.js");
const notifier = new NotifierService();
const http = require("http");
const server = http.createServer(app);
notifier.connect(server);
const port = process.env.PORT || "4000";
app.set("port", port);

server
  .listen(port)
  .on("listening", () =>
    console.log("info", `HTTP server listening on port ${port}`)
  );

module.exports = app;

But now how do I send the websocket message from the /newPurchase route in app.js on the backend? If I create a new instance of NotifierService in app.js in order to use the notifierService.send method in the /newPurchase route, then the new NotifierService instance won't have access to the websocket connections because it would be a different instance than the one initiated on www.js.

Front End:

App.js

import React from "react";

import { BrowserRouter as Router, Switch, Route } from "react-router-dom";
import LiveStorePurchases from "./LiveStorePurchases";

function App(props) {
  return (
    <div className="App">
      <Router>
        <Switch>
          <Route exact path="/livePurchases/:storeId">
            <LiveStorePurchases />
          </Route>
        </Switch>
      </Router>
    </div>
  );
}

export default App;

LivePurchaseServer.js

import React, { useEffect, useState } from "react";
import { useParams } from "react-router-dom";

export default function LiveStorePurchases() {
  let { storeId } = useParams();
  const URL = "ws://127.0.0.1:4000?storeId=" + storeId;

  const [ws, setWs] = useState(new WebSocket(URL));

  useEffect(() => {
    ws.onopen = (e) => {
      newFunction(e);

      function newFunction(e) {
        alert("WebSocket Connected");
      }
    };

    ws.onmessage = (e) => {
      const message = e.data;
      alert(message);
    };

    return () => {
      ws.onclose = () => {
        alert("WebSocket Disconnected");
        setWs(new WebSocket(URL));
      };
    };
  }, [ws.onmessage, ws.onopen, ws.onclose, ws, URL]);

  return (
    <div
      style={{
        color: "red",
        fontSize: "4rem",
      }}
    >
      store: {storeId}
    </div>
  );
}