How do I trigger events from a handler so that another handler can listen to it in Express JS

I'm building a web app wherein when someone posts a post, all subscribers receive a notification. I'm using SSE (server sent events) in Node js. My example code looks like this.

postEvent.js

const EventEmitter = require("node:events");
class PostEvent extends EventEmitter {}
module.exports = new PostEvent();

post.js

const PostEvent = require('./postEvent');
app.post('/post', (req, res) => {
   // get data from req and save to the database
  PostEvent.emit('posted');
})

sse.js

const PostEvent = require('./postEvent');
app.get('/sse', (req, res) => {
   res.set('Content-Type', 'text/event-stream'); // for streaming server response indefinitely
   setInterval(() => res.write(`data: my sse data every second\n\n`), 1000); // this works
   PostEvent.on('posted', () => res.write('data: posted\n\n')); // this doesn't
})

The problem here is that the PostEvent.on('posted', callback) doesn't run when I send a fetch request to the /post route. Am I doing something wrong or the event system doesn't work like this?
Is there anything else that I can do to implement this functionality?

Note: Express boilerplate code has not been mentioned and the client side functions properly with EventSource when streams are sent.

Edit: Can we do this event system in an http or sse environment at all? I'm not making a request to /sse, but I'm trying to run the function in it.

I was not able to reproduce your problem when using only one file for both routes: https://prnt.sc/sOzp0xHvpbDM

My bet is that when having it split in 2 files (post.js and sse.js) different instances of the PostEvent are being passed. If that is the case then you will need to instantiate and then pass the same in both spots.

const express = require("express");
const PostEvent = require('./postEvent');

const app = express();

app.post('/post', (req, res) => {
  PostEvent.emit('posted');
  res.json("event posted");
})

app.get('/sse', (req, res) => {
  res.set('Content-Type', 'text/event-stream');
  setInterval(() => res.write(`data: my sse data every second\n\n`), 1000);
  PostEvent.on('posted', () => res.write(`data: posted in ${new Date().toLocaleTimeString()} \n\n`));
})

const PORT = 3000;
app.listen(PORT, function (err) {
  if (err) console.log(err);
  console.log("Server listening on PORT", PORT);
});