An easy to use Server Side Events library for Node.js + frontend

Usage no npm install needed!

<script type="module">
  import easyServerSentEvents from 'https://cdn.skypack.dev/easy-server-sent-events';



An easy to use Server Side Events library for Express/Node.js + a frontend-library for web browsers.

Automatically resumes broken connections, provides broadcasting capabilities and simplifies the syntax on both backend and frontend.


npm install easy-server-sent-events


Here's an example of how to use the module on the backend in Node.js:

  • It is Express middleware, so you'll have to use express.
  • You don't have to use express-session but it makes it much easier, when you want to target the sending of events to specific clients.
const express = require('express');
const session = require('express-session');
const app = express();
const sse = require('easy-server-sent-events');

// express-session middleware 
// (Please note: It might be better to store sessions in a db)
  secret: 'something hard to guess',
  resave: false,
  saveUninitialized: true,
  cookie: { secure: false }

// You can change the following options:
// endpoint: which endpoint you want to use
// script: at which route a clientside library should be served
// These are the default values (they will be set even if omitted):
const options = {
  endpoint: '/api/sse',
  script: '/sse.js'

// Calling the module returns an object with four properties:
// SSE:
//     the middleware - use with an express app
// send: 
//     a function that sends events from the server:
//     send(to, eventType, data)
// openSessions: 
//     a function returns how many sessions are open
// openConnections: 
//     a function that returns how man connections that are open
const {SSE, send, openSessions, openConnections} = sse(options);

// Other middleware goes here
// For example static middleware (if needed)

// Start web server
app.listen(4000, () => console.log('Listening on port 4000'));

// Example of using send(to, eventType, data)
// Here we send messages to all connected clients
// We randomly choose between the event types
// 'message' and 'other' (you can name your event types how you like)
// and send a message (an object with the properties cool and content)
function test() {
    Math.random() < .5 ? 'message' : 'other',
      cool: true, 
      content: 'This is a message sent ' + new Date().toLocaleTimeString() 
  // log how many openSessions and openConnections we have
    'openSessions', openSessions(),
    'openConnections', openConnections()

// send an event every 3 seconds
setInterval(test, 3000);

How do I send events to specific clients?

  • You send events to specific clients by replacing the "all" value with a filter.
  • The filter receives the request object originally sent from the client.
  • Thus - if you use the express-session middleware you will have a session object attached to the request object.
Filter by session properties

The following will only send messages to a user with the username 'root':

  req => req.session.user && req.session.user.username === 'root', 
  'This is a super secret message.'
Alternate ways to filter

If you decide to not use express-session the property req.query.browserId could be handy for identifying different clients. It is set by this module and unique for each client.


No build system/vanilla JS

If you are using this module without a build system that handles import statements you can include the frontend library with a script tag:

<script src="/sse.js"></script>

With a build system (with React, Vue etc)

In build systems that support import statements use:

import SSE from 'easy-server-sent-events/sse';

Basic usage

Create a new instance of the library and add eventlisteners

// Create an instance
// a connect it to the endpoint
// '/api/sse' (default)
const sse = new SSE('/api/sse');

sse.listen('message', (data) => {
  console.log('message', data);

sse.listen('other', (data) => {
  console.log('other', data);

Restart after login/logout etc

After login/logout and other events in your application that might change the session object on the backend, immediately restart your instance to make sure it has access to these changes:


Remove an event listener

If you want to remove and event listener you can save the result of a call to listen in a variable and call unlisten later:

const sse = new SSE('/api/sse');

let messageListener = sse.listen('message', (data) => {
  console.log('message', data);

// later...

Usage with the create-react-app dev server: Proxying

Proxying SSE (as well as web sockets) is hard/not doable with the built in proxy system in create-react-app.

We recommend that you use the npm package react-amazing proxy that solves all problems with letting SSE through the proxy.

Older browsers

Microsoft Edge and Microsoft IE does not support server sent events because of lack of support for the EventSource object. There is a polyfill available: