Friday, March 1, 2024

Find out how to Use Server-sent Occasions in Node.js — SitePoint

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On this article, we’ll discover learn how to use server-sent occasions to allow a shopper to obtain computerized updates from a server through an HTTP connection. We’ll additionally have a look at why that is helpful, and we’ll present sensible demonstrations of learn how to use server-sent occasions with Node.js.

Desk of Contents

Why Server-sent Occasions Are Helpful

The Internet relies on request-response HTTP messages. Your browser makes a URL request and a server responds with knowledge. Which will result in additional browser requests and server responses for photos, CSS, JavaScript and many others. The server can not provoke messages to the browser, so how can it point out that knowledge has modified? Thankfully, you possibly can add options similar to stay information bulletins, climate stories, and inventory costs with server-sent occasions.

Implementing stay knowledge updates utilizing normal net applied sciences has all the time been potential:

  • The Nineteen Nineties Internet used a full-page or body/iframe refresh.
  • The 2000s Internet launched Ajax, which may use lengthy polling to request knowledge and replace the suitable DOM components with new info.

Neither possibility is good, because the browser should set off a refresh. If it makes requests too usually, no knowledge could have modified so each the browser and server do pointless work. If it makes requests too slowly, it might miss an essential replace and the inventory value you’re watching has already crashed!

Server-sent occasions (SSE) enable a server to push knowledge to the browser at any time:

  • The browser nonetheless makes the preliminary request to ascertain a connection.
  • The server returns an event-stream response and retains the connection open.
  • The server can use this connection to ship textual content messages at any level.
  • The incoming knowledge raises a JavaScript occasion within the browser. An occasion handler operate can parse the info and replace the DOM.

In essence, SSE is an endless stream of information. Consider it as downloading an infinitely giant file in small chunks that you may intercept and skim.

SSE was first carried out in 2006 and all main browsers help the usual. It’s presumably much less well-known than WebSockets, however server-sent occasions are less complicated, use normal HTTP, help one-way communication, and supply computerized reconnection. This tutorial gives instance Node.js code with out third-party modules, however SSE is accessible in different server-side languages together with PHP.

Server-sent Occasions Fast Begin

The next demonstration implements a Node.js net server which outputs a random quantity between 1 and 1,000 at a random interval of not less than as soon as each three seconds.

Under is our Node.js SSE demonstration (you possibly can open it in a separate browser tab in case you desire).

The code makes use of the usual Node.js http and url modules for creating an online server and parsing URLs:

import http from "node:http";
import url from "node:url";

The server examines the incoming URL request and reacts when it encounters a /random path:

const port = 8000;

http.createServer(async (req, res) => {

  const uri = url.parse(req.url).pathname;

  swap (uri) {
    case "/random":


console.log(`server working: http://localhost:${port}nn`);

It initially responds with the SSE HTTP event-stream header:

operate sseStart(res) {
  res.writeHead(200, {
    Content material-Kind: "textual content/event-stream",
    Cache-Management: "no-cache",
    Connection: "keep-alive"

One other operate then sends a random quantity and calls itself after a random interval has elapsed:

operate sseRandom(res) {
  res.write("knowledge: " + (Math.ground(Math.random() * 1000) + 1) + "nn");
  setTimeout(() => sseRandom(res), Math.random() * 3000);

Should you run the code regionally, you possibly can check the response utilizing cURL in your terminal:

$> curl -H Settle for:textual content/event-stream http://localhost:8000/random
knowledge: 481

knowledge: 127

knowledge: 975

Press Ctrl | Cmd and C to terminate the request.

The browser’s client-side JavaScript connects to the /random URI utilizing an EventSource object constructor:

const supply = new EventSource("/random");

Incoming knowledge triggers a message occasion handler the place the string following knowledge: is accessible within the occasion object’s .knowledge property:

supply.addEventListener('message', e => {
  console.log('RECEIVED', e.knowledge);

Essential notes

  • Like Fetch(), the browser makes an ordinary HTTP request, so you could have to deal with CSP, CORS and optionally go a second { withCredentials: true } argument to the EventSource constructor to ship cookies.
  • The server should retain particular person res response objects for each linked consumer to ship them knowledge. It’s achieved within the code above by passing the worth in a closure to the subsequent name.
  • Message knowledge can solely be a string (maybe JSON) despatched within the format knowledge: <message>nn. The terminating carriage returns are important.
  • The server can terminate an SSE response at any time with res.finish(), however…
  • When a disconnect happens, the browser routinely makes an attempt to reconnect; there’s no want to jot down your personal reconnection code.

Superior Server-sent Occasions

SSE requires no extra code than that proven above, however the next sections talk about additional choices.

One vs many SSE channels

A server may present any variety of SSE channel URLs. For instance:

  • /newest/information
  • /newest/climate
  • /newest/stockprice

This can be sensible if a single web page exhibits one subject, however much less so if a single web page exhibits information, climate, and inventory costs. In that state of affairs, the server should preserve three connections for every consumer, which may result in reminiscence issues as site visitors will increase.

An alternate possibility is to supply a single endpoint URL, similar to /newest, which sends any knowledge sort on one communication channel. The browser may point out the matters of curiosity within the URL question string — for instance, /newest?sort=information,climate,stockprice — so the server can restrict SSE responses to particular messages.

Sending totally different knowledge on a single channel

Messages from the server can have an related occasion: handed on the road above the knowledge: to establish particular sorts of info:

occasion: information
knowledge: SSE is nice!

occasion: climate
knowledge: { "temperature": "20C", "wind": "10Kph", "rain": "25%" }

occasion: inventory
knowledge: { "image": "AC", "firm": "Acme Corp", "value": 123.45, "improve": -1.1 }

These will not set off the client-side "message" occasion handler. You could add handlers for every sort of occasion. For instance:

supply.addEventListener('information', e => {

    .textContent = e.knowledge;


supply.addEventListener('climate', e => {

  const w = JSON.parse(e.knowledge);

    .textContent = `${ w.temperature } with ${ w.wind } wind`;


supply.addEventListener('inventory', e => {

  const s = JSON.parse(e.knowledge);

  doc.getElementById(`stock-${ s.image }`)
    .textContent = `${ s.share }: ${ s.value } (${ s.improve }%)`;


Utilizing knowledge identifiers

Optionally, the server may also ship an id: after a knowledge: line:

occasion: information
knowledge: SSE is nice!
id: 42

If the connection drops, the browser sends the final id again to the server within the Final-Occasion-ID HTTP header so the server can resend any missed messages.

The latest ID can also be accessible client-side within the occasion object’s .lastEventId property:

supply.addEventListener('information', e => {

  console.log(`final ID: ${ e.lastEventId }`);

    .textContent = e.knowledge;


Specifying retry delays

Though reconnection is computerized, your server could know that new knowledge is just not anticipated for a particular interval, so there’s no have to retain an energetic communication channel. The server can ship a retry: response with a milliseconds worth both by itself or as a part of a ultimate message. For instance:

retry: 60000
knowledge: Please do not reconnect for an additional minute!

On receipt, the browser will drop the SSE connection and try and reconnect after the delay interval has elapsed.

Different occasion handlers

In addition to "message" and named occasions, you can too create "open" and "error" handlers in your client-side JavaScript.

An "open" occasion triggers when the server connection is established. It could possibly be used to run extra configuration code or initialize DOM components:

const supply = new EventSource('/sse1');

supply.addEventListener('open', e => {

  console.log('SSE connection established.');


An "error" occasion triggers when the server connection fails or terminates. You’ll be able to look at the occasion object’s .eventPhase property to examine what occurred:

supply.addEventListener('error', e => {

    if (e.eventPhase === EventSource.CLOSED) {
      console.log('SSE connection closed');
    else {
      console.log('error', e);


Keep in mind, there’s no have to reconnect: it happens routinely.

Terminating SSE communication

The browser can terminate an SSE communication utilizing the EventSource object’s .shut() technique. For instance:

const supply = new EventSource('/sse1');

setTimeout(() => supply.shut(), 3_600_000);

The server can terminate the connection by:

  1. firing res.finish() or sending a retry: delay, then
  2. returning an HTTP standing 204 when the identical browser makes an attempt to reconnect.

Solely the browser can re-establish a connection by creating a brand new EventSource object.


Server Aspect Occasions present a technique to implement stay web page updates that are presumably simpler, extra sensible, and extra light-weight than Fetch()-based Ajax polling. The complexity is on the server finish. You could:

  1. preserve all consumer’s energetic connections in reminiscence, and
  2. set off knowledge transmissions when one thing adjustments.

However that is absolutely below your management, and scaling needs to be no extra complicated than every other net utility.

The one downside is that SSE doesn’t assist you to ship messages from the browser to the server (aside from the preliminary connection request). You possibly can use Ajax, however that’s too gradual for apps similar to motion video games. For correct two-way communication, you require WebSockets. We’ll have a brand new tutorial on that quickly!

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