What are Server-Sent Events in JavaScript

Server-Sent Events in JavaScript

In today’s world, where real-time communication is the norm, web applications often require constant updates from the server. To achieve this, web developers use a variety of techniques, such as polling, long-polling, and web sockets. However, these techniques have their limitations, such as increased latency, high bandwidth usage, and complexity. In this context, Server-Sent Events (SSE) provide a simple and efficient way to push data from the server to the client in real-time.

What are Server-Sent Events?

Server-Sent Events (SSE) is a standard protocol that allows the server to send data to the client in real-time, over a single HTTP connection. It is a unidirectional communication channel, where only the server can initiate the communication. The client listens to the server for updates, and the server sends the data as a stream of text. The data can be in any format, such as JSON, XML, or plain text, and can be sent at any time, without the need for the client to send a request.

SSE is based on the EventSource API, which is a JavaScript interface for receiving server-sent events. The EventSource API is supported by all modern browsers, including Chrome, Firefox, Safari, and Edge.

How to use Server-Sent Events in JavaScript

Using Server-Sent Events in JavaScript is straightforward. Here is a step-by-step guide:

Step 1: Create a Server-Sent Event endpoint

The first step is to create a Server-Sent Event endpoint on the server-side. This endpoint should return a stream of data in the SSE format. Here is an example of a Node.js endpoint:

const http = require('http');

http.createServer((req, res) => {
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive'

  setInterval(() => {
    const data = { message: 'Hello, world!' };
    res.write(`data: ${JSON.stringify(data)}nn`);
  }, 1000);

In this example, we create an HTTP server that returns a stream of data every second. The response headers indicate that the content type is text/event-stream, and the connection should be kept alive.

Step 2: Connect to the Server-Sent Event endpoint

The next step is to connect to the Server-Sent Event endpoint using the EventSource API. Here is an example of how to connect to the Node.js endpoint we created earlier:

const eventSource = new EventSource('http://localhost:3000');

eventSource.addEventListener('message', (event) => {
  const data = JSON.parse(event.data);

In this example, we create a new EventSource object that connects to the Server-Sent Event endpoint. We then listen for the ‘message’ event, which is triggered when the server sends a message. We parse the message data as JSON and log it to the console.

Step 3: Close the connection

When the client no longer needs to receive updates, it should close the connection to the Server-Sent Event endpoint. Here is an example of how to close the connection:


Benefits of using Server-Sent Events

Using Server-Sent Events in web applications has several benefits:

  • Low latency: Server-Sent Events provide real-time updates with low latency, as there is no need to wait for a response from the server.
  • Low bandwidth usage: Server-Sent Events use a single HTTP connection, which reduces bandwidth usage and server load.
  • Simple implementation: Server-Sent Events are easy to implement, as they use a simple API and do not require complex setup or configuration.


In summary, Server-Sent Events provide a simple and efficient way to push data from the server to the client in real-time. They are easy to implement and have several benefits over other real-time communication techniques. By using Server-Sent Events, web developers can create web applications that provide real-time updates to users, without the need for complex setup or configuration.