eventsource

What is eventsource?

The eventsource npm package is a Node.js implementation of the EventSource client, which is used for receiving server-sent events (SSE). It allows clients to subscribe to a stream of updates from a server over an HTTP connection. The package is designed to be compliant with the W3C EventSource specification and is typically used for real-time data feeds, such as live notifications, stock prices updates, or any other continuous data stream.

What are eventsource's main functionalities?

Connecting to an SSE server

This code demonstrates how to connect to an SSE server and listen for messages. The 'onmessage' handler is called whenever a new message is received, and the 'onerror' handler is called if there's an error with the connection.

const EventSource = require('eventsource');
const es = new EventSource('http://example.com/events');
es.onmessage = function(event) {
  console.log(event.data);
};
es.onerror = function(err) {
  console.error('EventSource failed:', err);
};

Listening for specific event types

This code snippet shows how to listen for specific event types using the 'addEventListener' method. In this example, the client listens for 'userupdate' events and processes the received data.

const EventSource = require('eventsource');
const es = new EventSource('http://example.com/events');
es.addEventListener('userupdate', function(event) {
  const userData = JSON.parse(event.data);
  console.log('User update:', userData);
});

Reconnecting after connection loss

This example illustrates the automatic reconnection feature of the EventSource API. The 'onopen' handler is called when the connection is successfully established, and the 'onerror' handler indicates that the connection has been lost and a reconnection attempt will be made.

const EventSource = require('eventsource');
const es = new EventSource('http://example.com/events');
es.onopen = function() {
  console.log('Connection to server opened.');
};
es.onerror = function() {
  console.log('Connection lost, the browser will automatically attempt to reconnect.');
};

Other packages similar to eventsource

sse-client

The sse-client package is another implementation of the EventSource API for Node.js. It provides similar functionality for connecting to SSE servers and handling server-sent events. Compared to eventsource, it may have different API nuances or additional features, but the core functionality remains the same.

oceanwind

Oceanwind is not a direct alternative to eventsource, but it is an example of a package that uses server-sent events to provide real-time updates to clients. It is a library for managing real-time communication between servers and clients, and it may include an SSE implementation as part of its feature set.

README

eventsource

WhatWG/W3C-compatible server-sent events/eventsource client. The module attempts to implement an absolute minimal amount of features/changes beyond the specification.

If you're looking for a modern alternative with a less constrained API, check out the eventsource-client package.

Installation

npm install --save eventsource

Supported engines

• Node.js >= 20
• Chrome >= 63
• Safari >= 11.3
• Firefox >= 65
• Edge >= 79
• Deno >= 1.30
• Bun >= 1.1.23

Basically, any environment that supports:

• fetch
• ReadableStream
• TextDecoderStream
• URL
• Event, MessageEvent, EventTarget

If you need to support older runtimes, try the 2.x branch/version range (note: 2.x branch is primarily targetted at Node.js, not browsers).

Usage

import {EventSource} from 'eventsource'

const es = new EventSource('https://my-server.com/sse')

/*
 * This will listen for events with the field `event: notice`.
 */
es.addEventListener('notice', (event) => {
  console.log(event.data)
})

/*
 * This will listen for events with the field `event: update`.
 */
es.addEventListener('update', (event) => {
  console.log(event.data)
})

/*
 * The event "message" is a special case, as it will capture events _without_ an
 * event field, as well as events that have the specific type `event: message`.
 * It will not trigger on any other event type.
 */
es.addEventListener('message', (event) => {
  console.log(event.data)
})

/**
 * To explicitly close the connection, call the `close` method.
 * This will prevent any reconnection from happening.
 */
setTimeout(() => {
  es.close()
}, 10_000)

TypeScript

Make sure you have configured your TSConfig so it matches the environment you are targetting. If you are targetting browsers, this would be dom:

{
  "compilerOptions": {
    "lib": ["dom"],
  },
}

If you're using Node.js, ensure you have @types/node installed (and it is version 18 or higher). Cloudflare workers have @cloudflare/workers-types etc.

The following errors are caused by targetting an environment that does not have the necessary types available:

error TS2304: Cannot find name 'Event'.
error TS2304: Cannot find name 'EventTarget'.
error TS2304: Cannot find name 'MessageEvent'.

Migrating from v1 / v2

See MIGRATION.md for a detailed migration guide.

Extensions to the WhatWG/W3C API

Message and code properties on errors

The error event has a message and code property that can be used to get more information about the error. In the specification, the Event

es.addEventListener('error', (err) => {
  if (err.code === 401 || err.code === 403) {
    console.log('not authorized')
  }
})

Specify fetch implementation

The EventSource constructor accepts an optional fetch property in the second argument that can be used to specify the fetch implementation to use.

This can be useful in environments where the global fetch function is not available - but it can also be used to alter the request/response behaviour.

Setting HTTP request headers

const es = new EventSource('https://my-server.com/sse', {
  fetch: (input, init) =>
    fetch(input, {
      ...init,
      headers: {
        ...init.headers,
        Authorization: 'Bearer myToken',
      },
    }),
})

HTTP/HTTPS proxy

Use a package like node-fetch-native to add proxy support, either through environment variables or explicit configuration.

// npm install node-fetch-native --save
import {fetch} from 'node-fetch-native/proxy'

const es = new EventSource('https://my-server.com/sse', {
  fetch: (input, init) => fetch(input, init),
})

Allow unauthorized HTTPS requests

Use a package like undici for more control of fetch options through the use of an Agent.

// npm install undici --save
import {fetch, Agent} from 'undici'

await fetch('https://my-server.com/sse', {
  dispatcher: new Agent({
    connect: {
      rejectUnauthorized: false,
    },
  }),
})

License

MIT-licensed. See LICENSE.

Changelog

4.0.0 (2025-05-13)

⚠ BREAKING CHANGES

• FetchLikeInit is now removed. Use EventSourceFetchInit.
• Drop support for Node.js v18, as it is end-of-life.

Features

• require node.js v20 or higher (91a3a48)

Bug Fixes

• drop FetchLikeInit type. Use EventSourceFetchInit instead. (6786e46)