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 >= 71
• Safari >= 11.3
• Firefox >= 65
• Edge >= 79
• Deno >= 1.30
• Bun >= 1.1.23

Basically, any environment that supports:

• fetch
• ReadableStream
• TextDecoder
• URL
• Event, MessageEvent, EventTarget
• Symbol.for

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 undici to add proxy support, either through environment variables or explicit configuration.

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

const proxyAgent = new EnvHttpProxyAgent()

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

Using HTTP2

Use a package like undici that supports HTTP2.

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

const http2Dispatcher = new Agent({allowH2: true})

const es = new EventSource('https://my-server.com/sse', {
  fetch: (url, init) => fetch(url, {...init, dispatcher: http2Dispatcher}),
})

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'

const unsafeAgent = new Agent({
  connect: {
    rejectUnauthorized: false,
  },
})

await fetch('https://my-server.com/sse', {
  dispatcher: unsafeAgent,
})

Feature checking

For library authors, you may want to feature-check if an EventSource implementation supports passing a custom fetch implementation (such as this library does). We declare (from v4.1.0 and onwards) a non-enumerable symbol on the EventSource class to indicate this, named eventsource.supports-fetch-override. It can be used downstream such as:

function yourLibrary(options) {
  const OurEventSource = options.polyfills.EventSource || globalThis.EventSource

  if (OurEventSource && Symbol.for('eventsource.supports-fetch-override') in OurEventSource) {
    // We can safely assume this supports overriding/specifying `fetch`!
    const es = new OurEventSource('https://some.url', {
      fetch: (input, init) =>
        fetch(input, {
          ...init,
          headers: {
            ...(init.headers || {}),
            'x-some-header': 'foo',
          },
        }),
    })
    // …
  }
}

License

MIT-licensed. See LICENSE.