EventSource

Last updated Apr 23, 2026

Background

The EventSource ↗︎ interface is a server-sent event API that allows a server to push events to a client. The EventSource object is used to receive server-sent events. It connects to a server over HTTP and receives events in a text-based format.

Constructor

let eventSource = new EventSource(url, options);
  • url USVString - The URL to which to connect.
  • options EventSourceInit - An optional dictionary containing any optional settings.

By default, the EventSource will use the global fetch() function under the covers to make requests. If you need to use a different fetch implementation as provided by a Cloudflare Workers binding, you can pass the fetcher option:

export default {
  async fetch(req, env) {
    let eventSource = new EventSource(url, { fetcher: env.MYFETCHER });
    // ...
  }
};

Note that the fetcher option is a Cloudflare Workers specific extension.

Properties

  • eventSource.url USVString read-only
  • The URL of the event source.
  • eventSource.readyState USVString read-only
  • The state of the connection.
  • eventSource.withCredentials Boolean read-only
  • A Boolean indicating whether the EventSource object was instantiated with cross-origin (CORS) credentials set ( true), or not ( false).

Methods

  • eventSource.close()
  • Closes the connection.
  • eventSource.onopen
  • An event handler called when a connection is opened.
  • eventSource.onmessage
  • An event handler called when a message is received.
  • eventSource.onerror
  • An event handler called when an error occurs.

Events

  • message
  • Fired when a message is received.
  • open
  • Fired when the connection is opened.
  • error
  • Fired when an error occurs.

Class Methods

  • EventSource.from(readableStreamReadableStream) : EventSource
  • This is a Cloudflare Workers specific extension that creates a new EventSource object from an existing ReadableStream. Such an instance does not initiate a new connection but instead attaches to the provided stream.

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description: An interface for asynchronously fetching resources via HTTP requests inside of a Worker. title: Fetch image: https://developers.cloudflare.com/workers/runtime-apis/fetch/og.png?v=1475ba548190f898

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