← Workers / workers / runtime-apis

EventSource

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.

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.

• 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).

• 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.

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

• 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.