---
description: EventSource is a server-sent event API that allows a server to push events to a client.
title: EventSource
image: https://developers.cloudflare.com/og-docs.png
---

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# EventSource

Last updated Aug 25, 2026|Copy as Markdown|[View as Markdown](https://66f4ea29.previews.developers.cloudflare.com/workers/runtime-apis/eventsource/index.md)|[Agent setup](https://66f4ea29.previews.developers.cloudflare.com/agent-setup/)

## Background

The [EventSource ↗](https://developer.mozilla.org/en-US/docs/Web/API/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

```js
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:

```js
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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