cloudflare-workers-sse

Elegant Server-Sent Events (SSE) Streaming for Cloudflare Workers.

• Provides a simple, straightforward, and functional approach to streaming SSE messages using async generators.
• Easily integrates with existing Workers.
• Works seamlessly with OpenAI streaming.
• Adheres to the SSE specification and is thoroughly tested.

Installation

With npm

npm install cloudflare-workers-sse

With Yarn

yarn add cloudflare-workers-sse

With pnpm

pnpm add cloudflare-workers-sse

With Bun

bun add cloudflare-workers-sse

Usage

The implementation of SSE involves two components: a client that receives messages and a server (in this case, a worker) that publishes them.

Let's start with the worker.

import { sse } from "cloudflare-workers-sse";

export default {
  fetch: sse(handler),
};

async function* handler(request: Request, env: Env, ctx: ExecutionContext) {
  yield {
    event: "greeting",
    data: { text: "Hi there!" },
  };
}

And that's basically it. All messages yielded are streamed to a client listening for them. Once there are no more messages, the stream is closed.

Although the simplest client-side implementation can be achieved using EventSource, for more advanced scenarios — such as using POST requests or handling authentication — it is recommended to use libraries such as @microsoft/fetch-event-source.

const eventSource = new EventSource(
  "https://<YOUR_WORKER_SUBDOMAIN>.workers.dev"
);
eventSource.addEventListener("greeting", (event) => {
  // handle the greeting message
});

Message Type

All messages yielded by a handler should conform to the SSEMessage interface.

interface SSEMessage {
  id?: string;
  event?: string;
  data?: null | boolean | number | bigint | string | Jsonifiable;
  retry?: number;
}

data can be of any primitive type (except Symbol) or an object, in which case it will be converted to JSON. More information about Jsonifiable can be found here. If data is omitted or set to undefined or null, the empty data field will be added.

retry sets the reconnection time in milliseconds. If specified, must be a positive integer, otherwise a RangeError is thrown.

Handling Errors

Errors thrown by a handler or during serialization can be caught via the onError callback. It may also return a message to stream to the client before the connection closes.

import { sse } from "cloudflare-workers-sse";

export default {
  fetch: sse(handler, {
    onError: (error, request, env, ctx) => ({ event: "error_occurred" }),
  }),
};

Custom Headers

By default, only essential headers such as Content-Type, Cache-Control, and Connection are included in the response. To send additional headers, use the customHeaders option.

import { sse } from "cloudflare-workers-sse";

export default {
  fetch: sse(handler, {
    customHeaders: { "access-control-allow-origin": "https://example.com" },
  }),
};

Middleware

If your worker requires additional logic — such as request validation, response modification, or other pre/post-processing — you can implement a middleware.

import { type FetchHandler, sse } from "cloudflare-workers-sse";

export default {
  fetch: middleware(sse(handler)),
};

function middleware<Env>(nextHandler: FetchHandler<Env>): FetchHandler<Env> {
  return async function middlewareHandler(
    request: Request,
    env: Env,
    ctx: ExecutionContext
  ) {
    // a before logic
    const response = await nextHandler(request, env, ctx);
    // an after logic
    return response;
  };
}