fetch-event-stream

A tiny (736b) utility for Server Sent Event (SSE) streaming via fetch and Web Streams API

Allows any HTTP method
Built with native Web Streams API
Works with browser, Node.js, Cloudflare Workers, Deno, and Bun
Supports WebWorker or Service Worker environments
Accepts AbortController for cancellable streams

Why?

Even though EventSource exists in browsers (and Deno!), it only sends GET requests and does not allow for custom HTTP headers. Most APIs (eg, Anthropic, OpenAI) require POST requests with an Authorization header and a JSON payload.
Web Streams are new, not very well understood, and are sometimes confused with NodeJS Streams. Because of this, many other libraries embed large polyfills or manually reconstruct desired behaviors through non-standard approaches. These polyfills are generally not necessary anymore, but still make large impacts on SDK size; for example, openai is 17kB (gzip).

Install

Available on JSR, npm, and deno.land

$ npm install --save fetch-event-stream

Usage

import { events, stream } from 'fetch-event-stream';
// or
import { events, stream } from 'https://deno.land/x/fetch_event_stream';

API

events(res, signal?)

Convert a Response body containing Server Sent Events (SSE) into an Async Iterator that yields ServerSentEventMessage objects.

Example

// Optional
let abort = new AbortController();

// Manually fetch a Response
let res = await fetch('https://...', {
  method: 'POST',
  signal: abort.signal,
  headers: {
    'api-key': 'token <value>',
    'content-type': 'application/json',
  },
  body: JSON.stringify({
    stream: true, // <- hypothetical
    // ...
  }),
});

if (res.ok) {
  let stream = events(res, abort.signal);
  for await (let event of stream) {
    console.log('<<', event.data);
  }
}

res

Type: Response

The Response to consume. Must contain a body that follows the Server-Sent Event message protocol.

signal

Type: AbortSignal

Optional. Use the AbortController interface to stop iteration. The stream will be destroyed.

stream(input, init?)

Convenience function that will fetch with the given arguments and, if ok, will return the events async iterator.

Note: Accepts the same arguments as fetch but does not return a Response!

Important: Will throw the Response if received non-2xx status code.

Example

// NOTE: throws `Response` if not 2xx status
let events = await stream('https://api.openai.com/...', {
  method: 'POST',
  headers: {
    'Authorization': 'Bearer <token>',
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    stream: true,
    // ...
  }),
});

for await (let event of events) {
  console.log('<<', JSON.parse(event.data));
}

input

Type: Request | URL | string

Refer to fetch#resource documentation.

init

Type: RequestInit

Refer to fetch#options documentation.