asyncSSE

Fetch Server-Sent Events (SSE) as an async iterable.

• 🚀 Lightweight (<1KB) and dependency-free
• 🔄 Works with any SSE-compatible API
• 🌐 Browser and Node.js compatible
• 📦 Easy to use with ES modules

Installation

To use locally, install via npm:

npm install asyncsse

... and add this to your script:

import { asyncSSE } from "./node_modules/asyncsse/dist/asyncsse.js";

To use via CDN, add this to your script:

import { asyncSSE } from "https://cdn.jsdelivr.net/npm/asyncsse@1";

Usage

Browser (via CDN)

<script type="module">
  import { asyncSSE } from "https://cdn.jsdelivr.net/npm/asyncsse@1";

  // Example usage
  (async () => {
    for await (const { data, error } of asyncSSE("https://api.example.com/sse")) {
      if (error) throw new Error(error);
      console.log(data);
    }
  })();
</script>

Node.js or bundled projects

import { asyncSSE } from "asyncsse";

// Example usage
(async () => {
  for await (const { data, error } of asyncSSE("https://api.example.com/sse")) {
    if (error) throw new Error(error);
    console.log(data);
  }
})();

API

asyncSSE(url: string, options?: RequestInit, config?: SSEConfig): AsyncIterable<SSEEvent>

Fetches Server-Sent Events from the specified URL and returns an async iterable.

• url: The URL to fetch SSE from
• options: Optional fetch options
• config: Optional configuration object
  • fetch: Custom fetch implementation (defaults to global fetch)
  • onResponse: Async callback to inspect or modify the Response before streaming begins

Returns an async iterable that yields SSEEvent objects.

Events can have fields like event, data, id, and retry like this:

event: userupdate
id: 123
data: {"username": "testuser", "status": "online"}
retry: 5000

data: Simple message

asyncSSE parses these into a single object.

(async () => {
  for await (const event of asyncSSE(sseStream, {})) {
    console.log(event.event, event.data, event.id, event.retry);
  }
})();

This would output:

{ event: "userupdate", id: "123", data: '{"username": "testuser", "status": "online"}', retry: "5000" }
{ data: "Simple message" }

Comment lines with a colon (:) are comments and are ignored by the parser. They will not result in an event.

: This is a comment, it will be ignored
data: First event
: Another comment
event: important
data: Second event
id: 456

This would output:

{ data: "First event" }
{ event: "important", data: "Second event", id: "456" }

retry fields suggest a reconnection time (in milliseconds). asyncSSE itself does not automatically handle reconnection based on this value; it simply parses and provides it. Wait for this duration and reconnect if the connection is lost.

Examples

import { asyncSSE } from "asyncsse";

const apiKey = "YOUR_OPENAI_API_KEY";
const url = "https://api.openai.com/v1/chat/completions";

const options = {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    Authorization: `Bearer ${apiKey}`,
  },
  body: JSON.stringify({
    model: "gpt-4",
    stream: true,
    messages: [{ role: "user", content: "Hello, world!" }],
  }),
};

const config = {
  onResponse: async (response) => {
    console.log("Requests remaining:", response.headers.get("X-Ratelimit-Remaining-Requests"));
  },
};

// Fetch the stream, event by event
for await (const { data, error } of asyncSSE(url, options, config)) {
  if (error) throw new Error(error);
  console.log(JSON.parse(data));
}

Testing with Text Input

You can directly stream SSE events from a text string using the provided fetchText helper:

import { asyncSSE } from "https://cdn.jsdelivr.net/npm/asyncsse@1";
import { fetchText } from "https://cdn.jsdelivr.net/npm/asyncsse@1/dist/fetchtext.js";
You can run the tests using Deno:

const text = "data: Hello\n\ndata: World\n\n";

// Stream events from text
for await (const event of asyncSSE(text, {}, { fetch: fetchText })) {
  console.log(event);
}

This outputs:

{ data: "Hello" }
{ data: "World" }

This is particularly useful for testing SSE parsing without making actual HTTP requests.

Development

git clone https://github.com/sanand0/asyncsse.git
cd asyncsse

npm install
npm run lint && npm run build && npm test

npm publish
git commit . -m"$COMMIT_MSG"; git tag $VERSION; git push --follow-tags

Release notes

• 1.4.1: 31 Jul 2025. Standardized package.json & README.md
• 1.3.3: 9 Jun 2025. Improved documentation for error handling and added more examples.
• 1.3.2: 25 Dec 2024. Update repo links
• 1.3.1: 3 Nov 2024. Add fetchText helper for mocking SSE responses. Add source maps and TypeScript
• 1.2.1: 3 Nov 2024. Add config.fetch parameter for custom fetch implementations
• 1.1.0: 2 Nov 2024. Add config.onResponse callback
• 1.0.0: 15 Oct 2024. Initial release

License

This project is licensed under the MIT License - see the LICENSE file for details.