parse-sse

Parse Server-Sent Events (SSE) from a Response

A lightweight, spec-compliant parser for Server-Sent Events that works with the native Fetch API. Returns a standard ReadableStream for maximum composability.

Perfect for consuming streaming APIs from OpenAI, Anthropic, and other services.

Install

npm install parse-sse

Usage

import {parseServerSentEvents} from 'parse-sse';

const response = await fetch('https://api.example.com/events');

for await (const event of parseServerSentEvents(response)) {
	console.log(event.type);        // Event type (default: 'message')
	console.log(event.data);        // Event data
	console.log(event.lastEventId); // Last event ID (always present as string)
	console.log(event.retry);       // Retry interval in ms (if specified)
}

With Ky

import {parseServerSentEvents} from 'parse-sse';
import ky from 'ky';

const response = await ky('https://api.example.com/events');

for await (const event of parseServerSentEvents(response)) {
	const data = JSON.parse(event.data);
	console.log(data);
}

OpenAI Streaming

import {parseServerSentEvents} from 'parse-sse';

const response = await fetch('https://api.openai.com/v1/chat/completions', {
	method: 'POST',
	headers: {
		'Content-Type': 'application/json',
		'Authorization': `Bearer ${apiKey}`,
	},
	body: JSON.stringify({
		model: 'gpt-4',
		messages: [{role: 'user', content: 'Hello!'}],
		stream: true,
	}),
});

for await (const event of parseServerSentEvents(response)) {
	if (event.data === '[DONE]') {
		break;
	}

	const data = JSON.parse(event.data);
	console.log(data.choices[0]?.delta?.content);
}

Custom Event Types

import {parseServerSentEvents} from 'parse-sse';

const response = await fetch('https://api.example.com/events');

for await (const event of parseServerSentEvents(response)) {
	switch (event.type) {
		case 'update':
			console.log('Update:', event.data);
			break;
		case 'complete':
			console.log('Complete:', event.data);
			break;
		case 'error':
			console.error('Error:', event.data);
			break;
		default:
			console.log('Message:', event.data);
	}
}

Advanced: Stream Composability

Since parseServerSentEvents() returns a standard ReadableStream, you can use all stream methods:

import {parseServerSentEvents} from 'parse-sse';

const response = await fetch('https://api.example.com/events');
const eventStream = parseServerSentEvents(response);

// Tee the stream to consume it twice
const [stream1, stream2] = eventStream.tee();

// Process both streams in parallel
await Promise.all([
	(async () => {
		for await (const event of stream1) {
			console.log('Stream 1:', event.data);
		}
	})(),
	(async () => {
		for await (const event of stream2) {
			console.log('Stream 2:', event.data);
		}
	})(),
]);

Advanced: Using ServerSentEventTransformStream

For advanced use cases, you can use ServerSentEventTransformStream directly for custom stream pipelines:

import {ServerSentEventTransformStream} from 'parse-sse';

// Custom pipeline
myTextStream
	.pipeThrough(new ServerSentEventTransformStream())
	.pipeTo(myWritableStream);

import {ServerSentEventTransformStream} from 'parse-sse';

// With custom decoder
response.body
	.pipeThrough(new MyCustomDecoderStream())
	.pipeThrough(new ServerSentEventTransformStream());

import {ServerSentEventTransformStream} from 'parse-sse';

// Filter events in a pipeline
fetch(url)
	.then(r => r.body)
	.pipeThrough(new TextDecoderStream())
	.pipeThrough(new ServerSentEventTransformStream())
	.pipeThrough(new TransformStream({
		transform(event, controller) {
			if (event.type === 'update') {
				controller.enqueue(event);
			}
		}
	}));

API

parseServerSentEvents(response)

Parse a Server-Sent Events (SSE) stream from a Response object.

Returns a ReadableStream that yields parsed events as they arrive. The stream can be consumed using async iteration (for await...of) or stream methods like .pipeTo(), .pipeThrough(), and .tee().

response

Type: Response

A Response object with a text/event-stream body.

Returns

Type: ReadableStream<ServerSentEvent>

A stream of parsed events that can be consumed using async iteration or standard stream methods.

ServerSentEventTransformStream

TransformStream that parses Server-Sent Events.

Use this for advanced stream composition or when you have a text stream that's already decoded.

Important: This expects string chunks as input. If you have a byte stream, pipe it through TextDecoderStream first.

import {ServerSentEventTransformStream} from 'parse-sse';

// Correct - with TextDecoderStream for bytes
response.body
	.pipeThrough(new TextDecoderStream())
	.pipeThrough(new ServerSentEventTransformStream());

// Correct - if you already have text chunks
myTextStream
	.pipeThrough(new ServerSentEventTransformStream());

Input

Type: string

Text chunks (already decoded from bytes). If you pass byte chunks, a TypeError will be thrown.

Output

Type: ServerSentEvent

Parsed SSE events.

ServerSentEvent

A parsed Server-Sent Event.

Type: object

type

Type: string
Default: 'message'

The event type.

data

Type: string

The event data.

Multiple data: fields are joined with newlines.

lastEventId

Type: string

The last event ID in the stream.

This is connection-scoped state that persists across events. When an event includes an id: field, this value is updated and persists for all subsequent events until changed again.

Always present as a string (empty string if no ID has been set). Matches browser MessageEvent.lastEventId behavior.

Used for reconnection with Last-Event-ID header.

retry

Type: number | undefined

The retry interval in milliseconds, if specified.

Indicates how long to wait before reconnecting.

FAQ

Why not use EventSource?

The browser's built-in EventSource API has several limitations:

• Can't set custom headers (like Authorization)
• Only supports GET requests
• Doesn't work with the Fetch API
• No support for async iteration
• Can't be used with custom fetch implementations

This package works with any Response object, giving you full control over the request.

How is this different from other SSE parsers?

Most SSE parsers either:

• Implement their own HTTP client (limiting flexibility)
• Don't follow the spec correctly (especially for edge cases)
• Have dependencies or large bundle sizes
• Use callbacks instead of streams

This package focuses on doing one thing well: parsing SSE from a standard Response object using web platform standards (ReadableStream, TransformStream).

Can I use this with other HTTP clients?

Yes! Any HTTP client that returns a standard Response object will work:

// With Ky
import ky from 'ky';

const response = await ky(url);

// With native fetch
const response = await fetch(url);

// Both work the same way
for await (const event of parseServerSentEvents(response)) {
	console.log(event.data);
}

Related

• ky - Tiny and elegant HTTP client based on Fetch
• fetch-extras - Useful utilities for working with Fetch