@orkestrel/sse

A typed Server-Sent Events parser — incremental, spec-compliant parsing of event-stream chunks into typed events with data, event, id, and retry fields. Feed it string chunks as they arrive; a blank line dispatches the accumulated event, and a partial line or in-progress event split across chunk boundaries is buffered until the rest arrives. The id / retry fields also persist as sticky connection state — surfaced via the id / retry getters for reconnection — and an optional limit bounds total buffered characters. A pure functional primitive — no Emitter, no events, no server / HTTP / agent coupling; it never throws on malformed input, only a typed SSEError('OVERFLOW') when a configured limit is exceeded. Part of the @orkestrel line.

Install

npm install @orkestrel/sse

Requirements

• Node.js >= 24
• ESM + CJS (dual-format build)
• No runtime dependencies

Usage

import { createSSEParser, isSSEError } from '@orkestrel/sse'

const parser = createSSEParser({ limit: 1_000_000 })
parser.parse('data: a\ndata: b\n\n') // [{ data: 'a\nb' }] - the two data lines joined
parser.parse('event: ping\ndata: 1') // [] - buffered until its blank line
parser.parse('\n\n') // [{ data: '1', event: 'ping' }]

parser.id // '1' - sticky last-event-id, survives dispatch
parser.retry // undefined - sticky reconnection time, until a retry: field arrives

try {
	parser.parse('x'.repeat(2_000_000))
} catch (error) {
	if (isSSEError(error) && error.code === 'OVERFLOW') parser.reset()
}

parser.flush() // force out a trailing unterminated event at end-of-stream
parser.reset() // full reset - drops buffered state and sticky id/retry

Pair it with a TextDecoder({ stream: true }) when reading a byte stream so multi-byte UTF-8 characters split across reads are handled — the decoder handles partial characters, this parser handles partial lines and events.

The optional limit option caps total buffered characters; when set, a parse(chunk) call that would exceed it throws a typed SSEError('OVERFLOW') instead of growing unbounded, leaving parser state unchanged. Without flush(), a stream that ends without a final blank line has its last event discarded per spec — call flush() at end-of-stream to force it out.

Guide

For the full surface — the SSEParser class, its SSEEvent shape, the wire format it implements, and the createSSEParser factory — see guides/src/sse.md.

Package

Published as a single typed entry point per the exports field in package.json.

License

MIT © Orkestrel — see LICENSE.