sse-line-parser

A lightweight, zero-dependency SSE (Server-Sent Events) stream parser. It does one thing: parse a ReadableStream into structured event objects. No request management, no reconnection logic, no EventEmitter abstraction.

Installation

pnpm add sse-line-parser
# or
npm install sse-line-parser
# or
yarn add sse-line-parser

Quick Start

import parseSSEStream from "sse-line-parser";

const res = await fetch("/api/chat", { method: "POST", body: "..." });
const reader = res.body!.getReader();

await parseSSEStream({
  renderStream: reader,
  options: {
    onMessage(msg) {
      console.log(msg.event, msg.data);
    },
    onDone() {
      console.log("stream finished");
    },
    onError(err) {
      console.error(err);
    },
  },
});

What It Parses

Each SSE event block is parsed into a LineData object:

id: 1
event: message
data: {"text":"hello world"}

{
  id: 1,
  event: "message",
  data: { text: "hello world" }  // auto JSON.parse if content starts with { or [
}

Multi-line data fields are joined with \n before parsing:

data: {"text":"hel
data: lo world"}

{
  data: {
    text: "hello world";
  }
}

[DONE] signals end of stream and stops processing immediately:

data: [DONE]

API

parseSSEStream(props)

| Field | Type | Description | | ------------------- | ----------------------------------------- | ----------------------------------------- | | renderStream | ReadableStreamDefaultReader<Uint8Array> | Stream reader from res.body.getReader() | | options.onMessage | (msg: LineData) => void | Called for each complete event | | options.onDone | () => void | Called when stream ends normally | | options.onError | (err: Error) => void | Called on parse errors |

LineData

type LineData = {
  id?: number;
  event?: string;
  data: Record<string, any> | string | null;
  error?: string;
};

Design Philosophy

This library handles only stream parsing. It will not:

• Wrap fetch or manage requests
• Handle reconnection or retry fields
• Maintain connection state
• Introduce class-based or reactive abstractions

If you need those features, build them on top — the parser itself stays simple and composable.

Runtime Support

• Node.js >= 18
• Browser (native ReadableStream)
• Bun / Deno / Edge Runtime

License

MIT