SSE Feed

Server-Sent Events consumer supporting backpressure.

Introduction

EventSource API to consume SSE feed is not available in Node.js. There are plenty of polyfills for EventSource. There are some consumers of SSE connection. All of them suffer in some way:

not handling automatic reconnects,
or having a weird buffering behaviour,
or relying on Node-specific http API,
or not supporting backpressure.

This library tries to solve that. It does reconnect as EventSource does, it does not buffer input, it supports backpressure, it connects through a native fetch.

Installation

As usual, do

npm add sse-feed

Usage

First, create an instance of SSEFeed, then consume the stream as sseFeed.stream(). It presents a JS-native ReadableStream.

import { SSEFeed } from 'sse-feed'
const sseFeed = new SSEFeed('http://example.com/sse')
// Use iteration
for await (const event of sseFeed.stream()) {
  // Do something with the event
  console.log(event)
}
// ...or use more native streaming
sseFeed.stream().pipeTo(...)

Cancellation

When you are done with consuming the events, call sseFeed.close(). It tells the stream to gracefully stop.

"open" and "close" events

To keep track of the feed connectivity, you could subscribe to open and error events. SSEFeed implements EventTarget interface. These events are independent of SSE stream events.

import { SSEFeed } from 'sse-feed'
const sseFeed = new SSEFeed('http://example.com/sse')
sseFeed.stream().pipeTo(...)
sseFeed.addEventListener('open', openEvent => {
console.log('Opened')
})

open and error events could be emitted multiple times during the streaming as per the Server-Sent Events specification.

One SSEFeed instance - One stream

Calling sseFeed.stream() initiates a HTTP connection. The connection lifecycle is tracked by open and error events. Hence, there is a one-to-one correspondence between ReadableStream of sseFeed.stream() and an instance of SSEFeed. Hence, you can call sseFeed.stream() once per sseFeed instance.

If you need multiple consumers, you could use tee() method, like sseFeed.steram().tee(), or you could create another SSEFeed instance.

Custom fetch properties

SSEFeed constructor accepts an optional second parameter. It can be { withCredentials: true } like for EventSource. Or it can be fetch parameters like { method: 'POST' }.