WebSocket, SSE & XHR

Both a client and a server-side WebSocket. The client is the global WebSocket as Node's (undici's): the constructor returns CONNECTING and the handshake runs on the event loop; messages arrive as MessageEvents whose binary data is a Blob or an ArrayBuffer per binaryType, and close()/onclose carry a CloseEvent (code, reason, wasClean). Server-side, attach klain:ws's `new WebSocketServer({ server })` to an http.createServer (ws://) or https.createServer (wss://) server and `wss.on('connection', socket => …)` hands each upgraded connection a WSConnection that coexists with ordinary HTTP. The raw Node `server.on('upgrade', (req, socket, head) => …)` event is also available for hand-rolled or non-WebSocket upgrades.

urlstring | URL

The ws:// or wss:// endpoint (http:/https: are rewritten, as Node does).

protocolsstring | string[]

Subprotocols to offer; the server's choice is ws.protocol.

WebSocket — The client socket, an EventTarget with onopen/onmessage/onclose/onerror handlers.

const ws = new WebSocket('ws://127.0.0.1:8083/')
ws.onopen = () => { ws.send('ping') }
ws.onmessage = (ev) => { console.log(ev.data); ws.close() }

Server-sent events (SSE) as Node's (undici's) EventSource, over fetch — behind `--experimental-eventsource` in Node. Covers readyState/close(), onmessage/onopen/onerror, and addEventListener/removeEventListener for named (event: name) records — data/event/id/retry field parsing, multi-line data: concatenation, the CRLF/LF/CR line endings, and auto-reconnect honoring a server-sent retry: delay while replaying Last-Event-ID. A response that arrives with the wrong shape (non-200, or without text/event-stream) fails permanently instead of retrying.

urlstring | URL

The SSE endpoint to connect to.

initEventSourceInit

`{ withCredentials }`.

EventSource — The event source, with readyState, onmessage/onopen/onerror and addEventListener.

const messages = new EventSource('http://127.0.0.1:8765/stream')
messages.onopen = (ev) => { console.log(ev.type) }   // open
messages.onmessage = (ev) => { console.log(ev.data); messages.close() }

A legacy client modeled as the spec's synchronous mode: .send() runs the request to completion before returning, then fires every registered callback once. It's not actually blocking — .send() is built on the same non-blocking primitive fetch() uses, so it still yields the current fiber when called from inside an http.listen connection handler or a klain:sync worker goroutine (making pooled goroutines a real closed-loop load generator). Supports .open(method, url, async?, user?, password?) with async: false exact, .setRequestHeader(), .send()/.send(body), .abort(), .readyState, .status, .responseText/.response, .getResponseHeader()/.getAllResponseHeaders(), and the zero-argument callbacks. A network-level failure fires .onerror rather than throwing.

XMLHttpRequest — The request object, with onreadystatechange/onload/onerror callbacks.

const xhr = new XMLHttpRequest()
xhr.onload = () => { console.log('loaded') }
xhr.open('GET', 'http://127.0.0.1:8765/get')
xhr.send()
console.log(xhr.status)  // 200