#WebSocket

Fairyfly speaks WebSocket on both sides: upgrade HTTP clients with the websocket option (server), or dial out with new WebSocket(url) (client). Server and client share the frame protocol but have different APIs — this guide covers both.

#Quick look: echo server + client

Server (server.js) — the websocket block makes upgrades automatic; echo everything back:

// server.js
const peers = new Set();

http.serve(
  {
    port: 3000,
    websocket: {
      open: (socket) => {
        peers.add(socket);
        socket.send("welcome");
      },
      message: (socket, data) => socket.send(data),
      close: (socket) => peers.delete(socket),
    },
  },
  (url, method, body) => new Response("http here, ws upgrades on this port"),
);

Client (client.js) — connect, send, print:

// client.js
const ws = new WebSocket("ws://127.0.0.1:3000/ws");
ws.onopen = () => ws.send("hello");
ws.onmessage = (ev) => console.log("got:", ev.data);

ff server.js
# in another terminal:
ff client.js
# got: welcome
# (server echoes "hello" back: got: hello)

#Server: upgrading connections

Add a websocket block to the options. Its presence enables upgrades; the upgrade itself fires when a request arrives with a Sec-WebSocket-Key header — on any path, before your handler runs. The handler only ever sees plain HTTP, so it returns normal responses for every path:

http.serve(
  { port: 3000, websocket: { open, message, close } },
  (url, method, body) => new Response("not found", { status: 404 }),
);

Always check the type of data first — text and binary arrive differently:

message: (socket, data) => {
  if (typeof data === "string") console.log("text:", data);
  else console.log("binary bytes:", data.length);
},

#Server: sending and broadcast

Server sockets have exactly two methods — send (text) and sendBinary (bytes). There is no socket.id, no readyState, and no server-side close. Identify peers by object reference in a Set:

// server.js — chat room that broadcasts to everyone
const peers = new Set();

http.serve(
  {
    port: 3000,
    websocket: {
      open: (socket) => peers.add(socket),
      message: (socket, data) => {
        for (const peer of peers) peer.send(data);
      },
      close: (socket) => peers.delete(socket),
    },
  },
  (url, method, body) => new Response(`online: ${peers.size}`),
);

ff server.js
curl http://127.0.0.1:3000/   # online: 0  (peers counted via Set size)

Send binary with sendBinary:

socket.sendBinary(new Uint8Array([1, 2, 3]).buffer);

Caution: socket.send after the client disconnected fails — always remove sockets in close, and never store an index or id on them (none exists).

#Client: connecting and messaging

// client.js
const ws = new WebSocket("ws://127.0.0.1:3000/ws");

ws.onopen = () => {
  console.log("open, state:", ws.readyState); // 1
  ws.send("hello");
  ws.sendBinary(new Uint8Array([9, 9]).buffer);
};

ws.onmessage = (ev) => console.log("got:", ev.data);
ws.onclose = () => console.log("closed");
ws.onerror = (e) => console.error("ws error:", e.message);

Only ws:// and wss:// URLs are accepted. For wss:// to private CAs, the same trust rules as fetch apply (--ca for scripts, FF_CA_FILE for ff start).

#Client errors

Two shapes, depending on when the failure happens. Synchronous mistakes throw immediately — every message starts with a WebSocket: prefix:

new WebSocket("http://localhost:3000/");
// TypeError: WebSocket: invalid WebSocket URL (expected ws:// or wss://)
new WebSocket();
// TypeError: WebSocket: WebSocket constructor requires a URL

A failed connection at runtime (refused, unreachable) does not throw and does not fire onerror — you get a bare onclose. When the close does carry an error, onerror fires first with an event whose .message is a string (not an Error), immediately followed by onclose, whose event carries numeric .code and string .reason:

ws.onerror = (ev) => console.error("ws error:", ev.message);
ws.onclose = (ev) => console.log("closed:", ev.code, ev.reason);

#Practical example: presence counter

Track who's online and push the count to every client on join/leave:

// presence.js
const peers = new Set();
const count = () => `online: ${peers.size}`;
const broadcast = () => {
  for (const peer of peers) peer.send(count());
};

http.serve(
  {
    port: 3000,
    websocket: {
      open: (socket) => {
        peers.add(socket);
        broadcast();
      },
      message: (socket, data) => socket.send(`echo: ${data}`),
      close: (socket) => {
        peers.delete(socket);
        broadcast();
      },
    },
  },
  (url, method, body) => new Response(count()),
);

ff presence.js
curl http://127.0.0.1:3000/   # online: 0
# connect two clients -> each receives "online: 1", then "online: 2"

#Reference: limits

#Troubleshooting

Upgrade never fires — the websocket block must be present, and the client must send the handshake headers (Connection: Upgrade, Upgrade: websocket, Sec-WebSocket-Key — any real WS client does). No handler branch is needed: the upgrade happens before the handler runs.

socket.id is undefined — by design. Use a Set of socket objects; peers.has(socket) is the identity check.

Binary arrives as Uint8Array, not string — check typeof data === "string" first, per the table above.

Client connect fails — confirm the ws:///wss:// scheme, the 512-byte path limit, and (for wss://) the CA trust setup from the fetch guide.