Reconnect & resync

{ "id": 999, "cmd": "ping" }

{ "id": 999, "type": "pong", "ts": 1776949200000 }

let reconnectDelay = 1000;

function connect() {
  // Node / server: pass X-Api-Key as a header.
  // Browser:       use `${WSS}/ws/user?key=<encodeURIComponent(apiKey)>`
  //                because the WebSocket(…) ctor can't set custom headers.
  const ws = new WebSocket(`${WSS}/ws/user`, {
    headers: { 'X-Api-Key': apiKey },
  });

  ws.onopen = () => {
    reconnectDelay = 1000;
    // Always re-subscribe from scratch on a new connection.
    ws.send(JSON.stringify({
      id: nextId(),
      cmd: 'subscribe',
      params: { subscriptions: mySubscriptions },
    }));
  };

  ws.onclose = () => {
    // Drop sid → handler map; on next open you'll re-subscribe and
    // get fresh sids.
    sidHandlers.clear();
    setTimeout(connect, reconnectDelay);
    reconnectDelay = Math.min(reconnectDelay * 2, 30_000);
  };

  ws.onmessage = handleMessage;
}

if (delta.prevSeq !== lastSeq[delta.id]) {
  // Gap — ask the gateway to re-push a snapshot on this sid.
  ws.send(JSON.stringify({
    id: nextId(),
    cmd: 'get_book_snapshot',
    params: { sid: delta.sid },
  }));
}