ws.js

_init() {
  if (this._server) throw new Error('Server already initialized');

  this._server = http.createServer((req, res) => {
    const UPGRADE_REQUIRED = 426;
    const body = http.STATUS_CODES[UPGRADE_REQUIRED];
    res.writeHead(UPGRADE_REQUIRED, {
      'Content-Type': 'text/plain',
      'Upgrade': 'WebSocket',
    });
    res.end(body);
  });
}