function startServer()

in packages/relay-devtools-core/src/standalone.js [183:251]
52 lines of code
3 McCabe index (conditional complexity)

function startServer(port?: number = 8097) {
  const httpServer = require('http').createServer();
  const server = new Server({ server: httpServer });
  let connected: WebSocket | null = null;
  server.on('connection', (socket: WebSocket) => {
    if (connected !== null) {
      connected.close();
      log.warn(
        'Only one connection allowed at a time.',
        'Closing the previous connection'
      );
    }
    connected = socket;
    socket.onerror = error => {
      connected = null;
      onDisconnected();
      log.error('Error with websocket connection', error);
    };
    socket.onclose = () => {
      connected = null;
      onDisconnected();
      log('Connection to RN closed');
    };
    initialize(socket);
  });

  server.on('error', event => {
    onError(event);
    log.error('Failed to start the DevTools server', event);
    startServerTimeoutID = setTimeout(() => startServer(port), 1000);
  });

  httpServer.on('request', (request, response) => {
    // NPM installs should read from node_modules,
    // But local dev mode needs to use a relative path.
    const basePath = existsSync('./node_modules/relay-devtools-core')
      ? 'node_modules/relay-devtools-core'
      : '../relay-devtools-core';

    // Serve a file that immediately sets up the connection.
    const backendFile = readFileSync(`${basePath}/dist/backend.js`);

    response.end(
      backendFile.toString() +
        '\n;' +
        'RelayDevToolsBackend.connectToDevTools();'
    );
  });

  httpServer.on('error', event => {
    onError(event);
    statusListener('Failed to start the server.');
    startServerTimeoutID = setTimeout(() => startServer(port), 1000);
  });

  httpServer.listen(port, () => {
    statusListener('The server is listening on the port ' + port + '.');
  });

  return {
    close: function() {
      connected = null;
      onDisconnected();
      clearTimeout(startServerTimeoutID);
      server.close();
      httpServer.close();
    },
  };
}