Breaking changes

• The WebSocket constructor now throws a SyntaxError if any of the
  subprotocol names are invalid or duplicated (0aecf0c).

• The server now aborts the opening handshake if an invalid
  Sec-WebSocket-Protocol header field value is received (1877dde).

• The protocols argument of handleProtocols hook is no longer an Array but
  a Set (1877dde).

• The opening handshake is now aborted if the Sec-WebSocket-Extensions header
  field value is empty or it begins or ends with a white space (e814110).

• Dropped support for Node.js < 10.0.0 (552b506).

• The WebSocket constructor now throws a SyntaxError if the connection URL
  contains a fragment identifier or if the URL's protocol is not one of 'ws:',
  'wss:', or 'ws+unix:' (ebea038).

• Text messages and close reasons are no longer decoded to strings. They are
  passed as Buffers to the listeners of their respective events. The listeners
  of the 'message' event now take a boolean argument specifying whether or not
  the message is binary (e173423).

  Existing code can be migrated by decoding the buffer explicitly.

  websocket.on('message', function message(data, isBinary) {
    const message = isBinary ? data : data.toString();
    // Continue as before.
  });
  
  websocket.on('close', function close(code, data) {
    const reason = data.toString();
    // Continue as before.
  });

• The package now uses an ES module wrapper (78adf5f).

• WebSocketServer.prototype.close() no longer closes existing connections
  (df7de57).

  Existing code can be migrated by closing the connections manually.

  websocketServer.close();
  for (const ws of websocketServer.clients) {
    ws.terminate();
  }

• The callback of WebSocketServer.prototype.close() is now called with an
  error if the server is already closed (abde9cf).

• WebSocket.prototype.addEventListener() is now a noop if the type argument
  is not one of 'close', 'error', 'message', or 'open' (9558ed1).

• WebSocket.prototype.removeEventListener() now only removes listeners added
  with WebSocket.prototype.addEventListener() and only one at time (ea95d9c).

• The value of the onclose, onerror, onmessage, and onopen properties is
  now null if the respective event handler is not set (6756cf5).

• The OpenEvent class has been removed (21e6500).

Bug fixes

• The event listeners added via handler properties are now independent from the
  event listeners added with WebSocket.prototype.addEventListener()
  (0b21c03).