WebSockets server tutorial

server.js

// Read the subprotocol from the client request headers:
const protocol = req.headers['sec-websocket-protocol'];
// If provided, they'll be formatted as a comma-delimited string of protocol
// names that the client supports; we'll need to parse the header value, if
// provided, and see what options the client is offering:
const protocols = !protocol ? [] : protocol.split(',').map(s => s.trim());
// To keep it simple, we'll just see if JSON was an option, and if so, include
// it in the HTTP response:
if (protocols.includes('json')) {
  // Tell the client that we agree to communicate with JSON data
  responseHeaders.push(`Sec-WebSocket-Protocol: json`);
}