Server Side Events Server

sseServer.js

// this is the ExpressJS function you would put at /sse for setting up Server Side Events
async function(request, response) {
  
  // for simplicity, we'll use a timestamp as an object key and for the first server event id (see below)
  let timestamp = (+ new Date());
  
  // store the response so you can use it later in other parts of the code
  request.app.get('connections')[timestamp] = response;
   
  // server side events require mime type 'text/event-stream'
  response.set({
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive'
  });
  
  // keep the connection alive until we receive a close from the client
  var connectionAlive = true;
  request.connection.on('close',() => {    
     connectionAlive = false;
     delete request.app.get('connections')[timestamp];
  });
  
  /*
    server side events require an 'id' & two new lines at the end
  */
  let timestamp = (+ new Date());
  let event = `id: ${timestamp}
retry: 5000
data: put some data here

`;

  // send the event
  response.write(event);

  // if you don't keep the connection alive, the connection will automatically close
  // and the server won't be able to send events later
  while(connectionAlive) {    
    await sleep(5000);
  }
};

function sleep(ms) {
  return new Promise(resolve => {
    setTimeout(resolve, ms)
  });
};