How to notify front-end from backend?

Question

I am using multer for file uploading. On the server I have this:

stream.on('finish', () => {                                                                              
  console.log('finished')                                                                                
  next()
})

What would be the best way (with what command) to notify the frontend that 'finished' has happened? I want to know this in order to have a loading bar.

Answer 1

That's what websockets are for. The server can send data to the client, and vice-versa. Check out socket.io.

Server : socket.emit("upload finished")

Client : socket.on("upload finished", function...)

Answer 2

While Jeremy Thille 's answer works well, beware of broadcasting event to all.

Socket.emit() simply broadcasts message to all connected clients.

So you need to store the client id of the uploader and notify the progress only to that client and not all. Refer this.