WebSocket

let socket = new WebSocket("ws://javascript.info");

let socket = new WebSocket("wss://javascript.info/article/websocket/demo/hello");

socket.onopen = function(e) {
  alert("[open] Connection established");
  alert("Sending to server");
  socket.send("My name is John");
};

socket.onmessage = function(event) {
  alert(`[message] Data received from server: ${event.data}`);
};

socket.onclose = function(event) {
  if (event.wasClean) {
    alert(`[close] Connection closed cleanly, code=${event.code} reason=${event.reason}`);
  } else {
    // e.g. 
    // event.code  1006 
    alert('[close] Connection died');
  }
};

socket.onerror = function(error) {
  alert(`[error] ${error.message}`);
};

GET /chat
Host: javascript.info
Origin: https://javascript.info
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Key: Iv8io/9s+lYFgZWcXczP8Q==
Sec-WebSocket-Version: 13

101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: hsBlbuDTkk24srzEOTBUlZAlC2g=

GET /chat
Host: javascript.info
Upgrade: websocket
Connection: Upgrade
Origin: https://javascript.info
Sec-WebSocket-Key: Iv8io/9s+lYFgZWcXczP8Q==
Sec-WebSocket-Version: 13
Sec-WebSocket-Extensions: deflate-frame
Sec-WebSocket-Protocol: soap, wamp

101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: hsBlbuDTkk24srzEOTBUlZAlC2g=
Sec-WebSocket-Extensions: deflate-frame
Sec-WebSocket-Protocol: soap

socket.bufferType = "arraybuffer";
socket.onmessage = (event) => {
  // event.data () arraybuffer ()) 
};

// 100 ms 
setInterval(() => {
  if (socket.bufferedAmount == 0) {
    socket.send(moreData());
  }
}, 100);

socket.close([code], [reason]);

// :
socket.close(1000, "Work complete");

// 
socket.onclose = event => {
  // event.code === 1000
  // event.reason === "Work complete"
  // event.wasClean === true (clean close)
};

// 
socket.onclose = event => {
  // event.code === 1006
  // event.reason === ""
  // event.wasClean === false (no closing frame)
};

<!-- message form -->
<form name="publish">
  <input type="text" name="message">
  <input type="submit" value="Send">
</form>

<!-- div with messages -->
<div id="messages"></div>

let socket = new WebSocket("wss://javascript.info/article/websocket/chat/ws");

// 
document.forms.publish.onsubmit = function() {
  let outgoingMessage = this.message.value;

  socket.send(outgoingMessage);
  return false;
};

//  - div#message 
socket.onmessage = function(event) {
  let message = event.data;

  let messageElem = document.createElement('div');
  messageElem.textContent = message;
  document.getElementById('messages').prepend(messageElem);
}

const ws = new require('ws');
const wss = new ws.Server({noServer: true});

const clients = new Set();

http.createServer((req, res) => {
  //  websocket 
  // -websocket 
  wss.handleUpgrade(req, req.socket, Buffer.alloc(0), onSocketConnect);
});

function onSocketConnect(ws) {
  clients.add(ws);

  ws.on('message', function(message) {
    message = message.slice(0, 50); //  50 

    for(let client of clients) {
      client.send(message);
    }
  });

  ws.on('close', function() {
    clients.delete(ws);
  });
}