ws-keep-alive.ts

let timerID = 0
let ws = new WebSocket(url)
ws.onopen = () => {
  // some code
  keepAlive()
}
ws.onclose = () => {
  // some code
  cancelKeepAlive()
}
function keepAlive() {
  var timeout = 20000;
  if (ws.readyState == ws.OPEN) {
    ws.send('');
  }
  timerId = setTimeout(keepAlive, timeout);
}
function cancelKeepAlive() {
  if (timerId) {
    clearTimeout(timerId);
  }
}