Worker

html

<button onclick="sayHI()">Say HI</button>
<button onclick="unknownCmd()">Send unknown command</button>
<button onclick="stop()">Stop worker</button>
<output id="result"></output>
<script>
  function sayHI() {
    worker.postMessage({'cmd': 'start', 'msg': 'Hi'});
  }

  function stop() {
    // Calling worker.terminate() from this script would also stop the worker.
    worker.postMessage({'cmd': 'stop', 'msg': 'Bye'});
  }

  function unknownCmd() {
    worker.postMessage({'cmd': 'foobard', 'msg': '???'});
  }

  let worker = new Worker('work.js');

  worker.addEventListener('message', function(e) {
    document.getElementById('result').textContent = e.data;
  }, false);
</script>

work.js

addEventListener('message', function (e) {
  let data = e.data;
  switch (data.cmd) {
    case 'start':
      postMessage('WORKER STARTED: ' + data.msg);
      break;
    case 'stop':
      postMessage('WORKER STOPPED: ' + data.msg + '. (buttons will no longer work)');
      close(); // Terminates the worker.
      break;
    default:
      postMessage('Unknown command: ' + data.msg);
  }
}, false);