chromium/src/third_party/blink/web_tests/http/tests/serviceworker/resources/postmessage-port-worker.js

var port;

self.onmessage = function(e) {
  var message = e.data;
  if ('port' in message) {
    message.port.onmessage = function(e2) {
      port = e2.data.port;
      port.onmessage = on_message;
    };
  }
};

function on_message(e) {
  var message = e.data;
  if ('value' in message) {
    port.postMessage('Acking value: ' + message.value);
  } else if ('done' in message) {
    port.postMessage('quit');
  }
}