The ports read-only property of the ExtendableMessageEvent interface returns the array containing the MessagePort objects representing the ports of the associated message channel (the channel the message is being sent through.)

An array of MessagePort objects.

When the following code is used inside a service worker to respond to a push messages by sending the data received via PushMessageData to the main context via a channel message, the event object of onmessage will be a ExtendableMessageEvent.

let port;

self.addEventListener("push", (e) => {
  const obj = e.data.json();

  if (obj.action === "subscribe" || obj.action === "unsubscribe") {
    port.postMessage(obj);
  } else if (obj.action === "init" || obj.action === "chatMsg") {
    port.postMessage(obj);
  }
});

self.onmessage = (e) => {
  port = e.ports[0];
};

• Using Service Workers
• Service workers basic code example
• Is ServiceWorker ready?
• Channel Messaging