ExtendableMessageEvent: lastEventId property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2018.

Note: This feature is only available in Service Workers.

The lastEventID read-only property of the ExtendableMessageEvent interface represents, in server-sent events, the last event ID of the event source. This is an empty string.

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 an ExtendableMessageEvent.

js
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) => {
  console.log(e.lastEventId);
  port = e.ports[0];
};