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