FetchEvent.clientId

The clientId read-only property of the FetchEvent interface returns the id of the Client that the current service worker is controlling.

The Clients.get() method could then be passed this ID to retrieve the associated client.

Syntax

var myClientId = fetchEvent.clientId;

A DOMString that represents the client ID.

self.addEventListener('fetch', function(event) {
  console.log(event.clientId);
});

Specification
Service Workers 1 # fetch-event-clientid

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`clientId`	49	17	45 Extended Support Releases (ESR) before Firefox 78 ESR do not support service workers and the Push API.	No	36	11.1	49	49	45	36	11.3	5.0