A service worker is a script that your browser runs in the background, separate from a web page, opening the door to features that don't need a web page or user interaction. ... Things to note about a service worker: It's a JavaScript Worker, so it can't access the DOM directly.serviceWorker. The Navigator. serviceWorker read-only property returns the ServiceWorkerContainer object for the associated document, which provides access to registration, removal, upgrade, and communication with the ServiceWorker . The feature may not be available in private mode.