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A Web Worker is a JavaScript file running across processor cores(dedicated), browser tabs(shared), sessions(service), and iframes.

A worker script cannot access the DOM objects. Its global members are defined on the 'self' object instead of the 'window' object.

(Launch the FULL-screen mode and press F12 to open the console.)
RESETRUNFULL
// /shared/worker.js   (fixed)
console.log(globalThis.name, globalThis==self);     // myWorker true
// console.log(window);                             // error: undefined
console.log(self);                                  /* DedicatedWorkerGlobalScope {name: 'myWorker', 
                                                          onmessage: null, 
                                                          onmessageerror: null, 
                                                          cancelAnimationFrame: ƒ, 
                                                          close: ƒ, …} */

<!DOCTYPE><html><body style="height:0">
<p></p>
<script>
   document.querySelector("p").innerHTML = (globalThis==self)+" : "+(globalThis==window);
   var myWorker = new Worker("/shared/worker.js", { name : "myWorker" });
</script></body></html>

Note that window.postMessage() and the onmessage event work across parent-child windows and iframes too.

A ServiceWorker continues to run in the background even after the user has navigated away from the site. It requires a secure context, ie. HTTPS.

Worker Type Lifespan
Dedicated Web Worker Until closing of tab or navigation to another page.
Shared Worker Until closing of all tabs of the site on a browser.
Service Worker About one minute after the closing of all tabs of the site on a browser, on a desktop PC.
(as of 3 Oct, 2020)