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Dedicated Web Worker

A dedicated Web Worker is a thread spawned from the main JavaScript flow. With dedicated Web Workers you can run on multiple processor cores simultaneously.

A dedicated web worker inherits members from the WorkerGlobalScope, WindowOrWorkerGlobalScope, DedicatedWorkerGlobalScope interfaces, including: console, location, navigator, performance, caches, indexedDB, isSecureContext, origin, atob(), btoa(), fetch(), queueMicrotask(), setInterval(), setTimeout().

The following example tries to find prime numbers fast by running the computation (not the most efficient algorithm!) in parallel threads.
RESETRUNFULL
// /shared/prime.js  (fixed)
onmessage = ev=>{
   n = ev.data;
   var isPrime=true;
   if (n>2)
      for (i=2; i<Math.sqrt(n)+1; i++)
         if (n%i==0){isPrime=false; break;}
   postMessage({n,isPrime});
}

<!DOCTYPE html><html><body>
   <p>Prime numbers: <output></output></p>
   <button onclick="startWorker()">Start a Worker</button> 
   <button onclick="workers.pop().terminate();">Stop a Worker</button>
   <script>
      var c=0;
      var last = 2;
      var workers =[];
      var startTS=Date.now();
      function startWorker() {
         if(!Worker) {
            document.querySelector("p").innerHTML=
               "Your browser does not support Web Workers...";
         }
         var w = new Worker("/shared/prime.js");
         w.onmessage = function (e) {
            if (last==2) last++;
            else last+=2;
            if (last>1000000){
               workers.forEach(wk=>wk.terminate());
               document.querySelector("p").innerHTML=
                  `${Math.round((Date.now()-startTS)/1000)}s taken by ${workers.length} workers to get ${c} prime numbers.`;
            }
            if (e.data.isPrime){
               c++;
               if (c%1000==0) document.querySelector("output").innerHTML = e.data.n;
            }
            w.postMessage(last);
         }
         workers.push(w);
         w.postMessage(last);
      }
   </script>
</body></html>

You can import one or more scripts into the worker's scope:

importScripts('foo.js', 'bar.js');

The navigator.hardwareConcurrency read-only property returns the number of logical processor threads available to run software threads on the user's computer.