Web Workers

const worker = new Worker(url)
worker.onmessage = (e) => {
  if (e.data.progress) setProgress(e.data.progress)
  if (e.data.done) show(e.data.count)
}
worker.postMessage({ limit: 2_000_000 })

// inside the worker — heavy loop never blocks the UI:
self.onmessage = (e) => {
  let count = 0
  for (let n = 2; n <= e.data.limit; n++)
    if (isPrime(n)) count++
  self.postMessage({ done: true, count })
}