public/js/lib/primes.js

// Deliberately slow trial division: a stand-in for any CPU-heavy work
// (parsing a big file, image filters, diffing, search indexing).
export function countPrimes(limit) {
  let count = 0;
  for (let n = 2; n < limit; n++) {
    let prime = true;
    for (let d = 2; d * d <= n; d++) {
      if (n % d === 0) {
        prime = false;
        break;
      }
    }
    if (prime) count++;
  }
  return count;
}