Calculate primes till certain number, checking against the already calculated, and only with the ones higher than the sqrt

primes.js

function primes(max) {
  let calculated = [1, 2, 3]; // already calculated primes
  let num = 4;                // current number being checked
  let factors;                // factors for the given number
  let sqrt;                   // sqrt for a given number
  // if the input is too low to be calculated, just return the default primes
  if (max < 5) return calculated;
  // otherwhise calculate for each number till you reach max
  while (num < max) {
    // discard even numbers
    if (num % 2 == 1) {
      sqrt = Math.sqrt(num);  // calculate the sqrt only once per number
      const factors = calculated
        .filter(el=> el >= sqrt)    // discard candidate if lower than the number sqrt
        .filter(el=>num % el == 0); // discard candidate if not a factor
      // if we found no factor then it's a prime!
      if (factors.length == 0) calculated.push(num);
    }
    num++;
  }
  return calculated;
}
//const p = primes(10000);