sieve_of_erathosphenes.js

const sieveOfErathosphenes = num => {
  const arr = [false, false]

  for (let idx = 2; idx < num; idx ++) {
    arr[idx] = true
  }

  for (let i = 2; i <  Math.sqrt(num); i++) {
    for (let j = 2; j * i <= num; j++) {
      arr[i * j] = false
    }
  }
  
  return arr.map((el, idx) => el ? idx : 0).filter(el => el)
}

const result = sieveOfErathosphenes(30)

console.time('test')
console.log(result) // [ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ] test: 6.255ms
console.timeEnd('test')