function isPrime (n) {
  if (n < 2) return false

  const limit = Math.floor(Math.sqrt(n))

  for (let i = 2; i <= limit; i += 1) {
    if (n % i === 0) {
      return false
    }
  }
  return true
}

function reducePrimeOccurrences (A, B) {
  const counts = {}

  for (let i = 0; i < B.length; i += 1) {
    const num = B[i]
    counts[num] = counts[num] ? counts[num] + 1 : 1
  }

  return A.reduce((acc, el) => isPrime(counts[el] || 0) ? acc : [...acc, el], [])
}

const A = [2, 3, 9, 2, 5, 1, 3, 7, 10]
const B = [2, 1, 3, 4, 3, 10, 6, 6, 1, 7, 10, 10, 10]
const C = reducePrimeOccurrences(A, B)