findPrimeFactors.ts

function isPrime(numberToCheck: number) {
  if (numberToCheck <= 1) return false;
  for (let possibleDivisor = 2; possibleDivisor <= numberToCheck; possibleDivisor++) {
    if (numberToCheck % possibleDivisor === 0) return false;
  }
  return true;
}

const numbers = [99999, 12, 15];
const allFactors = [];

for (let numberToFactorise of numbers) {
  for (let possibleFactor = 1; possibleFactor <= numberToFactorise; possibleFactor++) {
    if (numberToFactorise % possibleFactor === 0) allFactors.push(numberToFactorise);
  }
}

const allPrimeFactors = allFactors.filter(isPrime);
console.log(allPrimeFactors);