The sum of the first 10 truncatable primes

truncatablePrimes.js

let tPrimesCounter = 0;
let sum = 0;
let num = 7;

function isPrime(num, div = 2) {
  if (num < div || num % div === 0) {
    return false;
  }
  if (div === Math.ceil(Math.sqrt(num))) {
    return true;
  }
  return isPrime(num, div + 1);
}

while (tPrimesCounter < 10) {
  num += 1;
  subNum = num;
  const subNumLen = String(subNum).length;
  let index = 0;
  while (index < subNumLen) {
    subNumR = subNum % Math.pow(10, subNumLen - index);
    subNumL = Math.floor(subNum / Math.pow(10, index));
    if (!isPrime(subNumL) || !isPrime(subNumR)) {
      break;
    }
    index += 1;
  }
  if (index === subNumLen) {
    tPrimesCounter += 1;
    sum += num;
  }
}

console.log('Sum: ', sum);