summarize prime number, but to check whether it is prime or not, based on previous prime number

sum_prime.js

function sumPrimes(num) {
  prime = [2,3];
  sum = 2 + 3;
  for(n = 4; n <= num; n++){
    var prime_flag = true;
    for(prime_d = 0; prime_d < prime.length; prime_d++){
      if(n % prime[prime_d] === 0){
        prime_flag = false;
      }
    }
    if(prime_flag){
      sum += n;
      prime.push(n);
    }
  }
  return sum;
}

sumPrimes(10);