Use a isPrime method to return true on primes. Then loop through and add result to output if isPrime.

Find prime numbers

function sumPrimes(num) {
  function isPrime(num) {
    if(num < 2) return false;
    for (var i = 2; i < num; i++) {
        if(num%i==0)
            return false;
    }
    return true;
  }

  var output = 0;

  for(var i = 0; i <= num; i++){
      if(isPrime(i)) {
        output =+ output + i;
      }
  }
  return output;
}

sumPrimes(10);