sum_all_primes.js

function sumPrimes(num) {
  var p = [];
  
  p[num] = undefined; // initializing array till 0 -> num
  
  p[0] = false; // not prime
  p[1] = false; // not prime
  
  for (var i = 2; i * i <= num; i+=1) {
    if (p[i] === undefined) { // if its a prime
      // cross off all the composites divisible by i
      for (var j = 2; i * j <= num; j+=1) {
        p[i*j] = false;
      }
    } 
  }
  
  var sum = 0;
  for (var k = 2; k <= num; k++) {
    if (p[k] === undefined) {
      sum += k;
    }
  }
  
  return sum;
}

sumPrimes(10);