Solution for http://www.codewars.com/dojo/katas/521ef596c106a935c0000519/solutions/javascript Using algorithm: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

prime.js

function prime(num) {  
  var primes = [],
    i, j;
  
  if (num < 2) return [];
  
  for (i = 2; i <= num; i++) { primes[i] = true; }

  for (i = 2; i <= Math.floor(Math.sqrt(num)); i++) {    
    var n = 0;
    if (primes[i]) {
      for (j = Math.pow(i, 2); j <= num; j = Math.pow(i,2)+n*i) { primes[j] = false; n++ }
    }
  }
  
  primes = primes.map(function(p, i) {
    if (p)return i;
  }).filter(function(p) { return p; });
  
  return primes;
}