The Sieve of Eratosthenes.

prime_sieve.js

function Eratosthenes(max) {
  var elim = {},
      primes = [];
  for (var i=2; i<=max; i++) {
    if ( !elim[i] ) {
      primes.push(i);
      for (var j=i*2; j<=max; j+=i) {
        elim[j] = true;
      }
    }
  }
  return primes;
}