fastest js prime generator been able to get http://jsperf.com/123123123456

prime-generator.js

// Thanks to @isntitvacant (https://github.com/chrisdickinson) for optimizing the
// bit shift performance tweaks.

var SB = require('buffer').SlowBuffer;
var ITER = 2e4;
var SIZE = 1e3;

function genPrimes(max) {
  var primes = new Array();
  var len = (max >>> 3) + 1;
  var sieve = new SB(len);
  var cntr = 0;
  var x = 2;
  var j;

  sieve.fill(0xff, 0, len);

  for (cntr = 0, x = 2; x <= max; ++x) {
    if (sieve[x >>> 3] & (1 << (x & 7))) {
      primes[cntr++] = x
      for(j = 2 * x; j <= max; j += x)
        sieve[j >>> 3] &= ~(1 << (j & 7))
    }
  }

  return primes;
}


var t = process.hrtime();

for (var i = 0; i < ITER; i++)
  genPrimes(SIZE);

t = process.hrtime(t);

console.log(((t[0] * 1e6 + t[1] / 1e3) / ITER).toFixed(1) + ' us/op');