primetester.js

function primeTester(num) {
  var range = new Array;
  for (var i=1; i <= num; i++) {
    range.push(i);
  }

  var factors = new Array;
  for (var i=0; i < range.length; i++) {
    var current = range[i];
    if (num % current === 0) {
      factors.push(current);
    }
  }

  if (factors.length === 2) {
    return true;
  }
  return false;
}