isPrime.coffee

isPrime = (n) ->
    return true if n is 2 or n is 3
    return false if n % 2 is 0

    for i in [3..Math.sqrt n]
        return false if n % i is 0

    true

primesUnder100 = (n for n in [1..100] when isPrime n)
console.log(primesUnder100)

isPrime.js

var isPrime, n, primesUnder100;

isPrime = function(n) {
  var i, _i, _ref;
  if (n === 2 || n === 3) {
    return true;
  }
  if (n % 2 === 0) {
    return false;
  }
  for (i = _i = 3, _ref = Math.sqrt(n); 3 <= _ref ? _i <= _ref : _i >= _ref; i = 3 <= _ref ? ++_i : --_i) {
    if (n % i === 0) {
      return false;
    }
  }
  return true;
};

primesUnder100 = (function() {
  var _i, _results;
  _results = [];
  for (n = _i = 2; _i <= 100; n = ++_i) {
    if (isPrime(n)) {
      _results.push(n);
    }
  }
  return _results;
})();

console.log(primesUnder100);