Find Prime Numbers Less Than n

function primeNumbers(num) {

    var primes = [2,3,5,7];

    for(var i = 9; i < num; i+=2)   //loop is set to increment by odd numbers only
    {
        if(isPrime(i))
        {
            primes.push(i);
        }
    }

    return primes.join('\n');

    function isPrime(n)
    {
        if(!(n % 3) || !(n % 5) || !(n % 7)) return false;  //any multiple of 3, 5 or 7 is not prime

        for(var j = 3, len = Math.ceil(n / 2); j < len; j+=2)  //loops through all odd numbers less than half of the argument number
        {
            if(!(n % j)) return false;
        }

        return true;
    }

}

console.log(primeNumbers(2000));