Function to determine number of circular primes in a range.

circularprimes.js

/*
A number is called a circular prime if it is prime and all of its rotations are primes.

For example, the number 197 has two rotations: 971, and 719. Both of them are prime.
Another example: 1193 is a circular prime, since 1931, 9311 and 3119 all are also prime.
*/

function isPrime(n) {
  if (n == 1) return false;
  for (var i = 2; i <= Math.sqrt(n); i++) {
    if (n % i == 0) return false;
  }
  return true;
}

function rotate(n) {
  var nn = ""+n;
  var n2 = nn[nn.length-1]+nn.substring(0,nn.length-1);
  return parseInt(n2);
}

function circular(N) {
  var outbound = 0;
  for (var i = 2; i <= N; i++) {
    var isMatch = true;
    var check = i;
    for (j = 0; j < (""+i).length; ++j) {
      if (!isPrime(check)) {
        isMatch = false;
        break;
      }
      check = rotate(check);
    }
    if (isMatch) outbound++;
  }
  return outbound;
}

console.log(circular(1));
console.log(circular(100));
console.log(circular(10000));
console.log(circular(1000000));