Count the number of prime numbers less than a non-negative number

primes.js

var countPrimes = function(n) {
    var counter = 0;
    var input = n - 1;

    var isPrime = function(input) {
        var square;

        if (isNaN(input) || !isFinite(input) || input % 1 || input < 2) {

            return false;

        } 

        square = Math.sqrt(input);

        for (var i = 2; i <= square; i++) {

            if (input % i === 0) {

                return false;

            }
        }

        return true;
    };

    while (input > 1) {

        if (isPrime(input)) {

            counter += 1;

        }

        input -= 1;
    }

    return counter;
};