How to find prime numbers between 0 - 100?

Answer 1

Using recursion combined with the square root rule from here, checks whether a number is prime or not:

function isPrime(num){

    // An integer is prime if it is not divisible by any prime less than or equal to its square root
    var squareRoot = parseInt(Math.sqrt(num));
    var primeCountUp = function(divisor){
        if(divisor > squareRoot) {
            // got to a point where the divisor is greater than 
            // the square root, therefore it is prime
            return true;
        }
        else if(num % divisor === 0) {
            // found a result that divides evenly, NOT prime
            return false;
        }
        else {
            // keep counting
            return primeCountUp(++divisor);
        }
    };

    // start @ 2 because everything is divisible by 1
    return primeCountUp(2);

}