euler.3.js

var sys = require('sys');

var Euler3 = function() {

    var isPrimeNumber = function(num) {
	//1 is NOT a prime number
	if(num == 1) return false;
	//2 is a prime number.
	if(num == 2) return true;
	//Exit early for even numbers (except 2 which is the only equal prime number).
	if((num % 2)==0) {
	    return false;
	}

	//There is no reason to check numbers above the square root of the number. The reason is that if a divisor above the square root exists, one below will as well.
	for(var i=2;i<=Math.sqrt(num);i++) {
	    if(isDivisor(num,i)) {
		return false;
	    }
	}
	//If no divisors was found the number must be a prime.
	return true;
    }

    var isDivisor = function(number, candidate) {
	//If a division has no remainder the candidate is a divisor of number. 
	return ((number%candidate)==0);
    }

    var getLargestPrimeFactor = function(ofNum) {
	for(var i=2;i<Math.round(ofNum/2);i++) {
	    if(isDivisor(ofNum, i)) {
		candidate = ofNum/i;
		if(isPrimeNumber(candidate)) {
		    sys.puts('Result: ' + candidate);
		    break;
		}
	    }
	}
    }
    getLargestPrimeFactor(600851475143);
};

new Euler3();