50.js

/*
 * Problem 50 - Project Euler
 * Sean Lee 02/03/12
 */

var isPrime = []
var primes = []

var genprimes = function(upperbound){ //Sieve of Erastothenes
	for (var i = 0; i< upperbound; i++){
		isPrime.push(1);
	}
	isPrime[0] = 0;
	isPrime[1] = 0;

	var i = 2;

	var loopbound = Math.sqrt(upperbound); 
	while( i <= loopbound){
		for( var k = 2*i; k < upperbound; k = k + i){
			isPrime[k] = 0;
		}
		i++;
	}
	for ( var i = 2; i < upperbound; i ++){
		if(isPrime[i] === 1) primes.push(i);
	}
}
function find(upperbound){
	var sums = []
	var longest = [0,0];
	var i = 0;
	while (true){
		for( var a in sums){
			sums[a] += primes[i];
			if (sums[a] > upperbound) return longest;
			if (isPrime[sums[a]]){
				var length = sums.length + 1 - a;
				if (length > longest[1]){
					longest = [sums[a], length];
				}
			}
		}
		sums.push(primes[i]);
		i++;
	}
}
genprimes(1000000);
console.log(find(1000000)[0]);