primes.js

var primes = [2];

var n = 3;  // The number we are testing. Will be incremented.

function checkNextNumber() {
	// It's sufficient to check if there are any prime factors up to sqrt(n)

	n++;

	var limitForChecking = Math.floor(Math.sqrt(n));
	var isPrime = true;

	for(var i=0;i<primes.length;i++) {
		if(primes[i] > limitForChecking) {
			break;
		} else if(n % primes[i] === 0) {
			isPrime = false;
			break;
		}
	}

	if(isPrime) {
		console.log("Found prime:", n);
		primes.push(n);
	}

	setTimeout(checkNextNumber, 0);
}

checkNextNumber();