Largest prime factor for n

largest_prime_factor.js

// find largest prime factor of n
function primeFactorization(n) {
    // begin by fetching the first 100 primes. if this proves insufficient we'll get more by multiplying the count by 10 each time.
    var primeCount = 100;
    var primes = getPrimes(primeCount);
    var factors = [];
    var temp = n;
    for (var i = 0; primes.indexOf(temp) < 0; i++) {
        if (i > primes.length - 1) {
            primeCount *= 10;
            primes = getPrimes(primeCount);
        }
        if (temp % primes[i] == 0) {
            temp = temp / primes[i];
            factors.push(primes[i]);
        }
    }
    // if the last division is a prime then add it as a factor.
    if (primes.indexOf(temp) >= 0) factors.push(temp);
    return Math.max.apply(null, factors);
}

function getPrimes(n) {
    var primes = [2, 3];
    for (var i = 5; primes.length < n; i += 2) {
        if (primes.every(function (p) { return i % p; })) {
            primes.push(i);
        }
    }
    return primes;
}

function test(actual, expected, err) {
    if (actual === expected) console.log("SUCCESS");
    else err(actual, expected);
}

function errMsg(actual, expected) {
    console.error("ERROR. Found: " + actual + " but expected: " + expected);
}

test(primeFactorization(600851475143), 6857, errMsg);

for a way more efficient implementation of getPrimes: https://gist.github.com/YanivHaramati/177030fbefa0d0d2879f