Greatest Prime Factor

I'm trying to complete an algorithm challenge to find the largest prime factor of 600851475143. I'm not necessarily asking for the answer. Just trying to figure out why this code isn't working. Why does it return 'undefined' instead of a number?

let isPrime = n => {
    let div = n - 1;
    while (div > 1) {
        if (n % div == 0) return false;
        div--;
    }
    return true;
};

let primeFactor = x => {
    for (let i = Math.floor(x / 2); i > 1; i--) {
        if (x % i == 0 && isPrime(i) == true) {
            return i;
        }
    }
};

console.log(primeFactor(35)); // 7
console.log(primeFactor(13195)); // 29
console.log(primeFactor(600851475143)); // undefined