function findPrimeFactor(num) {

    let prime;

    for (let factor = 3; factor < num; factor += 2) {
        if (num % factor === 0) {
            for (let i = 3; i < factor; i += 2) {
                if (factor % i === 0) {
                    break;
                }
                else {
                    prime = factor;
                }
            }
        }
    }
    return prime;
}