Prime Factorization with flatMap

PrimeFactorizationWithflatMap.js

// ======================
// Prime factorization with flatMap
//======================
// flatMap can be used as a way to add and remove items (modify the number of items) during a map.
// In other words, it allows you to map many items to many items (by handling each input item separately), rather than always one-to-one.
// it works like the opposite of filter.

var numbersTofactor = [20,1,17,2,3,15,16,100,157];
var factors = numbersTofactor.flatMap((n) => {
    var allFactors = [];
    const m = factor(allFactors,n); // [], 20
    return [m];
});

function factor(allFactors,num) {
    let nextGCF = findGCF(num); // 5
    if(nextGCF !== 0) {
        allFactors.push(nextGCF);
        if(nextGCF == num) return allFactors;
        let x = num / nextGCF; // x = 4
        return factor(allFactors,x);
    } else {
        // cant be factored give up and return num and 1
        return num == 1 ? [num] : [1,num];
    }
}

function findGCF(num) {
    const primes = [2,3,5,7];
    if(primes.includes(num)) return num;
    let smallest = 0;
    let gcf = 0;
    primes.forEach((p) => {
        // can be factored
        if(num % p == 0) {
            // find smallest factor
            if((num / p) < smallest || smallest == 0) {
                smallest = num / p;
                gcf = p;
            }
        }
    });
    return gcf;
}