Two ways with Two functions to calculate the GCF for (two numbers) only.

Great Common Factor(divisor).js

/* 

Greatest Common Factor

GCF function takes two numbers and generates the greatest common factor.
*/

function GCF(num1, num2) {
        var arr1 = findFactors(num1);
        var arr2 = findFactors(num2);

        var cf = []; // store common factors.

        for (var i = 0; i < arr1.length; i ++) {
            for (var y = 0; y < arr2.length; y++) {
                // get the common factors (the number which exist on both arrays)
                if (arr1[i] === arr2[y]) {
                    cf.push(arr1[i]);
                }
            }
        }
        //sort common factors and grap the first/largest one. 
       var gcf = cf.sort(function(a,b){a-b})[0];
       return gcf;
    }

/* findFactors take a number to calculate its factors (Factors are the numbers you multiply together
to get another number:)

*/
    function findFactors(num) {
        var arr = [];
        
        // loop over the num to find all the numbers that accept to divide the (num).  
        for(var i = 1; i <= num; i++) {
            if(num%i === 0 ) { // if remainder is 0, this means it accept dividing by this number
                arr.push(i); // you can push (i) it is divisable by (num)
            }
        }

        return arr;
    }

function GCF2(num1, num2) {
    var gcf =  recursion(num1, num2);
    return gcf;
}

function recursion(num1, num2) {
    // the equation is num1 =  Math.floor(num1/num2)*num2 + remainder;
    // but I chanegd it to be able to store the remainder .. which will be our GCF.
    var remainder = num1 - Math.floor(num1/num2)*num2;
    
    // if remainder if 0, then we need the previous remainder which is num2
    if(remainder === 0) {
        return num2;
    }
    
    
    num1 = num2;
    num2 = remainder;

    return recursion(num1, num2); // re calculate with the new values.
}


//geting the performance of each one, it seams that GCF2 is faster than GCF (and this is logical :D).
function GCF_perf(n,m){
    var t0 = performance.now();
    var result = GCF(n,m);
    var t1 = performance.now();
    console.log('Took', (t1 - t0).toFixed(4), 'milliseconds to generate:', result);
}

function GCF2_perf(n,m){
    var t0 = performance.now();
    var result =  GCF2(n,m);
    var t1 = performance.now();
    console.log('Took', (t1 - t0).toFixed(4), 'milliseconds to generate:', result);
}

too complicated