greatestCommonPrimeDivisor

function greatestCommonPrimeDivisor(a, b) {
    var aCPDs = getCPDs(a);
    var bCPDs = getCPDs(b);
    var max = -1;
    
    for (var i = 0; i < aCPDs.length; i++) {
        for (var j = 0; j < bCPDs.length; j++) {
            if (aCPDs[i] === bCPDs[j] && max < aCPDs[i]) {
                max = aCPDs[i];
            }
        }
    }
    
    function getCPDs (n) {
       var cpds = [];
        
        for (var i = 2; i < n; i++) {
            if (n % i === 0 && checkP(i)) {
                cpds.push(i);
            }
        }
        
        return cpds;
    }
    
    function checkP (n) {
        for (var i = 2; i < n; i++) {
            if (n % i === 0) {
                return false
            }
        }
        
        return true;
        
    }
    
    return max;
}