// Sort Numbers in Descending Order
function sortDescending(num) {
    return Number(num.toString().split('').sort((a, b) => b - a).join(''));
//or: return parseInt(num.toString().split('').sort().reverse().join(''));
}

console.log(sortDescending(123)); // 321
console.log(sortDescending(1254859723)); // 9875543221