/* 
1) passing array
   > quicksort([4,3,2,1]); // => [1,2,3,4]
*/
function quicksort(tail) {
  if (tail.length === 0) return [];
  let head = tail.splice(0, 1)[0];
  return quicksort(tail.filter( _ => _ <= head))
         .concat([head])
         .concat(quicksort(tail.filter( _ => _ > head )))
}

/* 
2) via arguments
   > quicksort(4,3,2,1); // => [1,2,3,4]
   or:
   > quicksort(...[4,3,2,1]); // => [1,2,3,4]
*/
function quicksort(x, ...xs) {
  if (arguments.length === 0) return [];
  return quicksort(...xs.filter( _ => _ <= x))
         .concat([x])
         .concat(quicksort(...xs.filter( _ => _ > x )))
}