diff --git a/6.quickSort.md b/6.quickSort.md
index 28ad477..e04c952 100644
--- a/6.quickSort.md
+++ b/6.quickSort.md
@@ -79,6 +79,10 @@ function partition2(arr, low, high) {
 }
 
 function quickSort2(arr, low, high) {
+  const len = arr.length;
+  low = typeof low != 'number' ? 0 : low,
+  high = typeof high != 'number' ? len - 1 : high;
+  
   if (low < high) {
     let pivot = partition2(arr, low, high);
     quickSort2(arr, low, pivot - 1);