just slice arr once

use variable `rightIndex` to make it more efficient, just slice arr once.

snippets/dropRightWhile.md

@@ -7,8 +7,9 @@ Returns the remaining elements.
 
 ```js
 const dropRightWhile = (arr, func) => {
-  while (arr.length > 0 && !func(arr[arr.length - 1])) arr = arr.slice(0, -1);
-  return arr;
+  let rightIndex = arr.length;
+  while (rightIndex-- && !func(arr[rightIndex]));
+  return arr.slice(0, rightIndex + 1);
 };
 ```