Quicksort implementation in JavaScript
I'm pretty sure the js convention is still to prefix &quot;private&quot; stuff with an underscore.
class QuickSort {
  sort(a) {...}
  _sort(a, lo, hi) {...}
  ...}

The only other thing that sticks out to me is your use of ++ and -- in the while loops. While perfectly idiomatic, I think it would express the logic better to move both conditionals together in the while(condition), and the increment action into the while's body. Of course it's always nice if one can abstract away a loop altogether...
Other people can speak more specifically about javascript than I can.