Array.prototype._reverse = function() {
  const len = this.length;
  const mid = Math.floor(len / 2);

  // l: left pointer; r: right pointer;
  let l = 0,
      r = len - 1,
      temp;
  

  while (l <= mid || r >= mid) {
    temp = this[l];
    this[l] = this[r];
    this[r] = temp;
    l++;
    r--;
  }
};