JavaScript Solution

[J_Scott_Chapman]

  let findMaxSlidingWindow = function(arr, window_size) {
  var result = [];
  let window = [];
  if(window_size > arr || window_size < 0) return null;
  for (let i = 0; i < arr.length; i++) {
      window.push(arr[i]);
      if(window.length === window_size) {
          result.push(Math.max(...window));
          window.shift();
      }
  }
  return result;
};