Without heaps of course.

function find_sliding_window_median(nums, k) {
    let windowSum = 0, start = 0;
    for(let i=0; i<nums.length; i++){
      windowSum += nums[i]
      if(i >= k-1){
        nums[i-1] = (windowSum / k)
        windowSum -= nums[start]
        start++
      }
    }
    nums.length -=1
    return nums
  }