my approach. when mid is found use two pointers to check either side of mid.

function find_range(arr, key) {
  let left=0, right=arr.length-1
  while(left <= right){
    let mid = (left+right) >>> 1
    if(arr[mid] < key) left = mid + 1
    else if(arr[mid] > key) right = mid - 1  
    else{
      let lft = mid, rgt = mid
      while(lft >= 0 && arr[lft-1] == arr[mid]) lft--
      while(rgt <= arr.length-1 && arr[rgt+1] == arr[mid]) rgt++
      return [lft,rgt]
    }
  }
  return [-1,-1];
}