how to check if document exists in firestore code example

Example: realtime firebase database : How to check if document exists

this.realtimedbService.getOneItem(key,  'colection').valueChanges()
              .subscribe(
                  result => {
                      if (result === null) {
                          console.log('NOT EXIST');
                                     }
                      if (result !== null) {
                        console.log('EXIST');

                      }
                  },
                  error => {
                      console.log(" EE : ");
                      console.log(error);
                  }
              );