Very clever. I had to debug it to wrap it around my head.
For future readers:

• i is the index (number-1) of the room, thus i+1 is room number
• rooms is holding for each room, the next day available (the last departure for each room)
• the map {} is checking for each arrival, the first room available at this arrival date, and puts the departure date in this room

Thanks for enjoying :)

Nice one. Thank you!

Great challenge! I recommend checking a similar, but easier kata at first: https://www.codewars.com/kata/536a155256eb459b8700077e
That would give you a good basis for solving this one.