hard stuff

nights.js

function combineCustomNights(customNights) {
  const result = [];
  let currentRange = null;

  customNights.sort((a, b) => new Date(a.date) - new Date(b.date));

  customNights.forEach((night) => {
    if (!currentRange) {
      currentRange = { start: night.date, end: night.date, rooms: { ...night } };
      delete currentRange.rooms.date;
    } else {
      const nextDate = new Date(currentRange.end);
      nextDate.setDate(nextDate.getDate() + 1);

      if (night.date === currentRange.end || night.date === nextDate.toISOString().slice(0, 10)) {
        currentRange.end = night.date;
        Object.keys(night).forEach((key) => {
          if (key !== 'date') {
            currentRange.rooms[key] = (currentRange.rooms[key] || 0) + night[key];
          }
        });
      } else {
        result.push(currentRange);
        currentRange = { start: night.date, end: night.date, rooms: { ...night } };
        delete currentRange.rooms.date;
      }
    }
  });

  if (currentRange) {
    result.push(currentRange);
  }

  return result;
}

const customNights = [
  { "date": "2023-03-14", "standardRooms": 1 },
  { "date": "2023-03-15", "suite": 1 },
  { "date": "2023-03-15", "standardRooms": 1, "doubleRooms": 2 },
  { "date": "2023-03-17", "standardRooms": 1 }
];

console.log(combineCustomNights(customNights));

output is:
[
{
"start": "2023-03-14",
"end": "2023-03-15",
"rooms": {
"standardRooms": 2,
"suite": 1,
"doubleRooms": 2
}
},
{
"start": "2023-03-17",
"end": "2023-03-17",
"rooms": {
"standardRooms": 1
}
}
]