Group an array of objects based on multi keys in Javascript?

[midojeg753]

I would like to share with you an existence problem

I have array as follows

const data = [
 {
   "countries": ["France", "USA", "Canada"],
   "city": "Paris",
   "stadium": "Parce de france"
 },
 {
   "countries": ["France", "Itlay", "Spain"],
   "city": "Roma",
   "arena": "La cigal"
 },
  {
   "countries": ["France"],
   "city": "Roma",
   "stadium": "Olymico"
 },
 {
   "countries": ["France"],
   "city": "Paris",
   "arena": "Velodrome"
 },
]

I want to group it first by city then by stadium and arena if exist How can I achieve this

output like :

{
   "city": "Paris",
   "arena": [
     {
       "value":"Velodrome",
       "countries": ["France"]
     }
   ],
   "stadium": [
     {
       "value":"Parce de france",
       "countries": ["France", "USA", "Canada"]}
   ]
 }
 {
   "city": "Roma",
   "arena": [
     {
       "value":"La cigal",
       "countries": ["France", "Itlay", "Spain"]
     }
   ],
   "stadium": [
     {
       "value":"Olymico",
       "countries": ["France"]
     }
   ]
 }
]

I was able to group by city with reduce

let group = function(array, key) {
  return array.reduce(function(result, item) {
    (result[item[key]] = result[item[key]] || []).push(item);
    return result;
  }, {});
};

How can I group the data first by city and then by stadium, including any arenas if they exist?

 

[lepaj24341]

The required output format is a little strange, but you can indeed do it with a single reduce() statement:

const data = [{
  "countries": ["France", "USA", "Canada"],
  "city": "Paris",
  "stadium": "Parce de france"
}, {
  "countries": ["France", "Itlay", "Spain"],
  "city": "Roma",
  "arena": "La cigal"
}, {
  "countries": ["France"],
  "city": "Roma",
  "stadium": "Olymico"
}, {
  "countries": ["France"],
  "city": "Paris",
  "arena": "Velodrome"
}];

const result = Object.values(
  data.reduce((a, {city, arena, stadium, countries}) => {
    a[city] ??= { city, arena: [], stadium: [] };
    if (arena) a[city].arena.push({ value: arena, countries });
    if (stadium) a[city].stadium.push({ value: stadium, countries });
    return a;
  }, {})
);

console.log(result);