JavaScript group a JSON object by two properties and count

Suppose, we have an array of objects like this −

const arr = [
   {"location":"Kirrawee","identity_long":"student"},
   {"location":"Kirrawee","identity_long":"visitor"},
   {"location":"Kirrawee","identity_long":"visitor"},
   {"location":"Kirrawee","identity_long":"worker"},
   {"location":"Sutherland","identity_long":"student"},
   {"location":"Sutherland","identity_long":"resident"},
   {"location":"Sutherland","identity_long":"worker"},
   {"location":"Sutherland","identity_long":"resident"},
   {"location":"Miranda","identity_long":"resident"},
   {"location":"Miranda","identity_long":"worker"},
   {"location":"Miranda","identity_long":"student"},
   {"location":"Miranda","identity_long":""},
   {"location":"Miranda","identity_long":"worker"},
   {"location":"Miranda","identity_long":"resident"}
];

We are required to write a JavaScript function that takes in one such array of objects. The function should prepare a new array of objects in which all (identical) the objects are grouped together based on the location property.

And the objects should be assigned a count property that contains the number of times it appeared in the original array of objects.

Therefore, for the above array, the output should look like −

const output = [
   {"location":"Kirrawee","identity":"student","count":1},
   {"location":"Kirrawee","identity":"visitor","count":2},
   {"location":"Kirrawee","identity":"worker","count":1},
   {"location":"Sutherland","identity":"student","count":1},
   {"location":"Sutherland","identity":"resident","count":2},
   {"location":"Sutherland","identity":"worker","count":1},
   {"location":"Miranda","identity":"resident","count":2},
   {"location":"Miranda","identity":"worker","count":2},
   {"location":"Miranda","identity":"student","count":1}
];

Example

The code for this will be −

const arr = [
   {"location":"Kirrawee","identity_long":"student"},
   {"location":"Kirrawee","identity_long":"visitor"},
   {"location":"Kirrawee","identity_long":"visitor"},
   {"location":"Kirrawee","identity_long":"worker"},
   {"location":"Sutherland","identity_long":"student"},
   {"location":"Sutherland","identity_long":"resident"},
   {"location":"Sutherland","identity_long":"worker"},
   {"location":"Sutherland","identity_long":"resident"},
   {"location":"Miranda","identity_long":"resident"},
   {"location":"Miranda","identity_long":"worker"},
   {"location":"Miranda","identity_long":"student"},
   {"location":"Miranda","identity_long":""},
   {"location":"Miranda","identity_long":"worker"},
   {"location":"Miranda","identity_long":"resident"}
];
const groupArray = (arr = []) => {
   // create map
   let map = new Map()
   for (let i = 0; i < arr.length; i++) {
      const s = JSON.stringify(arr[i]);
      if (!map.has(s)) {
         map.set(s, {
            location: arr[i].location,
            identity: arr[i].identity_long,
            count: 1,
         });
      } else {
         map.get(s).count++;
      }
   }
   const res = Array.from(map.values())
   return res;
};
console.log(groupArray(arr));

Output

And the output in the console will be −

[
   { location: 'Kirrawee', identity: 'student', count: 1 },
   { location: 'Kirrawee', identity: 'visitor', count: 2 },
   { location: 'Kirrawee', identity: 'worker', count: 1 },
   { location: 'Sutherland', identity: 'student', count: 1 },
   { location: 'Sutherland', identity: 'resident', count: 2 },
   { location: 'Sutherland', identity: 'worker', count: 1 },
   { location: 'Miranda', identity: 'resident', count: 2 },
   { location: 'Miranda', identity: 'worker', count: 2 },
   { location: 'Miranda', identity: 'student', count: 1 },
   { location: 'Miranda', identity: '', count: 1 }
]