Get new List with id and number of times object repeats in a List with dart.

main.dart

String checkCountOf = 'id';

List objects = [
  {"id": "123"},
  {"id": "123"},
  {"id": "20"},
  {"id": "35"},
  {"id": "20"},
];

void main() {
  var output = [];
  for (final obj in objects) {
    var index = output.indexWhere((e) {
      return e[checkCountOf] == obj[checkCountOf];
    });
    if (index >= 0) {
      output[index]['count']++;
    } else {
      output.add({
        checkCountOf: obj[checkCountOf],
        "count": 1,
      });
    }
  }

  print(output);
  // [{id: 123, count: 2}, {id: 20, count: 2}, {id: 35, count: 1}]
}