How to use .firstWhereOrNull with Map?

Question

I know someone already asked the same question. But i still confuse about something. I followed the provided answer and use this:

extension ExtendedMap on Map {
  /// The first entry satisfying test, or null if there are none.
  MapEntry? firstWhereOrNull(bool Function(MapEntry entry) test) {
    for (var entry in this.entries) {
      if (test(entry)) return entry;
    }
    return null;
  }
}

And i try:

someMap?.firstWhereOrNull((date) => (date > recentSunday )&& (date <= nextSunday)

But i got this error:

The operator '>' isn't defined for the type 'MapEntry<dynamic, dynamic>'.Try defining the operator '>'.

The operator '<=' isn't defined for the type 'MapEntry<dynamic, dynamic>'.Try defining the operator '<='.

I'm confused why this happened. I thought it should work if any boolean function is passed as argument.Can anyone explain why it is asked to define the operator?

Answer 1

The callback has type bool Function(MapEntry entry), so date is of type MapEntry. If you want to access the value associated with this entry, use date.value:

final result = someMap?.firstWhereOrNull((date) => (date.value > recentSunday )&& (date.value <= nextSunday));
                                                        ^^^^^^                         ^^^^^^

Complete example:

final someMap = {
  'halloween': 1,
  'christmas': 12,
  'easter': 123,
};

final recentSunday = 10;
final nextSunday = 30;

final result = someMap?.firstWhereOrNull((date) => (date.value > recentSunday )&& (date.value <= nextSunday));

print(result);

Console output:

MapEntry(christmas: 12)

Note that the returned value is also a MapEntry.