main.dart

import 'dart:convert';

void main() {
  // Json obtained through some api:
  const jsonString = '[{"page":1, "items": [1, 2]}, {"page":2, "items": [3, 4]}]';
  final items = json.decode(jsonString);
  final x = items.expand((p) => p['items']);
  //final x = items.expand((p) => p['items'] as List<int>);
  //final x = items.expand((p) => p['items'].map((i) => i as int));
  //final x = items.map((p) => p['items'] as List).toList().expand((i) => i).toList();
  print(x);
  
  // When the list is not dynamic, it works:
  const foo = [{"page":1, "items": [1, 2]}, {"page":2, "items": [3, 4]}];
  final y = foo.expand((p) => p['items']).toList();
  //final y = foo.map((p) => p['items']).toList().expand((i) => i).toList();
  print(y); // => [1, 2, 3, 4];

  // example from dart's website:
  const pairs = [[1, 2], [3, 4]];
  final flattened = pairs.expand((pair) => pair).toList();
  print(flattened); // => [1, 2, 3, 4];
}