language-dartlisted

# Dart ## Null safety - Sound null safety is on by default (Dart ≥2.12) — no unsound nulls sneak through. - Non-nullable is the default; append `?` only when a value can genuinely be absent (`String? name`). - `late` defers initialization for values assigned before first read (DI fields, `initState` fields) — a `late` field read before assignment throws. - `!` (null-assertion) asserts non-null; prefer narrowing (`if (x != null)`) or `?.`/`??` over `!`, since `!` throws at runtime. - `??=` for lazy default assignment; `??` for fallback expressions. ```dart String greet(String? name) => 'Hello, ${name ?? 'stranger'}'; ``` ## Async — Futures and Streams - `Future<T>` wraps a value that arrives later; `async`/`await` reads like sync code. - Unhandled Future errors surface as unhandled exceptions — always `try`/`catch` around `await`, or attach `.catchError`. - `Stream<T>` models a sequence over time; `await for` consumes it, `StreamController` produces it. - `Completer<T>` bridges callback-based APIs into a `Future`. - `Future.wait([...])` for concurrent futures; don't `await` sequentially in a loop when the work is independent. ```dart Future<User> fetchUser(String id) async { try { final res = await api.get('/users/$id'); return User.fromJson(res.data); } on DioException catch (e) { throw UserFetchException(id, e); } } ``` ## Widget composition - `StatelessWidget` is the default; reach for `StatefulWidget` only when the widget owns mutable state across re