Future<String> testFuture() {
  return Future.value('success');
}
main() {
  testFuture().then((s) {
    print(s);
  });
}

/// Register callbacks to be called when this future completes.
  ///
  /// When this future completes with a value,
  /// the [onValue] callback will be called with that value.
  /// If this future is already completed, the callback will not be called
  /// immediately, but will be scheduled in a later microtask.
  ///
  /// If [onError] is provided, and this future completes with an error,
  /// the `onError` callback is called with that error and its stack trace.
  /// The `onError` callback must accept either one argument or two arguments
  /// where the latter is a [StackTrace].
  /// If `onError` accepts two arguments,
  /// it is called with both the error and the stack trace,
  /// otherwise it is called with just the error object.
  /// The `onError` callback must return a value or future that can be used
  /// to complete the returned future, so it must be something assignable to
  /// `FutureOr<R>`.
  ///
  /// Returns a new [Future]
  /// which is completed with the result of the call to `onValue`
  /// (if this future completes with a value)
  /// or to `onError` (if this future completes with an error).
  ///
  /// If the invoked callback throws,
  /// the returned future is completed with the thrown error
  /// and a stack trace for the error.
  /// In the case of `onError`,
  /// if the exception thrown is `identical` to the error argument to `onError`,
  /// the throw is considered a rethrow,
  /// and the original stack trace is used instead.
  ///
  /// If the callback returns a [Future],
  /// the future returned by `then` will be completed with
  /// the same result as the future returned by the callback.
  ///
  /// If [onError] is not given, and this future completes with an error,
  /// the error is forwarded directly to the returned future.
  ///
  /// In most cases, it is more readable to use [catchError] separately,
  /// possibly with a `test` parameter,
  /// instead of handling both value and error in a single [then] call.
  ///
  /// Note that futures don't delay reporting of errors until listeners are
  /// added. If the first `then` or `catchError` call happens
  /// after this future has completed with an error,
  /// then the error is reported as unhandled error.
  /// See the description on [Future].
  Future<R> then<R>(FutureOr<R> onValue(T value), {Function? onError});

Future<String> testFuture() {
  return Future.value('success');
}
main() {
  testFuture().then((s) {
    print(s);
  }, onError: (e) {
    print('onError:');
    print(e);
  });
}

Future<String> testFuture() {
  return Future.value('success');
}
main() {
  testFuture().then((s) {
    print(s);
  }).catchError((e) {
    print('catchError:');
    print(e);
  });
}