Working with Nullable Values

Dart gives you several tools for nullable values.

Null-aware access: ?.

void main() {
  String? nickname;
  String? city = 'Oslo';

  print(nickname?.length);
  print(city?.length);
  print(city?.toUpperCase());
}

null
4
OSLO

?. returns null instead of failing when the value is null.

Default values: ?? and ??=

void main() {
  String? saved;
  final name = saved ?? 'Guest';
  print(name);

  final quantity = int.tryParse('abc') ?? 1;
  print(quantity);

  String? theme;
  theme ??= 'light';
  print(theme);
}

Guest
1
light

• a ?? b — a if it isn't null, otherwise b.
• x ??= value — assigns only if x is currently null.

Type promotion

After you check that a local variable isn't null, Dart treats it as non-nullable automatically:

void describe(String? input) {
  if (input == null) {
    print('Nothing entered');
    return;
  }
  print('You typed ${input.length} characters');
}

void main() {
  describe('hello');
  describe(null);
}

You typed 5 characters
Nothing entered

This is called type promotion. The early return for the missing case — a guard clause — keeps the main code simple.

The ! operator (use sparingly)

! asserts that a value isn't null. If it is, the program throws:

void main() {
  String? value;
  print(value!.length);
}

Unhandled exception:
Null check operator used on a null value

Treat ! as a warning sign — there's almost always a safer option.

late

late declares a non-nullable variable that will be assigned before it's used, but not right away:

late final String description;

void main() {
  description = 'Set once, later';
  print(description);
}

Set once, later

Reading a late variable before it's assigned throws a LateInitializationError.

Practice

Write int parseAge(String text) that returns the number in text, or 0 if it isn't valid. Call it with '36' and 'abc'.

int parseAge(String text) => int.tryParse(text) ?? 0;

void main() {
  print(parseAge('36'));
  print(parseAge('abc'));
}

36
0