Named and Optional Parameters

Dart functions support named and optional parameters — you'll see them in almost every Flutter widget.

Named parameters

Named parameters go inside { } and are passed by name, in any order:

String createUser({required String name, String role = 'member', bool active = true}) {
  return '$name ($role, active: $active)';
}

void main() {
  print(createUser(name: 'Ada'));
  print(createUser(name: 'Linus', active: false));
  print(createUser(role: 'admin', name: 'Grace'));
}

Ada (member, active: true)
Linus (member, active: false)
Grace (admin, active: true)

• Named parameters are optional unless marked required.
• An optional named parameter needs a default value, or a nullable type.
• Names make calls self-explanatory: compare createUser('Linus', 'member', false) — what does false mean?

Optional positional parameters

Parameters in [ ] are positional but optional:

String greet(String name, [String greeting = 'Hello']) => '$greeting, $name!';

void main() {
  print(greet('Ada'));
  print(greet('Ada', 'Welcome'));
}

Hello, Ada!
Welcome, Ada!

A function can have positional parameters followed by either optional positional or named parameters — not both.

Named parameters in Flutter

This is how Flutter widgets read:

Text(
  'Hello',
  style: TextStyle(fontSize: 24),
  textAlign: TextAlign.center,
)

Common mistakes

• Forgetting a required argument: Error: Required named parameter 'name' must be provided.
• Giving an optional named parameter a non-nullable type with no default — Dart rejects it, since it could be missing.

Practice

Write String formatPrice(double amount, {String currency = 'USD'}) returning '12.5 USD'. Call it once with only an amount and once with currency: 'EUR'.

String formatPrice(double amount, {String currency = 'USD'}) => '$amount $currency';

void main() {
  print(formatPrice(12.5));
  print(formatPrice(12.5, currency: 'EUR'));
}

12.5 USD
12.5 EUR