Dart Lesson 21 of 102 3 min read

Function Parameters in Dart: Positional, Named, Optional and Default

Master Dart function parameters: required positional, optional positional, named parameters, the required keyword and default values.

Dart gives you several ways to pass values into a function. Choosing well makes your functions easier to call correctly.

Required positional parameters #

The plain kind. Arguments are matched by position, and every one must be supplied.

String fullName(String first, String last) {
  return '$first $last';
}

void main() {
  print(fullName('Ada', 'Lovelace'));
}

Ada Lovelace

Optional positional parameters #

Wrap parameters in [ ] to make them optional. Without a default they are null when left out, so their type needs a ?.

String greet(String name, [String? title]) {
  if (title == null) return 'Hello, $name';
  return 'Hello, $title $name';
}

void main() {
  print(greet('Sharma'));
  print(greet('Sharma', 'Dr.'));
}

Hello, Sharma
Hello, Dr. Sharma

Default values #

Give an optional parameter a default with =. Then it is never null.

double priceWithTax(double price, [double rate = 0.13]) {
  return price * (1 + rate);
}

void main() {
  print(priceWithTax(100));
  print(priceWithTax(100, 0.05));
}

112.99999999999999
105.0

(The first result shows floating-point rounding. Use toStringAsFixed(2) when displaying money.)

Named parameters #

Wrap parameters in { } and the caller must write the name. The order no longer matters, and the call explains itself.

void createUser({required String name, int age = 18, String? city}) {
  print('$name, $age, ${city ?? 'unknown city'}');
}

void main() {
  createUser(name: 'Ravi');
  createUser(age: 30, name: 'Mina', city: 'Pokhara');
}

Ravi, 18, unknown city
Mina, 30, Pokhara

Each named parameter must be one of three things:

• marked required, so the caller has to pass it,
• given a default value, or
• nullable (String?), in which case it is null when omitted.

Mixing positional and named #

Positional parameters come first, then the named ones.

void sendMessage(String text, {required String to, bool urgent = false}) {
  print('${urgent ? '[URGENT] ' : ''}To $to: $text');
}

void main() {
  sendMessage('Lunch?', to: 'Sam');
  sendMessage('Server is down', to: 'Ops', urgent: true);
}

To Sam: Lunch?
[URGENT] To Ops: Server is down

You cannot use [ ] and { } in the same function.

Which kind should I use? #

Compare resize(800, 600, true, false) with resize(width: 800, height: 600, keepRatio: true, animate: false). Named parameters are the reason Flutter code is readable.

Try it yourself #

Write String formatPrice(double amount, {String currency = 'Rs.', int decimals = 2}). Calling formatPrice(1499.5) should return Rs. 1499.50, and formatPrice(20, currency: r'$', decimals: 0) should return $ 20.