Optional Parameters in Dart

Optional Positional Parameters

Wrap parameters in square brackets [ ] to make them optional. Optional positional parameters must come after required ones:

String greet(String name, [String greeting = 'Hello']) {
  return '$greeting, $name!';
}

void main() {
  print(greet('Alice'));            // Hello, Alice!
  print(greet('Bob', 'Welcome'));   // Welcome, Bob!
}

Optional Named Parameters

Wrap parameters in curly braces { } to make them named. Named parameters can be passed in any order:

void createUser({String name = 'Guest', int age = 0, String role = 'user'}) {
  print('Name: $name | Age: $age | Role: $role');
}

void main() {
  createUser();
  createUser(name: 'Alice', age: 25);
  createUser(role: 'admin', name: 'Bob', age: 30);
}

Name: Guest | Age: 0 | Role: user
Name: Alice | Age: 25 | Role: user
Name: Bob | Age: 30 | Role: admin

Default Values

Both positional and named optional parameters support default values:

double power(double base, [int exponent = 2]) {
  double result = 1;
  for (int i = 0; i < exponent; i++) result *= base;
  return result;
}

print(power(3));     // 9.0  (3^2)
print(power(3, 3));  // 27.0 (3^3)

Example

void main() {
  // Build a formatted message
  print(buildMessage('Error'));
  print(buildMessage('Success', emoji: '✅'));
  print(buildMessage('Warning', emoji: '⚠️', repeat: 3));
}

String buildMessage(String msg, {String emoji = '❌', int repeat = 1}) {
  return ('$emoji $msg ' * repeat).trim();
}

❌ Error
✅ Success
⚠️ Warning ⚠️ Warning ⚠️ Warning

🧠 Quiz

Summary

Optional positional parameters use [ ] and must follow required parameters. Optional named parameters use { } and can be passed in any order. Both support default values. Use named parameters for clarity when a function has many optional arguments.