Dart Function Parameters

Parameter Types

void greet(String name) {
  print('Hello $name');
}

void greet(String name, [int? age]) {
  print('Hello $name');
}

void greet({String? name, int? age}) {
  print('Hello $name');
}

void greet([String name = 'Guest']) {
  print('Hello $name');
}

🔹 Required Parameters

Basic parameters that must be provided:

void main() {
  // Function with required parameters
  void calculateArea(double width, double height) {
    double area = width * height;
    print('Area: $area');
  }
  
  void displayPersonInfo(String name, int age, String city) {
    print('Name: $name, Age: $age, City: $city');
  }
  
  // Must provide all required parameters
  calculateArea(10.0, 5.0);
  displayPersonInfo('Alice', 25, 'New York');
  
  // This would cause an error:
  // calculateArea(10.0); // Missing height parameter
}

🔹 Optional Positional Parameters

Parameters enclosed in square brackets [] are optional:

void main() {
  // Function with optional parameters
  void greetPerson(String name, [String? title, int? age]) {
    String greeting = 'Hello';
    if (title != null) greeting += ' $title';
    greeting += ' $name';
    if (age != null) greeting += ', age $age';
    print(greeting);
  }
  
  // Different ways to call the function
  greetPerson('John');                    // Only required parameter
  greetPerson('Jane', 'Dr.');             // With title
  greetPerson('Bob', 'Mr.', 30);          // With title and age
  greetPerson('Alice', null, 25);         // Skip title, provide age
}

🔹 Named Parameters

Parameters enclosed in curly braces {} can be called by name:

void main() {
  // Function with named parameters
  void createUser({String? name, int? age, String? email, bool isActive = true}) {
    print('Creating user:');
    print('  Name: ${name ?? 'Unknown'}');
    print('  Age: ${age ?? 'Not specified'}');
    print('  Email: ${email ?? 'Not provided'}');
    print('  Active: $isActive');
    print('---');
  }
  
  // Call with named parameters (order doesn't matter)
  createUser(name: 'Alice', age: 25);
  createUser(email: '[email protected]', name: 'Bob', isActive: false);
  createUser(age: 30);
  createUser(); // All parameters are optional
}

🔹 Required Named Parameters

Use the 'required' keyword to make named parameters mandatory:

void main() {
  // Function with required named parameters
  void bookFlight({
    required String from,
    required String to,
    required DateTime date,
    String? passengerName,
    int passengers = 1
  }) {
    print('Flight Booking:');
    print('  From: $from');
    print('  To: $to');
    print('  Date: ${date.toString().split(' ')[0]}');
    print('  Passengers: $passengers');
    if (passengerName != null) {
      print('  Passenger: $passengerName');
    }
  }
  
  // Must provide required named parameters
  bookFlight(
    from: 'New York',
    to: 'London',
    date: DateTime(2024, 6, 15),
    passengerName: 'John Doe'
  );
  
  // This would cause an error:
  // bookFlight(from: 'Paris'); // Missing required parameters
}

🔹 Default Parameter Values

Provide default values for optional parameters:

void main() {
  // Function with default values
  void printMessage(String message, [String prefix = 'INFO', bool timestamp = false]) {
    String output = '[$prefix] $message';
    if (timestamp) {
      output = '${DateTime.now().toString().split('.')[0]} $output';
    }
    print(output);
  }
  
  // Named parameters with defaults
  void configureApp({
    String theme = 'light',
    bool notifications = true,
    int maxUsers = 100,
    String language = 'en'
  }) {
    print('App Configuration:');
    print('  Theme: $theme');
    print('  Notifications: $notifications');
    print('  Max Users: $maxUsers');
    print('  Language: $language');
  }
  
  // Using default values
  printMessage('Application started');
  printMessage('Error occurred', 'ERROR');
  printMessage('Debug info', 'DEBUG', true);
  
  configureApp();
  configureApp(theme: 'dark', maxUsers: 500);
}

🔹 Parameter Best Practices