Functions and Parameters

Flexible, readable function signatures

Dart functions have a return type (inferred for local functions), a name and parameters. Parameters come in three kinds. Required positional parameters come first: double area(double w, double h). Optional positional parameters go in square brackets with defaults: String greet(String name, [String greeting = 'Hello']). Named parameters go in braces, are optional unless marked required, and are passed by name in any order: void connect({required String host, int port = 443}), called as connect(host: 'api.example.com'). Named parameters dominate Flutter APIs because they make calls with many options readable. Single-expression functions can use arrow syntax (int square(int x) => x * x;). Functions are first-class objects: they can be assigned to variables, passed as arguments and returned. Top-level functions, static methods and instance methods can all be torn off into function values. Use void for functions that return nothing and Never for functions that always throw.

Kinds of parameters

Required positional parameters first, then either optional positional or named parameters.

Parameter styles in practice

Named parameters with required and defaults read clearly at the call site.

double lineTotal(double unitPrice, int quantity, [double discount = 0]) =>
    unitPrice * quantity * (1 - discount);

String formatPrice(num amount, {String currency = 'INR', int decimals = 2}) =>
    '${amount.toStringAsFixed(decimals)} $currency';

void sendInvoice({required String to, required double amount, bool urgent = false}) {
  print('Sending ${formatPrice(amount)} invoice to $to${urgent ? ' (urgent)' : ''}');
}

void main() {
  print(lineTotal(49.5, 3));                 // optional positional defaults to 0
  print(lineTotal(49.5, 3, 0.1));
  print(formatPrice(1499.5, decimals: 0));
  sendInvoice(amount: 1499.5, to: 'asha@example.com');   // any order
  // sendInvoice(to: 'x');                  // compile error: amount is required
}

Prefer named parameters for booleans

createUser('Asha', true, false) is unreadable. createUser('Asha', isAdmin: true, sendWelcome: false) explains itself and cannot mix up the two flags.

त्वरित जाँच: How do you make a named parameter mandatory in Dart?

• Mark it with the required keyword
• Put it in square brackets
• Give it a default value
• Make it final