Dart: Functions

What you will learn

How to define functions, use arrow syntax for short bodies, and work with optional parameters.

Basic function

String greet(String name) {
  return 'Hello, $name!';
}

void main() {
  print(greet('Alice'));
}

Syntax: return type, function name, parameters with types, body in { }. void means no return value.

Arrow functions (single expression)

int add(int a, int b) => a + b;
String greet(String name) => 'Hello, $name!';

=> expr is shorthand for { return expr; }. Use it when the function body is a single expression.

Optional positional parameters

Wrap in [ ] to make parameters optional:

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

greet('Alice');           // Hello, Alice!
greet('Alice', 'Hi');     // Hi, Alice!

Named parameters

Wrap in { } for named (keyword) parameters:

void configure({String? host, int port = 8080}) {
  print('$host:$port');
}

configure(host: 'localhost', port: 3000);
configure(port: 4000);

Named parameters are optional by default. Use required to make them mandatory:

void register({required String username, required String email}) { }  // both required

Common mistakes

• Forgetting required on a named parameter that must always be provided
• Using => for multi-statement functions — => only works for single expressions
• Confusing [ ] (optional positional) with { } (named parameters)
• Forgetting that void functions cannot be used in expressions

Quick check below!