Function Definition (Return Type / Args / return)

// Define a function as: returnType functionName(paramType paramName, ...) { body }
returnType functionName(paramType paramName) {
  // body
  return value;
}

// Use void when the function has no return value
void functionName(paramType paramName) {
  // body (no return needed)
}

// Use arrow syntax (=>) when the body is a single expression
returnType functionName(paramType paramName) => expression;

sg_function_basic.dart

// sg_function_basic.dart — basic examples of defining, calling, and returning values from functions
// Uses character data from Steins;Gate to demonstrate each concept

// -----------------------------------------------
// void function — no return value
// -----------------------------------------------

// Takes a character name and prints a greeting (no return value)
void greet(String name) {
  print('$name: El Psy Kongroo');
}

// -----------------------------------------------
// Function with a return value
// -----------------------------------------------

// Takes an experiment name and researcher name, and returns a report string
String makeReport(String experimentName, String researcher) {
  return '[$experimentName] Researcher: $researcher';
}

// -----------------------------------------------
// Arrow syntax — use when the body is a single expression
// -----------------------------------------------

// Returns the sum of two integers (arrow syntax)
int addLabMembers(int a, int b) => a + b;

// -----------------------------------------------
// Default parameters (named parameters)
// -----------------------------------------------

// Prints character info. divergence is optional and defaults to 1.000000
void printCharacter({
  required String name,
  required int labMemberNo,
  double divergence = 1.000000,
}) {
  print('Lab Member No.$labMemberNo  $name  (World line: $divergence)');
}

// -----------------------------------------------
// Default parameters (optional positional)
// -----------------------------------------------

// Takes a machine name and destination, and returns a launch message
// If destination is omitted, 'El Psy Kongroo' is used
String launch(String machineName, [String destination = 'El Psy Kongroo']) {
  return '$machineName → Departing to $destination';
}

// -----------------------------------------------
// main function — entry point of the program
// -----------------------------------------------

void main() {
  // Call a void function
  greet('Okabe Rintaro');
  greet('Makise Kurisu');

  print('');

  // Call functions that return a value
  var report1 = makeReport('Time Leap Machine Experiment', 'Hashida Itaru');
  var report2 = makeReport('Future Gadget No.8', 'Okabe Rintaro');
  print(report1);
  print(report2);

  print('');

  // Call a function using arrow syntax
  var total = addLabMembers(8, 3);
  print('Total lab members: $total');

  print('');

  // Call with named parameters (specify argument names explicitly)
  printCharacter(name: 'Okabe Rintaro', labMemberNo: 1);
  printCharacter(name: 'Makise Kurisu', labMemberNo: 4, divergence: 1.048596);
  printCharacter(name: 'Shiina Mayuri',  labMemberNo: 2, divergence: 0.571024);

  print('');

  // Call with optional positional parameters
  print(launch('C204'));                    // uses the default value
  print(launch('IBN 5100', '1975'));        // provides destination explicitly
}

dart run sg_function_basic.dart
Okabe Rintaro: El Psy Kongroo
Makise Kurisu: El Psy Kongroo

[Time Leap Machine Experiment] Researcher: Hashida Itaru
[Future Gadget No.8] Researcher: Okabe Rintaro

Total lab members: 11

Lab Member No.1  Okabe Rintaro  (World line: 1.0)
Lab Member No.4  Makise Kurisu  (World line: 1.048596)
Lab Member No.2  Shiina Mayuri  (World line: 0.571024)

C204 → Departing to El Psy Kongroo
IBN 5100 → Departing to 1975

Since Dart 3, you can use record types ((type1, type2)) to return multiple values from a function.

sg_function_record.dart

// sg_function_record.dart — example of returning multiple values using a record type
// Uses world line data from Steins;Gate to demonstrate the concept

// -----------------------------------------------
// Function that returns multiple values as a record
// -----------------------------------------------

// Takes a divergence number and returns the world line name and branch year as a pair
(String name, String branchYear) getWorldLineInfo(double divergence) {
  if (divergence >= 1.0) {
    return ('Beta world line', '1974');
  } else if (divergence >= 0.5) {
    return ('Alpha world line', '1975');
  } else {
    return ('Gamma world line', '1973');
  }
}

void main() {
  // Receive the record and destructure it into variables
  var (name, year) = getWorldLineInfo(1.048596);
  print('World line: $name  Branch year: $year');

  var (name2, year2) = getWorldLineInfo(0.571024);
  print('World line: $name2  Branch year: $year2');

  // You can also store the whole record in a variable and access fields individually
  var info = getWorldLineInfo(0.337187);
  print('World line: ${info.$1}  Branch year: ${info.$2}');
}

dart run sg_function_record.dart
World line: Beta world line  Branch year: 1974
World line: Alpha world line  Branch year: 1975
World line: Gamma world line  Branch year: 1973

Dart functions are defined as returnType functionName(params) { ... } and return a value with return. Use void when there is no return value, and the => arrow syntax when the body is a single expression. In addition to required positional parameters, you can use named parameters wrapped in {} and optional positional parameters wrapped in [] to improve readability at the call site. Since Dart 3, record types allow you to return multiple values from a single function. For variables and types, see Variables and Types (var / final / const). For how functions interact with null safety, see Null Safety.