Functions

A function is a named, reusable block of code that performs a task.

Real-world analogy: a function is a recipe. You write the steps once, then just say "make pancakes" whenever you want them.

Syntax

ReturnType name(Type parameter) {
  // code
  return value;
}

Example

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

void main() {
  print(greet('Ada'));
  print(greet('Linus'));
}

Expected output

Hello, Ada!
Hello, Linus!

• The return type (String) comes first; void means no value is returned.
• Each parameter has a type and a name.
• return sends the result back to the caller.

Arrow functions

A function that just returns one expression can use =>:

int square(int n) => n * n;
bool isAdult(int age) => age >= 18;

void main() {
  print(square(7));
  print(isAdult(16));
}

49
false

Functions without a return value

void printBanner(String text) {
  print('*** $text ***');
}

void main() {
  printBanner('Sale');
}

*** Sale ***

Functions can be declared before or after main in the file.

Common mistakes

• Declaring a return type but forgetting return on some path — Dart reports that the function might complete without returning a value.
• Passing the wrong type: square('7') doesn't compile.

Practice

Write double area(double width, double height) and bool isSquare(double width, double height). Print both for a 4 by 4 rectangle.

double area(double width, double height) => width * height;

bool isSquare(double width, double height) => width == height;

void main() {
  print(area(4, 4));
  print(isSquare(4, 4));
}

16.0
true