Arrow Functions in Dart

What are Arrow Functions?

An arrow function (also called a lambda or fat-arrow function) is a concise syntax for writing single-expression functions. Instead of { return expression; }, you write => expression.

Syntax

// Full function
int add(int a, int b) {
  return a + b;
}

// Arrow function (equivalent)
int add(int a, int b) => a + b;

Examples

// Math functions
int square(int n) => n * n;
double circleArea(double r) => 3.14159 * r * r;
bool isAdult(int age) => age >= 18;
String greet(String name) => 'Hello, $name!';

void main() {
  print(square(5));                          // 25
  print(circleArea(3).toStringAsFixed(2));   // 28.27
  print(isAdult(20));                        // true
  print(greet('Dart'));                      // Hello, Dart!

  // Arrow function as variable
  var double_ = (int n) => n * 2;
  print(double_(7));                         // 14
}

25
28.27
true
Hello, Dart!
14

Arrow Functions as Arguments

Arrow functions are commonly passed as callbacks to higher-order functions:

List<int> numbers = [1, 2, 3, 4, 5, 6, 7, 8];

// Filter even numbers
var evens = numbers.where((n) => n % 2 == 0).toList();
print(evens);  // [2, 4, 6, 8]

// Map to squares
var squares = numbers.map((n) => n * n).toList();
print(squares); // [1, 4, 9, 16, 25, 36, 49, 64]

// Reduce to sum
var sum = numbers.reduce((a, b) => a + b);
print(sum);     // 36

⚠️ Common Mistakes

• Using arrow syntax for multi-statement functions — => only works for a single expression.
• Forgetting that arrow functions implicitly return the expression value.

🧠 Quiz

Summary

Arrow functions use => expression as a shorthand for single-expression functions. They implicitly return the expression value. They are widely used as callbacks in where(), map(), reduce(), and other higher-order functions.