Dart Lesson 22 of 102 2 min read

Arrow Functions in Dart (=> Syntax)

Write short functions in Dart with the arrow syntax. Learn what => means, where it can be used and where a block body is needed.

When a function body is a single expression, Dart lets you replace the braces and return with =>.

Block body and arrow body #

These two functions are identical:

int doubleIt(int n) {
  return n * 2;
}

int doubleItShort(int n) => n * 2;

void main() {
  print(doubleIt(21));
  print(doubleItShort(21));
}

42
42

Read => expression as “returns this expression”.

More examples #

bool isAdult(int age) => age >= 18;

String initials(String first, String last) => '${first[0]}${last[0]}';

double circleArea(double r) => 3.14159 * r * r;

void log(String message) => print('[LOG] $message');

void main() {
  print(isAdult(20));
  print(initials('Grace', 'Hopper'));
  print(circleArea(2));
  log('Started');
}

true
GH
12.56636
[LOG] Started

log returns nothing, but => is still allowed because print(...) is a single expression.

One expression only #

An arrow body cannot hold statements. No if statement, no for loop, no variable declarations. You can use anything that produces a value, such as a ternary or a switch expression.

String sign(int n) => n > 0 ? 'positive' : n < 0 ? 'negative' : 'zero';

String size(int cm) => switch (cm) {
      < 150 => 'short',
      < 180 => 'medium',
      _ => 'tall',
    };

void main() {
  print(sign(-4));
  print(size(172));
}

negative
medium

Arrows with collections #

Arrow syntax is most common in small functions passed to other functions.

void main() {
  var prices = [120, 80, 300];

  var withTax = prices.map((p) => p * 1.13).toList();
  var cheap = prices.where((p) => p < 150).toList();

  print(withTax.map((p) => p.toStringAsFixed(1)).toList());
  print(cheap);
}

[135.6, 90.4, 339.0]
[120, 80]

Arrows in classes #

Getters and short methods are usually written this way.

class Rectangle {
  final double width, height;
  Rectangle(this.width, this.height);

  double get area => width * height;
  bool get isSquare => width == height;
}

void main() {
  var r = Rectangle(4, 4);
  print('${r.area} ${r.isSquare}');
}

16.0 true

When not to use an arrow #

If the expression runs past one line or needs a comment to be understood, switch to a block body. Shorter is only better while it is still easy to read.

Try it yourself #

Rewrite these as arrow functions: a function that returns the larger of two numbers, one that checks whether a string is longer than 8 characters, and one that returns the last element of a list.