Extension Methods in Dart

What are Extension Methods?

Extension methods let you add new functionality to existing classes — including built-in types like String, int, List — without modifying or subclassing them. They are purely syntactic sugar: the compiler converts the call to a static function call.

Syntax

extension ExtensionName on ExistingType {
  ReturnType newMethod() {
    // 'this' refers to the instance
    return ...;
  }
}

Example

extension StringExtension on String {
  String capitalize() {
    if (isEmpty) return this;
    return this[0].toUpperCase() + substring(1).toLowerCase();
  }

  bool get isValidEmail => contains('@') && contains('.');

  String repeat(int times) => this * times;
}

void main() {
  print('hello'.capitalize());           // Hello
  print('WORLD'.capitalize());           // World
  print('user@example.com'.isValidEmail); // true
  print('ha'.repeat(3));                 // hahaha
}

Hello
World
true
hahaha

Extending Built-in Types

extension IntExtension on int {
  bool get isEven => this % 2 == 0;
  bool get isOdd => this % 2 != 0;
  int get squared => this * this;
  List<int> to(int end) => List.generate(end - this + 1, (i) => this + i);
}

extension ListExtension<T> on List<T> {
  T? get secondOrNull => length >= 2 ? this[1] : null;
}

void main() {
  print(7.isOdd);         // true
  print(4.squared);       // 16
  print(1.to(5));         // [1, 2, 3, 4, 5]

  var items = ['a', 'b', 'c'];
  print(items.secondOrNull);  // b
}

🧠 Quiz

Summary

Extension methods add new methods to existing types using the extension on syntax. Use this to access the instance. They work on built-in types, third-party types, and your own classes. Extensions make code more expressive without modifying the original class.