Dart Extension Methods

last modified January 15, 2025

In this article, we show how to use extension methods in Dart language. Extension methods allow you to add functionality to existing classes without modifying their source code or creating a subclass.

Extension methods are particularly useful when you want to add utility methods to classes from external libraries or built-in types like String, int, or List.

Dart Extension Methods Simple Example

The following example demonstrates how to create and use a simple extension method in Dart.

main.dart

extension StringExtensions on String { bool get isPalindrome { return this == this.split(’’).reversed.join(’’); } }

void main() { final word = ‘racecar’; print(‘Is “$word” a palindrome? ${word.isPalindrome}’); }

In this program, we define an extension method isPalindrome for the String class. The method checks if a string reads the same backward as forward.

extension StringExtensions on String { bool get isPalindrome { return this == this.split(’’).reversed.join(’’); } }

We define an extension named StringExtensions that adds a isPalindrome getter to the String class.

void main() { final word = ‘racecar’; print(‘Is “$word” a palindrome? ${word.isPalindrome}’); }

We use the isPalindrome extension method to check if the string ‘racecar’ is a palindrome.

$ dart main.dart Is “racecar” a palindrome? true

Dart Extension Methods with Parameters

Extension methods can also accept parameters, allowing you to add more flexible functionality.

main.dart

extension NumberExtensions on int { int power(int exponent) { return this ^ exponent; } }

void main() { final number = 2; print(’$number raised to the power of 3 is ${number.power(3)}’); }

In this program, we define an extension method power for the int class. The method calculates the power of a number.

extension NumberExtensions on int { int power(int exponent) { return this ^ exponent; } }

We define an extension named NumberExtensions that adds a power method to the int class.

void main() { final number = 2; print(’$number raised to the power of 3 is ${number.power(3)}’); }

We use the power extension method to calculate 2 raised to the power of 3.

$ dart main.dart 2 raised to the power of 3 is 8

Dart Extension Methods with Generic Types

Extension methods can also be defined for generic types, making them reusable across different data types.

main.dart

extension ListExtensions<T> on List<T> { List<T> duplicate() { return […this, …this]; } }

void main() { final numbers = [1, 2, 3]; print(‘Duplicated list: ${numbers.duplicate()}’); }

In this program, we define an extension method duplicate for the List class. The method duplicates the elements of the list.

extension ListExtensions<T> on List<T> { List<T> duplicate() { return […this, …this]; } }

We define an extension named ListExtensions that adds a duplicate method to the List class. The method works for any type T.

void main() { final numbers = [1, 2, 3]; print(‘Duplicated list: ${numbers.duplicate()}’); }

We use the duplicate extension method to duplicate the elements of a list of integers.

$ dart main.dart Duplicated list: [1, 2, 3, 1, 2, 3]

Dart Extension Methods with Static Helpers

Extension methods can also include static helper methods, which are useful for utility functions.

main.dart

extension StringHelpers on String { static String capitalize(String input) { return input[0].toUpperCase() + input.substring(1); } }

void main() { final name = ‘alice’; print(‘Capitalized name: ${StringHelpers.capitalize(name)}’); }

In this program, we define a static helper method capitalize within an extension for the String class. The method capitalizes the first letter of a string.

extension StringHelpers on String { static String capitalize(String input) { return input[0].toUpperCase() + input.substring(1); } }

We define an extension named StringHelpers that includes a static capitalize method.

void main() { final name = ‘alice’; print(‘Capitalized name: ${StringHelpers.capitalize(name)}’); }

We use the static capitalize method to capitalize the first letter of the string ‘alice’.

$ dart main.dart Capitalized name: Alice

Source

Dart Extension Methods - Language Documentation

In this article, we have covered the basics of using extension methods in Dart. Extension methods are a powerful feature for adding functionality to existing classes without modifying their source code.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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