What are extensions?

Learn the Dart extension concept.

What are extensions?

Extension lets you add new methods or properties to an existing type (class, record, list). You add functionality from outside without subclassing. The extension StringOps on String { ... } declaration adds new methods to String.

Extensions are especially useful for adding functionality to types you don't own (library classes, dart:core types). Extension methods are resolved statically; that is, the called method is determined at compile time.

Extension syntax

An extension is declared with extension Name on Type { ... }. Name is the extension's name, Type is the extended type. Methods, getters, and setters are declared inside the curly braces.

For example, extension NumberOps on int { int square() => this * this; } adds a square() method to int. Usage is 5.square(). The extension name avoids conflicts; it doesn't affect the code.

Exercise A StringEcho extension

Define a StringEcho extension on String with a String echo() => '$this $this'; method. In main add print('Hello'.echo());.

Show example solution

extension StringEcho on String {
  String echo() => '$this $this';
}

void main() {
  print('Hello'.echo());
}

Test yourself

What does an extension provide?
- Does runtime type checking
- Creates a new class
- Changes the garbage collector
- Adds new methods to an existing type
Show answer

Adds new methods to an existing type — an extension adds new methods to an existing type from outside.

An extension declaration

extension NumberOps on ____ {
  int square() => this * this;
}

Show answer

extension NumberOps on int {
  int square() => this * this;
}

Match the extension parts

extension

Declaration keyword
on

Target type binder
Name

Extension name
Type

Extended type

Next lesson: Extension method

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