What are extensions?
- Dart course
- Expert
- Lesson 19/47
- 5 min
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.
Practice A StringEcho extension
Define a StringEcho extension on String with a String echo() => '$this $this'; method. In main add print('Hello'.echo());.
Starter code
void main() {
// define a StringEcho extension (on String; echo method)
// print 'Hello'.echo()
}Sample solution
extension StringEcho on String {
String echo() => '$this $this';
}
void main() {
print('Hello'.echo());
}Expected output
Hello Hello
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does an extension provide?
- Does runtime type checking
- Creates a new class
- Changes the garbage collector
- Adds new methods to an existing type Correct answer
Explanation: an extension adds new methods to an existing type from outside.
2 An extension declaration
extension NumberOps on ____ {
int square() => this * this;
}extension NumberOps on int {
int square() => this * this;
}Explanation: extension NumberOps on int applies to the int type.
3 Match the extension parts
| Term | Meaning |
|---|---|
extension |
Declaration keyword |
on |
Target type binder |
Name |
Extension name |
Type |
Extended type |