extends and super
Learn to subclass with extends and call the parent with super.
Subclassing with extends
To derive a class from another class you use the extends keyword. The derived class is a subclass; the one it derives from is the parent.
class Animal {
constructor(name) { this.name = name; }
}
class Dog extends Animal {}Dog inherits all members of Animal and can add its own.
Calling super
In a subclass constructor you must call the parent's constructor with super(...) before using this. This lets the parent do its setup first.
class Dog extends Animal {
constructor(name, breed) {
super(name);
this.breed = breed;
}
}Here super(name) runs the parent's constructor; then this.breed can be added.
Exercise Dog subclass
The Animal class below is given. Declare a subclass Dog with extends. Its constructor takes a name parameter, calls super(name), and assigns "Woof" to this.sound.
Show example solution
Inside the constructor first call super(name), then this.sound = "Woof".
Example solution:
class Dog extends Animal {
constructor(name) {
super(name);
this.sound = "Woof";
}
}Test yourself
-
What must be called before `this` in a subclass constructor?
constructor() { /* ... */ }- this()
- new Animal()
- super()
- extends
Show answer
super() — Correct! super() runs the parent constructor.
-
Extend the class
class Dog ____ Animal {}Show answer
class Dog extends Animal {} -
Match the concepts
- extends
- subclass declaration
- super()
- parent constructor call
- super.method()
- parent method call
- subclass
- derived class