Class Inheritance

Inheritance enables you to define a class that takes all the functionality from a parent class and allows you to add more.

Using class inheritance, a class can inherit all the methods and properties of another class.

Inheritance is a useful feature that allows code reusability.

To use class inheritance, you use the extends keyword. For example,

// parent class
class Person { 
    constructor(name) {
        this.name = name;
    }

    greet() {
        console.log(`Hello ${this.name}`);
    }
}

// inheriting parent class
class Student extends Person {

}

let student1 = new Student('Jack');
student1.greet();

Output

Hello Jack

In the above example, the Student class inherits all the methods and properties of the Person class. Hence, the Student class will now have the name property and the greet() method.

Then, we accessed the greet() method of Student class by creating a student1 object.

JavaScript super() keyword

The super keyword used inside a child class denotes its parent class. For example,

// parent class
class Person { 
    constructor(name) {
        this.name = name;
    }

    greet() {
        console.log(`Hello ${this.name}`);
    }
}

// inheriting parent class
class Student extends Person {

    constructor(name) {
    
        console.log("Creating student class");
        
        // call the super class constructor and pass in the name parameter
        super(name);
    }

}

let student1 = new Student('Jack');
student1.greet();

Here, super inside Student class refers to the Person class. Hence, when the constructor of Student class is called, it also calls the constructor of the Person class which assigns a name property to it.

Overriding Method or Property

If a child class has the same method or property name as that of the parent class, it will use the method and property of the child class. This concept is called method overriding. For example,

// parent class
class Person { 
    constructor(name) {
        this.name = name;
        this.occupation = "unemployed";
    }
    
    greet() {
        console.log(`Hello ${this.name}.`);
    }
 
}

// inheriting parent class
class Student extends Person {

    constructor(name) {
        
        // call the super class constructor and pass in the name parameter
        super(name);
        
        // Overriding an occupation property
        this.occupation = 'Student';
    }
    
    // overriding Person's method
    greet() {
        console.log(`Hello student ${this.name}.`);
        console.log('occupation: ' + this.occupation);
    }
}

let p = new Student('Jack');
p.greet();

Output

Hello student Jack.
occupation: Student

Here, the occupation property and the greet() method are present in parent Person class and the child Student class. Hence, the Student class overrides the occupation property and the greet() method.

Uses of Inheritance

• Since a child class can inherit all the functionalities of the parent’s class, this allows code reusability.
• Once a functionality is developed, you can simply inherit it. No need to reinvent the wheel. This allows for cleaner code and easier to maintain.
• Since you can also add your own functionalities in the child class, you can inherit only the useful functionalities and define other required features.