Inheritance in JavaScript. Basic code example

Inheritance allows, when creating new types of objects, to imitate the functionality of already existing objects if necessary. For example, we might have a User object representing an individual user. And it can also be an Employee object, which represents the worker. But a worker can also be a user and therefore must have all of its properties and methods. For example:

Example of inheritance using prototypes

Let’s create a User object constructor:

   
    function User(name, age) {
        this.name = name;
        this.age = age;
        this.go = function () { document.write(this.name + " go"); }
        this.displayInfo = function () {
            document.write("Name: " + this.name + "; age: " + this.age);
        };
    }
    //Set properties for all user objects through the prototype
    User.prototype.maxage = 99;

Now let’s create the Employee constructor:

    function Employee(name, age, comp) {
        //we imitate the properties of the object using the call method
        User.call(this, name, age);

        //add a new property that is not in the inherited object
        this.company = comp;

        //reassign the displayInfo method that is in the inherited object
        this.displayInfo = function () {
            document.write("Name: " + this.name + "; age: " + this.age + "; company: " + this.company);
        };

    }

    //we inheritan the properties of the prototype
    Employee.prototype = Object.create(User.prototype);

Let’s call:

    var alex = new User("Alexander", 33);
    var bill = new Employee("Bill", 32, "Google");
    alex.go();//Alexander go
    bill.go();//Bill go
    alex.displayInfo();//Name: Alexander; age: 33;
    bill.displayInfo();//Name: Bill, age: 32; company: Google
    console.log(bill.maxage);//99

Example of inheritance using Class

class Animal {
  constructor(name) {
    this.name = name;
  }

  speak() {
    console.log(`${this.name} makes a sound.`);
  }
}

class Dog extends Animal {
  speak() {
    console.log(`${this.name} barks.`);
  }
}

const dog = new Dog("Buddy");
dog.speak(); // Output: "Buddy barks."

In this example, the Dog class inherits from the Animal class. The Dog class has its own speak method, which overrides the Animal class’s speak method.