How to call base class constructor from child class in TypeScript ?

In this article, we will learn how we can call the base class constructor from the child class. When a class inherits the properties of another class, it is called a child class and the class whose properties are inherited is called the parent class and the whole process is called Inheritance. In Inheritance, the child class acquires the properties of the base class or parent class. 

You can call the base class constructor from the child class by using the super() which will execute the constructor of the base class.

Example:

Javascript

class Person {

  // Properties of the Person class
  Name: string;
  Profession: string;

  // Constructor of Person class
  constructor(name: string, profession: string) {
    this.Name = name;
    this.Profession = profession;
  }
}

class Details extends Person {

  // Properties of the class
  Name: string;
  Profession: string;

  // Constructor of the Details class
  constructor(name: string, profession: string) {

    // Calling the base class constructor
    super(name, profession);

    // Setting the properties
    this.Name = name;
    this.Profession = profession;
  }

  details(): string {
    return this.Name + " is " +
      this.Profession;
  }
}

// Creating an object
var data =
  new Details("A", "Android Developer");
var data2 =
  new Details("B", "Web Developer");

// Accessing the function details()
// and printing
console.log(data.details());
console.log(data2.details());

Output:

A is Android Developer
B is Web Developer

Here, the Person class is our parent class and the Details class is our child class and also the Details class inherits the Person class. For Inheriting another class extends keyword is used. The Details class inherits the properties of the Person class. 

Now in the derived class, we have used the super() which will call the constructor of the base class or parent class. After this, we have created an instance of the Details class and passed two parameters name and profession to its constructor and after this, we have called the details method which will print the value provided into the constructor parameter.

Example 2:

Javascript

class Square {

  // Properties of the Square class
  side: number;

  // Constructor of the Square class
  constructor(side: number) {
    this.side = side;
  }
}

class Area extends Square {

  // Properties of the Area class
  side: number;

  // Constructor of the Area class
  constructor(side: number) {

    // Calling the base class constructor
    super(side);

    // Setting the properties
    this.side = side;

  }

  // Returns the area of square
  area(): string {
    return "The area of Square is " + t
      his.side * this.side;
  }
}

// Creating object of class Area
var data = new Area(7);

// Getting the property and
// printing the value
console.log(data.area());

Output:

The area of Square is 49