Classes are an essential part of object-oriented programming (OOP). Classes are used to define the blueprint for real-world object modeling and organize the code into reusable and logical parts.
A class definition can only include constructors and functions. These components are together called as the data members of a class. The classes contain constructors that allocates the memory to the objects of a class. Classes contain functions that are responsible for performing the actions to the objects.
Note: Instead of data properties, the body of the class only contains methods.
Syntax: Class Expression
Syntax: Class Declaration
Let us see the illustration for the class expression and class declaration.
Example - Class Declaration
Example - Class Expression
Instantiating an Object from class
Like other object-oriented programming languages, we can instantiate an object from class by using the new keyword.
The object can access the attributes and functions of a class. We use the '.' dot notation (or period) for accessing the data members of the class.
In the above example, we have declared a class Student. The constructor of the class contains two arguments name and age, respectively. The keyword 'this' refers to the current instance of the class. We can also say that the above constructor initializes two variables 'n' and 'a' along with the parameter values passed to the constructor.
The function stu() in the class will print the values of name and age.
The Name of the student is: Peter The Age of the student is: 20
Note: Including a constructor definition is mandatory in class because, by default, every class has a constructor.
The Static keyword
The static keyword is used for making the static functions in the class. Static functions are referenced only by using the class name.
Before the ES6, the implementation of inheritance required several steps. But ES6 simplified the implementation of inheritance by using the extends and super keyword.
Inheritance is the ability to create new entities from an existing one. The class that is extended for creating newer classes is referred to as superclass/parent class, while the newly created classes are called subclass/child class.
A class can be inherited from another class by using the 'extends' keyword. Except for the constructors from the parent class, child class inherits all properties and methods.
A class inherits from the other class by using the extends keyword.
In the above example, we have declared a class student. By using the extends keyword, we can create a new class User that shares the same characteristics as its parent class Student. So, we can see that there is an inheritance relationship between these classes.
The name of the student is: Sahil
Types of inheritance
Inheritance can be categorized as Single-level inheritance, Multiple inheritance, and Multi-level inheritance. Multiple inheritance is not supported in ES6.
It is defined as the inheritance in which a derived class can only be inherited from only one base class. It allows a derived class to inherit the behavior and properties of a base class, which enables the reusability of code as well as adding the new features to the existing code. It makes the code less repetitive.
In multiple inheritance, a class can be inherited from several classes. It is not supported in ES6.
In Multi-level inheritance, a derived class is created from another derived class. Thus, a multi-level inheritance has more than one parent class.
Let us understand it with the following example.
eating... barking... weeping...
Method Overriding and Inheritance
It is a feature that allows a child class to provide a specific implementation of a method which has been already provided by one of its parent class.
There are some rules defined for method overriding -
Let us try to understand the illustration for the same:
In the above example, the implementation of the superclass function has changed in the child class. You will get the following output after the successful execution of the above code:
It is the show() method from the child class
The super keyword
It allows the child class to invoke the properties, methods, and constructors of the immediate parent class. It is introduced in ECMAScript 2015 or ES6. The super.prop and super[expr] expressions are readable in the definition of any method in both object literals and classes.
In this example, the characteristics of the parent class have been extended to its child class. Both classes have their unique properties. Here, we are using the super keyword to access the property from parent class to the child class.
It is the show() method from the parent class It is the show() method from the child class