Introduction to Object Oriented Programming in JavaScript

As JavaScript is widely used in Web Development, in this article we would explore some of the Object Oriented mechanism supported by JavaScript to get most out of it. Some of the common interview question in JavaScript on OOPS includes,- “How Object-Oriented Programming is implemented in JavaScript? How they differ from other languages? Can you implement Inheritance in JavaScript and so on…”

There are certain features or mechanisms which makes a Language Object Oriented like:

Object
Classes
Encapsulation
Inheritance

Let’s dive into the details of each one of them and see how they are implemented in JavaScript.

Object– An Object is a unique entity which contains property and methods. For example “car” is a real life Object, which have some characteristics like color, type, model, horsepower and performs certain action like drive. The characteristics of an Object are called as Property, in Object Oriented Programming and the actions are called methods. An Object is an instance of a class. Objects are everywhere in JavaScript almost every element is an Object whether it is a function,arrays and string.
Note: A Method in javascript is a property of an object whose value is a function.
Object can be created in two ways in JavaScript:

Using an Object Literal

//Defining object 
let person = { 
    first_name:'Mukul', 
    last_name: 'Latiyan', 
  
    //method 
    getFunction : function(){ 
        return (`The name of the person is  
          ${person.first_name} ${person.last_name}`) 
    }, 
    //object within object 
    phone_number : { 
        mobile:'12345', 
        landline:'6789'
    } 
} 
console.log(person.getFunction());  
console.log(person.phone_number.landline); 

Output:

Using an Object Constructor:

//using a constructor 
function person(first_name,last_name){ 
   this.first_name = first_name; 
   this.last_name = last_name; 
} 
//creating new instances of person object 
let person1 = new person('Mukul','Latiyan'); 
let person2 = new person('Rahul','Avasthi'); 
  
console.log(person1.first_name); 
console.log(`${person2.first_name} ${person2.last_name}`); 

Output:

Using Object.create() method: The Object.create() method creates a new object, using an existing object as the prototype of the newly created object.

// Object.create() example a 
// simple object with some properties 
const coder = { 
    isStudying : false, 
    printIntroduction : function(){ 
        console.log(`My name is ${this.name}. Am I  
          studying?: ${this.isStudying}.`) 
    } 
} 
// Object.create() method 
const me = Object.create(coder); 
  
// "name" is a property set on "me", but not on "coder" 
me.name = 'Mukul';  
  
// Inherited properties can be overwritten 
me.isStudying = 'True';  
  
me.printIntroduction();  

Output:

Classes– Classes are blueprint of an Object. A class can have many Object, because class is a template while Object are instances of the class or the concrete implementation.
Before we move further into implementation, we should know unlike other Object Oriented Language there is no classes in JavaScript we have only Object. To be more precise, JavaScript is a prototype based object oriented language, which means it doesn’t have classes rather it define behaviors using constructor function and then reuse it using the prototype.
Note: Even the classes provided by ECMA2015 are objects.

JavaScript classes, introduced in ECMAScript 2015, are primarily syntactical sugar over JavaScript’s existing prototype-based inheritance. The class syntax is not introducing a new object-oriented inheritance model to JavaScript. JavaScript classes provide a much simpler and clearer syntax to create objects and deal with inheritance.
–Mozilla Developer Network

Example:
Lets use ES6 classes then we will look into traditional way of defining Object and simulate them as classes.

// Defining class using es6 
class Vehicle { 
  constructor(name, maker, engine) { 
    this.name = name; 
    this.maker =  maker; 
    this.engine = engine; 
  } 
  getDetails(){ 
      return (`The name of the bike is ${this.name}.`) 
  } 
} 
// Making object with the help of the constructor 
let bike1 = new Vehicle('Hayabusa', 'Suzuki', '1340cc'); 
let bike2 = new Vehicle('Ninja', 'Kawasaki', '998cc'); 
  
console.log(bike1.name);    // Hayabusa 
console.log(bike2.maker);   // Kawasaki 
console.log(bike1.getDetails()); 

Output:

Traditional Way.

// Defining class in a Traditional Way. 
function Vehicle(name,maker,engine){ 
    this.name = name, 
    this.maker = maker, 
    this.engine = engine 
}; 
  
Vehicle.prototype.getDetails = function(){ 
    console.log('The name of the bike is '+ this.name); 
} 
  
let bike1 = new Vehicle('Hayabusa','Suzuki','1340cc'); 
let bike2 = new Vehicle('Ninja','Kawasaki','998cc'); 
  
console.log(bike1.name); 
console.log(bike2.maker); 
console.log(bike1.getDetails()); 

Output:

As seen in the above example it is much simpler to define and reuse object in ES6. Hence, we would be using ES6 in all our examples.

Encapsulation – The process of wrapping property and function within a single unit is known as encapsulation.
Let’s understand encapsulation with an example.

//encapsulation example 
class person{ 
    constructor(name,id){ 
        this.name = name; 
        this.id = id; 
    } 
    add_Address(add){ 
        this.add = add; 
    } 
    getDetails(){ 
        console.log(`Name is ${this.name},Address is: ${this.add}`); 
    } 
} 
  
let person1 = new person('Mukul',21); 
person1.add_Address('Delhi'); 
person1.getDetails(); 

Output:

In the above example we simply create an person Object using the constructor and Initialize it property and use it functions we are not bother about the implementation details. We are working with an Objects interface without considering the implementation details.
Sometimes encapsulation refers to hiding of data or data Abstraction which means representing essential features hiding the background detail. Most of the OOP languages provide access modifiers to restrict the scope of a variable, but their are no such access modifiers in JavaScript but their are certain way by which we can restrict the scope of variable within the Class/Object.
Example:

// Abstraction example 
function person(fname,lname){ 
    let firstname = fname; 
    let lastname = lname; 
  
    let getDetails_noaccess = function(){ 
        return (`First name is: ${firstname} Last  
            name is: ${lastname}`); 
    } 
  
    this.getDetails_access = function(){ 
        return (`First name is: ${firstname}, Last  
            name is: ${lastname}`); 
    } 
} 
let person1 = new person('Mukul','Latiyan'); 
console.log(person1.firstname); 
console.log(person1.getDetails_noaccess); 
console.log(person1.getDetails_access()); 

Output:

In the above example we try to access some property(person1.firstname) and functions(person1.getDetails_noaccess) but it returns undefine while their is a method which we can access from the person object(person1.getDetails_access()), by changing the way to define a function we can restrict its scope.

Inheritance – It is a concept in which some property and methods of an Object is being used by another Object. Unlike most of the OOP languages where classes inherit classes, JavaScript Object inherits Object i.e. certain features (property and methods)of one object can be reused by other Objects.
Lets’s understand inheritance with example:

//Inhertiance example 
class person{ 
    constructor(name){ 
        this.name = name; 
    } 
    //method to return the string 
    toString(){ 
        return (`Name of person: ${this.name}`); 
    } 
} 
class student extends person{ 
    constructor(name,id){ 
        //super keyword to for calling above class constructor 
        super(name); 
        this.id = id; 
    } 
    toString(){ 
        return (`${super.toString()},Student ID: ${this.id}`); 
    } 
} 
let student1 = new student('Mukul',22); 
console.log(student1.toString()); 

Output:

In the above example we define an Person Object with certain property and method and then we inherit the Person Object in the Student Object and use all the property and method of person Object as well define certain property and methods for Student.
Note: The Person and Student object both have same method i.e toString(), this is called as Method Overriding. Method Overriding allows method in a child class to have the same name and method signature as that of a parent class.
In the above code, super keyword is used to refer immediate parent class instance variable.

In this article we introduced Object Oriented features in JavaScript, there is an entire book which explains Object Oriented Programming in JavaScript in great details,- ” Object-Oriented JavaScript by Stoyan Stefanov”

This article is contributed by Sumit Ghosh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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