JavaScript Object Constructors

Last Updated : 11 Jul, 2025

An object is the collection of related data or functionality in the form of key. These functionalities usually consist of several functions and variables. All JavaScript values are objects except primitives.

const GFG = {
    subject : "programming",
    language : "JavaScript",
}

Here, subject and language are the keys and programming and JavaScript are the values.

Object Constructor

A constructor function is a special type of function in JavaScript that is used to create and initialize objects. Constructors are used with the new keyword to create instances of a particular type (object). By using constructors, we can easily create multiple instances of the same type of object, all with their own unique properties and methods.

JavaScript

//Driver Code Starts
// Constructor function
//Driver Code Ends

function Person(name, age) {
    this.name = name;
    this.age = age;
    this.sayHello = function() {
        console.log(`My name is ${this.name} and I am ${this.age} years old.`);
    };
}

//Driver Code Starts

//Creating Instances with a Constructor
const p1 = new Person("Akash", 30);
const p2 = new Person("Anvesh", 25);

p1.sayHello();
p2.sayHello();
//Driver Code Ends

Output

My name is Akash and I am 30 years old.
My name is Anvesh and I am 25 years old.

this keyword

The this keyword refers to the object it belongs to, like OOPs languages C++, C#, JAVA etc. this keyword is used in different ways in different areas. While executing a function in JavaScript that has a reference to its current execution context, that is the reference by which the function or data member is called. See the previous example.

Adding Property to an Object

The property can be added to the object by using dot(.) operator or square bracket.,

const GFG = {
    articles: 'computer science',
    quantity: 3000,
};

The GFG has two properties "articles" and "quantity". Now we wish to add one more property name called subject.

Using dot (.) operator

GFG.subject: 'JavaScript';

Using square bracket:

GFG['subject']: 'JavaScript';

Here, subject is the property and 'JavaScript' is the value of the property. Adding a property to Constructor: We cannot add a property to an existing constructor like adding a property to an object (see previous point), for adding a property we need to declare under the constructor.

function GFG(a, b, c) {
    this.A = a;
    this.B = b;
    this.C = c;
    this.G = "GEEK";
}

Here, we add a property name G with value "GEEK", in this case the value "GEEK" is not passed as an argument.

Adding a Method to an Object

We can add a new method to an existing object.

GFG.n = function () {
    return this.A + this.B;
};

Here, the object is GFG.

Adding a Method to Constructor

function GFG(a, b, c) {
    this.A = a;
    this.B = b;
    this.C = c;
    this.n = function () {
        return this.A + this.B;
    }
}

Here, in the last line a method is added to an object.

Instantiating an object constructor

There are two ways to instantiate object constructor,

const object_name = new Object(); // or  
const object_name = new Object("java", "JavaScript", "C#");

const object_name = { };

In 1st method, the object is created by using new keyword like normal OOP languages, and "Java", "JavaScript", "C#" are the arguments, that are passed when the constructor is invoked. In 2nd method, the object is created by using curly braces "{ }".

Assigning properties to the objects: There are two ways to assigning properties to the objects.

Using dot (.) operator:

object_name . properties = value;

Using third bracket:

object_name [ 'properties'] = value;

Example 1: Object creation by using new keyword and assigning properties to the object using dot(.) operator.

javascript

let gfg = new Object();

gfg.a = "JavaScript";
gfg.b = "GeeksforGeeks";


//Driver Code Starts
console.log("Subject: " + gfg.a);
console.log("Author: " + gfg.b );
//Driver Code Ends

Output

Subject: JavaScript
Author: GeeksforGeeks

Example 2: Object creation using curly braces and assigning properties to the object using third bracket "[]" operator.

javascript

let gfg = { };

gfg['a'] = "JavaScript";
gfg['b']= "GeeksforGeeks";


//Driver Code Starts
console.log("Subject: " + gfg.a);
console.log("Author: " + gfg.b );
//Driver Code Ends

Output

Subject: JavaScript
Author: GeeksforGeeks

Example 3: This example shows how to use function() with object constructor.

javascript

let gfg = new Object();

gfg.a = "JavaScript";
gfg.b = "GeeksforGeeks";
gfg.c = function () {
    return (gfg.a +" "+ gfg.b);
};


//Driver Code Starts
console.log("Subject: " + gfg.a);
console.log("Author: " + gfg.b);
console.log("Adding the strings: "+ gfg.c() );
//Driver Code Ends

Output

Subject: JavaScript
Author: GeeksforGeeks
Adding the strings: JavaScript GeeksforGeeks

Example 4: Another way to create a function using function name.

javascript

let gfg = { };

gfg.a = "JavaScript";
gfg.b = "GeeksforGeeks";
gfg.c = add;

// Declare function add()
function add() {
    return (gfg.a +" "+ gfg.b);
};


//Driver Code Starts
console.log("Subject: " + gfg.a);
console.log("Author: " + gfg.b);
console.log("Adding the strings: "+ gfg.c());
//Driver Code Ends

Output

Subject: JavaScript
Author: GeeksforGeeks
Adding the strings: JavaScript GeeksforGeeks

Object.create vs. New in JavaScript

SoumikMondal

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Article Tags :

JavaScript Object Constructors

Object Constructor

this keyword

Adding Property to an Object

Adding a Method to an Object

Adding a Method to Constructor

Instantiating an object constructor

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