JavaScript Object Constructors

Last Updated : 23 Jun, 2026

An object in JavaScript is a collection of related data and functionality stored as key–value pairs, where values can be variables or functions (methods).

  • Objects group related properties and behaviors into a single structure.
  • JS has primitive and non-primitive types; objects are non-primitive.
const GFG = {
    subject : "programming",
    language : "JavaScript",
}
  • GFG is an object created using object literal syntax.
  • subject is a key with the value "programming".
  • language is a key with the value "JavaScript".
  • Each key–value pair represents a property of the GFG object.

Object Constructor

A constructor function in JavaScript is a special function used with the new keyword to create and initialize objects of a specific type, allowing multiple instances with similar structure but unique properties.

  • Used with the new keyword to create object instances.
  • Initializes object properties and methods.
  • Helps create multiple objects of the same type efficiently.
  • Each instance has its own separate data while sharing the same blueprint.
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

this keyword

The this keyword inside a constructor function refers to the newly created object instance. It is used to initialize the properties and methods of that object.

  • Inside a constructor, this refers to the newly created object.
  • It is used to initialize object properties and methods.
  • Its value depends on how the function is called.

Adding Properties in a Constructor Function

Here, subject is the property and "JavaScript" is its value, and properties in a constructor must be defined inside the constructor itself.

  • subject acts as the key, while "JavaScript" is the assigned value.
  • New properties can be added dynamically, but defining them inside the constructor is recommended
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,

function Person(name, age) {
    this.name = name;
    this.age = age;
}

const person1 = new Person("John", 25);
const person2 = new Person("Alice", 30);
  • new Person() creates a new object instance.
  • The constructor initializes the object's properties using the provided arguments.
  • Multiple objects can be created from the same constructor with different values.

Note: Objects can also be created using object literals ({}) or the built-in Object() constructor, but these are different from creating objects using a custom constructor function.

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

  • Using dot (.) operator:
object_name.properties = value;
  • Using square 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

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

Example 3: Adding Methods in a Constructor Function

javascript 
function Car(brand, model) {
    this.brand = brand;
    this.model = model;

    this.getDetails = function () {
        return `${this.brand} ${this.model}`;
    };

}
//Driver Code Starts

const car1 = new Car("Toyota", "Camry");

console.log(car1.getDetails()); 
//Driver Code Ends


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