The new Keyword in JavaScript: A Complete Beginner-Friendly Guide

If you've ever seen code like this:

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

const user = new Person("Sayantan");

You might have wondered — what exactly does new do here?

This article will break it down step-by-step so you truly understand how object creation works in JavaScript.

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🔹 What Does the new Keyword Do?

The new keyword is used to create instances of objects from a constructor function.

In simple terms:

It turns a normal function into a blueprint for creating objects.

Without new, the function behaves like a regular function. With new, it becomes a constructor.

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🔹 Constructor Functions

A constructor function is just a normal function, but by convention:

• It starts with a capital letter

• It is used with the new keyword

Example:

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

Now we can create objects like this:

const car1 = new Car("Toyota", "Fortuner");
const car2 = new Car("Tesla", "Model 3");

Each object created is called an instance.

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🔹 What Happens Internally When You Use new?

This is the most important part.

When you run:

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

JavaScript performs 4 internal steps:

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✅ Step 1: Create a New Empty Object

const obj = {};

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✅ Step 2: Link the Object to the Constructor’s Prototype

obj.__proto__ = Car.prototype;

This is what enables prototype inheritance.

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✅ Step 3: Bind this to the New Object

this = obj;

Inside the constructor:

this.brand = "Toyota";
this.model = "Fortuner";

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✅ Step 4: Return the Object

return obj;

So finally:

car1 = {
  brand: "Toyota",
  model: "Fortuner"
}

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🔹 How new Links Prototypes

Every function in JavaScript has a prototype property.

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

Person.prototype.sayHello = function () {
  console.log("Hello, I am " + this.name);
};

Now:

const p1 = new Person("Sayantan");
p1.sayHello();

Why does this work?

Because:

p1.__proto__ === Person.prototype  // true

So JavaScript looks up the prototype chain when a method is not found directly on the object.

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🔹 Instances Created from Constructors

Each object created using new:

• Has its own copy of properties

• Shares methods via prototype

Example:

function Student(name) {
  this.name = name;
}

Student.prototype.study = function () {
  console.log(this.name + " is studying");
};

const s1 = new Student("Amit");
const s2 = new Student("Rahul");

Key Points:

• s1 and s2 are different objects

• But both share the same study() method (memory efficient)

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🔹 Without new (Common Mistake)

function User(name) {
  this.name = name;
}

const u1 = User("Sayantan"); // ❌

Problems:

• this refers to global object (or undefined in strict mode)

• No object is created properly