The new Keyword in JavaScript

If you've ever written new Person("Alice") and wondered what's actually happening behind the scenes — this post is for you.

what is constructor function?
A constructor function is just a regular function, but by convention we capitalize its name to signal it should be called with new.

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

Inside it, this refers to the new object being built. That's the key idea.
What does new actually do?
When you write:

const alice = new Person("Alice", 30);

JavaScript silently does four things for you:

1. Creates a brand-new empty object

// internally: const this = {};

2.Links it to the constructor's prototype

// internally: this.__proto__ = Person.prototype;

This is how alice can call methods like greet() even though they aren't stored on alice itself.
3.Runs the constructor body with this as that new object

this.name = "Alice";

4.Returns the object automatically.You don't need a return statement — new handles it

Putting methods on the prototype
Instead of copying a method into every instance (wasteful!), you add it once to the prototype:

Person.prototype.greet = function() {
  console.log("Hi, I'm " + this.name);
};

const alice = new Person("Alice", 30);
const bob   = new Person("Bob",   25);

alice.greet(); // "Hi, I'm Alice"
bob.greet();   // "Hi, I'm Bob"

Both alice and bob share the same greet function. Neither instance stores a copy — they just look up the chain.
The prototype chain

When you call alice.greet(), JavaScript searches in this order:

Does alice have greet directly? → No
Does alice.proto (= Person.prototype) have greet? → Yes! Use it.

This chain of lookups is called the prototype chain.

Summary:

1. A constructor function is a blueprint for creating objects.
2. new creates an empty object, links its prototype, runs the constructor, and returns the result.
3. Own data lives on the instance; shared methods live on the prototype.
4. When a property isn't found on the instance, JavaScript walks up the prototype chain to find it.