this in Constructors and the new Keyword in JavaScript

this in Constructors and the new Keyword in JavaScript

When you call a function with new, JavaScript creates a brand-new object and sets this to it. Here is how constructors work.

What new Does

1. Creates a new empty object.
2. Sets this to that new object inside the function.
3. Runs the function body (which typically adds properties to this).
4. Returns this (the new object), unless the function returns its own object.

function Person(name, age) {
  this.name = name;
  this.age = age;
}
const p = new Person("Kunal", 30);
console.log(p.name); // "Kunal"
console.log(p.age);  // 30
``
### Forgetting new Is a Bug

```js
const p = Person("Kunal", 30); // no new
console.log(p); // undefined
console.log(window.name); // "Kunal" (polluted global!)
``
Without `new`, `this` is the global object (or `undefined` in strict mode). Properties get added to `window` instead of a new object.

### ES6 Classes Require new

```js
class Person {
  constructor(name) { this.name = name; }
}
const p = Person("Kunal"); // TypeError: Class constructor cannot be invoked without 'new'
``
### new Binding Precedence

`new` overrides `bind` for `this` (but preset arguments remain):

```js
const BoundPerson = Person.bind(null, "Kunal");
const p = new BoundPerson(30); // this is a new object, not null
``
### Returning from a Constructor

```js
function Person(name) {
  this.name = name;
  return { custom: true }; // returns this object, not this
}
const p = new Person("Kunal");
console.log(p.custom); // true (returned object)
console.log(p.name);   // undefined (not this)
``
If you return an object, that object is returned instead of `this`. If you return a primitive (or nothing), `this` is returned.

### The Takeaway

`new` creates a fresh object and sets `this` to it. Forgetting `new` with a function constructor pollutes the global object. ES6 classes throw if you forget `new`. `new` overrides `bind` for `this`. Returning an object from a constructor returns that object instead of `this`.