Everything You Need to Know About new in JavaScript

1. What does new do?

• When you use new, Javascript:
• Creates a new empty object: {}
• Sets the new object's prototype to the constructor's prototype (obj.__proto__ = Constructor.prototype)
• Runs the Constructor function with this bound to the new object
• Returns the new object (unless the constructor explicitly returns another object)

2. Using new with a Constructor Function

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

const user = new Person("Ankit");
console.log(user.name); // "Ankit"

If you call person("Ankit") without new:

• It won't create a new object.
• this will refer to the global object (or undefined in strict mode)
• name will be set on the wrong thing → Bug! That's why new is important.

3. Using new with a Class

class Car {
  constructor(brand) {
    this.brand = brand;
  }
}

const c1 = new Car("Tesla");
console.log(c1.brand); // Tesla

Your must use new with classes, or Js will throw an error.

4. What if the Constructor Returns Something?

• If it returns an object, that object is returned instead of the new one.
• If it returns a primitive (string, number, etc.), it's ignore, and the new object is returned.

function Test() {
  this.value = 10;
  return { msg: "I override!" };
}

const t = new Test();
console.log(t); // { msg: "I override!" }

5. Built-in Objects that use new

Many built-ins in JS can be created with new:

let now = new Date();
let regex = new RegExp("abc");
let obj = new Object();
let arr = new Array(3); // [empty × 3]

But some are fine without new:

let str = String(123); // "123"
let num = Number("5"); // 5

6. When NOT to use new:

• Don't use new with normal functions that are not constructors.

function greet() {
  console.log("Hello!");
}

new greet(); // Works but makes no sense → {}