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 → {}