How do constructor functions and the 'new' keyword work together? Constructor functions और 'new' keyword साथ में कैसे काम करते हैं?

A constructor function is a regular function used with 'new' to create objects following a consistent template. The 'new' keyword performs several steps automatically.

function Person(name, age) {
    this.name = name;
    this.age = age;
}
Person.prototype.greet = function() {
    return `Hi, I'm ${this.name}, ${this.age} years old`;
};

const john = new Person('John', 30);
console.log(john.greet()); // Hi, I'm John, 30 years old

// What 'new Person(...)' actually does step by step:
function simulateNew(Constructor, ...args) {
    // 1. Create a new empty object with its prototype set to Constructor.prototype
    const obj = Object.create(Constructor.prototype);
    // 2. Call the constructor with 'this' bound to the new object
    const result = Constructor.apply(obj, args);
    // 3. If the constructor explicitly returns an object, use that instead
    return (typeof result === 'object' && result !== null) ? result : obj;
}
const simulated = simulateNew(Person, 'Jane', 25);
console.log(simulated.greet()); // Hi, I'm Jane, 25 years old

// Common mistake: forgetting 'new'
function PersonNoNew(name) {
    this.name = name; // 'this' is NOT bound to a new object in non-strict mode
}
const oops = PersonNoNew('Bob'); // called WITHOUT new
console.log(oops); // undefined - function returned nothing
// In strict mode or with classes, this throws a TypeError instead - safer

// ES6 classes enforce 'new' automatically
class PersonClass {
    constructor(name) {
        this.name = name;
    }
}
// PersonClass('John'); // TypeError: Class constructor cannot be invoked without 'new'

// Checking if a function was called with 'new'
function SafeConstructor(name) {
    if (!(this instanceof SafeConstructor)) {
        throw new Error('Must be called with new');
    }
    this.name = name;
}