JavaScript Prototype and Inheritance Explained Simply

Prototype Chain

Every JavaScript object has an internal [[Prototype]] link — a reference to another object. When you access a property, JS looks up the chain until it finds it or reaches null. This is the foundation of all inheritance in JavaScript.

Understanding __proto__ and prototype

const animal = {
    breathes: true,
    eat() { return 'Eating...'; }
};

const dog = Object.create(animal); // dog.__proto__ = animal
dog.name  = 'Rex';
dog.bark  = function() { return 'Woof!'; };

console.log(dog.bark());     // Woof!   ← own property
console.log(dog.eat());      // Eating! ← found on animal (prototype)
console.log(dog.breathes);   // true    ← found on animal
console.log(dog.hasOwnProperty('name')); // true
console.log(dog.hasOwnProperty('eat'));  // false (on prototype)

Constructor Functions and prototype

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

// Methods on prototype — shared across ALL instances (not duplicated)
Person.prototype.greet = function() {
    return `Hi, I'm ${this.name}`;
};

Person.prototype.isAdult = function() {
    return this.age >= 18;
};

const p1 = new Person('Rahul', 25);
const p2 = new Person('Priya', 30);

console.log(p1.greet());  // Hi, I'm Rahul
console.log(p2.greet());  // Hi, I'm Priya
// Both share the SAME greet function (prototype) — no memory duplication

ES6 Classes — Syntactic Sugar

class Animal {
    #sound; // private field (ES2022)

    constructor(name, sound) {
        this.name  = name;
        this.#sound = sound;
    }

    speak() {
        return `${this.name} says ${this.#sound}`;
    }

    static create(name, sound) {
        return new Animal(name, sound);
    }
}

class Dog extends Animal {
    constructor(name) {
        super(name, 'Woof'); // call parent constructor
    }

    fetch(item) {
        return `${this.name} fetches ${item}`;
    }
}

const dog  = new Dog('Rex');
console.log(dog.speak()); // Rex says Woof
console.log(dog.fetch('ball')); // Rex fetches ball
console.log(dog instanceof Dog);    // true
console.log(dog instanceof Animal); // true

Prototype Chain Lookup

// Chain: dog → Dog.prototype → Animal.prototype → Object.prototype → null
const dog = new Dog('Rex');

// toString() found on Object.prototype
console.log(dog.toString()); // [object Object]

// Check chain manually
console.log(Object.getPrototypeOf(dog) === Dog.prototype);         // true
console.log(Object.getPrototypeOf(Dog.prototype) === Animal.prototype); // true
console.log(Object.getPrototypeOf(Animal.prototype) === Object.prototype); // true