Prototype

Prototype

Every object in JavaScript has a hidden property called __proto__ (or [[Prototype]]). This prototype is the mechanism JavaScript uses to share methods and properties between objects.

What Is a Prototype?

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

User.prototype.login = function() {
    console.log(this.name + ' logged in');
};

var user = new User('Shaun');

console.log(user);
// User { name: 'Shaun' }
// __proto__:
//   login: function()
//   constructor: function User()

The login method is not on the user object directly — it is on the prototype. JavaScript looks up the prototype chain to find it.

The Prototype Chain

user.login()
    ↓
Does user have a 'login' property? No.
    ↓
Check user.__proto__ (User.prototype) → Found! Execute it.

user.toString()
    ↓
Does user have 'toString'? No.
    ↓
Check User.prototype → No.
    ↓
Check Object.prototype → Found! Execute it.

Visualizing the Chain

user (instance)
  └── __proto__ → User.prototype
                    ├── login()
                    └── __proto__ → Object.prototype
                                      ├── toString()
                                      ├── hasOwnProperty()
                                      └── __proto__ → null (end of chain)

Checking Prototypes

// What is on the prototype?
console.log(User.prototype);
// { login: function, constructor: function User }

// Is this method on the instance or prototype?
console.log(user.hasOwnProperty('name'));    // true  (own property)
console.log(user.hasOwnProperty('login'));   // false (on prototype)

Why Prototypes Matter

Approach	Memory Usage	Shared?
Method in constructor	Duplicated per instance	No
Method on prototype	Stored once	Yes — all instances share it

Key Takeaways

• Every object has a __proto__ linking to its prototype
• JavaScript walks up the prototype chain to find properties and methods
• Prototype methods are shared across all instances — efficient for memory
• hasOwnProperty() checks if a property is on the instance vs the prototype