Prototypes

When creating an object with new:

• A new object is created.
• Properties are added to that object via this.
• That object's prototype is linked to Constructor.prototype.
• It is returned.

prototype

• prototype is an object that becomes the prototype of objects created using new.
• Every normal function in JavaScript automatically gets a prototype property.

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

Person.prototype.sayHi = function() {
    console.log("Hi")
}

const p1 = new Person("Chandan")

p1.__proto__ === Person.prototype

Prototype Chain Lookup

p1.sayHi()
// JavaScript looks for sayHi on p1 → not found
// looks on p1.__proto__ (Person.prototype) → found

p1.toString()
// not on p1 → not on Person.prototype
// looks on Object.prototype → found

p1.notReal()
// not found anywhere → TypeError

__proto__

• It is a reference to an object's internal [[Prototype]].
• __proto__ is the actual prototype link of an object.
• It points to another object.
• It belongs to every object.

Modern Prototype Access

Object.getPrototypeOf(p1)
Object.setPrototypeOf(p1, newProto)

Object.prototype is the top of the prototype chain.

Object.prototype.__proto__ === null // true

	Belongs to	Points to
prototype	Functions only	Object that becomes __proto__ of instances
__proto__	Every object	The actual prototype of that object

prototype vs. __proto__

• prototype belongs to functions.
• __proto__ belongs to objects.

function Person() {}

console.log(Person.prototype); // Object used for instances
console.log(Person.__proto__); // Prototype of the function object itself

• Person.__proto__ answers: “Who created this function object?”
• Person.prototype answers: “What prototype should objects created by new Person() have?”
• If Person.prototype is not changed, new objects simply inherit from the default Person.prototype object.
• Person.__proto__ is not passed to instances at all. It only determines what the function object (Person) inherits from.

p
│
└── __proto__
      │
      ▼
Person.prototype
      │
      └── __proto__
            │
            ▼
Object.prototype
      │
      └── __proto__
            │
            ▼
           null