proto and Object.getPrototypeOf

JavaScript course · Advanced · Lesson 14/100

Learn an object's prototype with __proto__ and Object.getPrototypeOf.

proto and Object.getPrototypeOf

You can reach an object's prototype in two ways. The legacy way is the __proto__ property; it is now deprecated. The recommended way is the Object.getPrototypeOf(obj) function.

const rex = new Animal('Rex');
Object.getPrototypeOf(rex); // Animal.prototype
rex.__proto__;              // same result, but deprecated

Both return the same object. Still, Object.getPrototypeOf is safer in new code.

Writing the prototype

To read the prototype, use Object.getPrototypeOf. To write it, two safe options exist: create a new object with Object.create(proto), or change an existing one with Object.setPrototypeOf(obj, proto).

const base = { greet() { return 'hello'; } };
const child = Object.create(base);
// child now uses base as its prototype

Avoid assigning to __proto__ directly; you may see it in old code, but modern projects avoid it.

Exercise Compare the prototypes of two objects

Define the Animal constructor: assign the name parameter to this.name. Create two objects a and b with new Animal('Rex') and new Animal('Max'). Print Object.getPrototypeOf(a) === Object.getPrototypeOf(b) with console.log.

Show example solution

function Animal(name) {
  this.name = name;
}
const a = new Animal('Rex');
const b = new Animal('Max');
console.log(Object.getPrototypeOf(a) === Object.getPrototypeOf(b));

Test yourself

Which is the recommended way to read an object's prototype?
- Object.getPrototypeOf(obj)
- obj.__proto__
- obj.prototype
- Object.prototype
Show answer

Object.getPrototypeOf(obj) — Correct! Object.getPrototypeOf is the recommended way.

Read the prototype

const proto = Object.____;

Show answer

const proto = Object.getPrototypeOf(rex);

Match methods to their uses

__proto__

deprecated
Object.getPrototypeOf

recommended read
Object.create

new object + prototype
Object.setPrototypeOf

change existing

proto and Object.getPrototypeOf