Object.getPrototypeOf()

getPrototypeOf(obj)

The Object.getPrototypeOf() method returns the prototype of the specified object. This gives you access to an object’s internal [[Prototype]], which determines the object’s inheritance chain. Understanding prototypes is fundamental to JavaScript’s object-oriented features.

Syntax

Object.getPrototypeOf(obj)

Parameters

Parameter	Type	Description
obj	object	The object whose prototype to return

Examples

Basic prototype retrieval

const proto = { greet() { return 'hello'; } };
const obj = Object.create(proto);

console.log(Object.getPrototypeOf(obj) === proto); // true

With a plain object

const user = { name: 'Alice' };
console.log(Object.getPrototypeOf(user) === Object.prototype); // true

With an array

const numbers = [1, 2, 3];
console.log(Object.getPrototypeOf(numbers) === Array.prototype); // true

With null prototype

const nullProto = Object.create(null);
console.log(Object.getPrototypeOf(nullProto)); // null

Checking class inheritance

class Animal {
  constructor(name) {
    this.name = name;
  }
}

const dog = new Animal('Rex');
console.log(Object.getPrototypeOf(dog) === Animal.prototype); // true

Common Patterns

Verify object type without instanceof

function isArrayLike(obj) {
  return Object.getPrototypeOf(obj) === Array.prototype;
}

console.log(isArrayLike([1, 2, 3])); // true
console.log(isArrayLike({ length: 3 })); // false

Walk the prototype chain

const grandparent = { a: 1 };
const parent = Object.create(grandparent);
const child = Object.create(parent);

let current = child;
const chain = [Global_Objects::eval];
while (current) {
  chain.push(current);
  current = Object.getPrototypeOf(current);
}

console.log(chain.length); // 3

See Also

• Object.create() — Create object with specified prototype
• Object.keys() — Get array of object keys