object.isPrototypeOf

The isPrototypeOf() method checks if an object exists in another object's prototype chain.

Syntax

prototypeObj.isPrototypeOf(object)

Parameters

object

The object whose prototype chain will be searched.

Return value

A Boolean indicating whether the calling object has the specified object in its prototype chain.

Errors thrown

TypeError

A TypeError is thrown if prototypeObj is undefined or null.

Description

The isPrototypeOf() method allows you to check whether or not an object exists within another object's prototype chain.

Examples

This example demonstrates that Baz.prototype, Bar.prototype, Foo.prototype and Object.prototype exist in the prototype chain for object baz:

function Foo() {}
function Bar() {}
function Baz() {}

Bar.prototype = Object.create(Foo.prototype);
Baz.prototype = Object.create(Bar.prototype);

var baz = new Baz();

console.log(Baz.prototype.isPrototypeOf(baz)); // true
console.log(Bar.prototype.isPrototypeOf(baz)); // true
console.log(Foo.prototype.isPrototypeOf(baz)); // true
console.log(Object.prototype.isPrototypeOf(baz)); // true

isPrototypeOf() method, along with the instanceof operator particularly comes in handy if you have code that can only function when dealing with objects descended from a specific prototype chain, e.g., to guarantee that certain methods or properties will be present on that object.

For example, check if baz object descends from Foo.prototype:

if (Foo.prototype.isPrototypeOf(baz)) {
  // do something safe
}

Specifications

Specification	Status	Comment
ECMAScript 3rd Edition (ECMA-262)	Standard	Initial definition.
ECMAScript 5.1 (ECMA-262) The definition of 'Object.prototype.isPrototypeOf' in that specification.	Standard
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Object.prototype.isPrototypeOf' in that specification.	Standard
ECMAScript 2017 Draft (ECMA-262) The definition of 'Object.prototype.isPrototypeOf' in that specification.	Draft

Browser compatibility

See also

• instanceof
• Object.getPrototypeOf()
• Object.setPrototypeOf()
• Object.prototype.__proto__