Web/JavaScript/Reference/Global objects/Object/isPrototypeOf

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

isPrototypeOf() differs from the instanceof operator. In the expression "object instanceof AFunction", the object prototype chain is checked against AFunction.prototype, not against AFunction itself.

Syntax

prototypeObj.isPrototypeOf(object)

Parameters

object: The object whose prototype chain will be searched.

Return value

A Boolean indicating whether the calling object lies in the prototype chain of the specified object.

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

Using isPrototypeOf

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
ECMAScript (ECMA-262)The definition of 'Object.prototype.isPrototypeOf' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`isPrototypeOf`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 9	Opera Full support 4	Safari Full support 3	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support Yes

Legend

Full support: Full support