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

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Note: 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.





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


A TypeError is thrown if prototypeObj is undefined or null.


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


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);

const foo = new Foo();
const bar = new Bar();
const baz = new Baz();

// prototype chains:
// foo: Foo <- Object
// bar: Bar <- Foo <- Object
// baz: Baz <- Bar <- Foo <- Object
console.log(Baz.prototype.isPrototypeOf(baz));    // true
console.log(Baz.prototype.isPrototypeOf(bar));    // false
console.log(Baz.prototype.isPrototypeOf(foo));    // false
console.log(Bar.prototype.isPrototypeOf(baz));    // true
console.log(Bar.prototype.isPrototypeOf(foo));    // false
console.log(Foo.prototype.isPrototypeOf(baz));    // true
console.log(Foo.prototype.isPrototypeOf(bar));    // true
console.log(Object.prototype.isPrototypeOf(baz)); // true

The isPrototypeOf() method — along with the instanceof operator — comes in particularly 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, to execute some code that's only safe to run if a baz object has Foo.prototype in its prototype chain, you can do this:

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


ECMAScript Language Specification
# sec-object.prototype.isprototypeof

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See also