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The isPrototypeOf() method tests for an object 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.




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.


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

For example, consider the following prototype chain:

function Fee() {
  // ...

function Fi() {
  // ...
Fi.prototype = new Fee();

function Fo() {
  // ...
Fo.prototype = new Fi();

function Fum() {
  // ...
Fum.prototype = new Fo();

Later on down the road, if you instantiate Fum and need to check if Fi's prototype exists within the Fum prototype chain, you could do this:

var fum = new Fum();
// ...

if (Fi.prototype.isPrototypeOf(fum)) {
  // do something safe

This, 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.


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

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

Document Tags and Contributors

 Last updated by: Sheppy,