Object.prototype.isPrototypeOf()

The isPrototypeOf() method tests for an object 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.

Description

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.

Specifications

Browser compatibility

See also

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