object

The object whose prototype chain will be searched.

A boolean indicating whether the calling object (this) lies in the prototype chain of object. Directly returns false when object is not an object (i.e. a primitive).

TypeError

Thrown if this is null or undefined (because it can't be converted to an object).

All objects that inherit from Object.prototype (that is, all except null-prototype objects) inherit the isPrototypeOf() method. This method allows you to check whether or not the object exists within another object's prototype chain. If the object passed as the parameter is not an object (i.e. a primitive), the method directly returns false. Otherwise, the this value is converted to an object, and the prototype chain of object is searched for the this value, until the end of the chain is reached or the this value is found.

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

class Foo {}
class Bar extends Foo {}
class Baz extends Bar {}

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
}

However, Foo.prototype existing in baz's prototype chain doesn't imply baz was created using Foo as its constructor. For example, baz could be directly assigned with Foo.prototype as its prototype. In this case, if your code reads private fields of Foo from baz, it would still fail:

class Foo {
  #value = "foo";
  static getValue(x) {
    return x.#value;
  }
}

const baz = { __proto__: Foo.prototype };

if (Foo.prototype.isPrototypeOf(baz)) {
  console.log(Foo.getValue(baz)); // TypeError: Cannot read private member #value from an object whose class did not declare it
}

The same applies to instanceof. If you need to read private fields in a secure way, offer a branded check method using in instead.

class Foo {
  #value = "foo";
  static getValue(x) {
    return x.#value;
  }
  static isFoo(x) {
    return #value in x;
  }
}

const baz = { __proto__: Foo.prototype };

if (Foo.isFoo(baz)) {
  // Doesn't run, because baz is not a Foo
  console.log(Foo.getValue(baz));
}

• instanceof
• Object.getPrototypeOf()
• Object.setPrototypeOf()
• Inheritance and the prototype chain