Object.prototype.isPrototypeOf()
The isPrototypeOf()
method checks if an object exists in
another object's prototype chain.
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.
Syntax
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
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);
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
}
Specifications
Specification |
---|
ECMAScript Language Specification # sec-object.prototype.isprototypeof |
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