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instanceof

The instanceof tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value. Its behavior can be customized with Symbol.hasInstance.

Try it

Syntax

object instanceof constructor

Parameters

object: The object to test.
constructor: Constructor to test against.

Description

The instanceof operator tests the presence of constructor.prototype in object's prototype chain. This usually means object was constructed with constructor.

// defining constructors
function C() {}
function D() {}

const o = new C();

// true, because: Object.getPrototypeOf(o) === C.prototype
o instanceof C

// false, because D.prototype is nowhere in o's prototype chain
o instanceof D

o instanceof Object           // true, because:
C.prototype instanceof Object // true

// Re-assign `constructor.prototype`: you should
// rarely do this in practice.
C.prototype = {};
const o2 = new C();

o2 instanceof C  // true

// false, because C.prototype is nowhere in
// o's prototype chain anymore
o instanceof C

D.prototype = new C()  // add C to [[Prototype]] linkage of D
const o3 = new D();
o3 instanceof D        // true
o3 instanceof C        // true since C.prototype is now in o3's prototype chain

Note that the value of an instanceof test can change if constructor.prototype is re-assigned after creating the object (which is usually discouraged). It can also be changed by changing object's prototype using Object.setPrototypeOf.

Classes behave in the same way, because classes also have the prototype property.

class A {}
class B extends A {}

const o1 = new A();
// true, because Object.getPrototypeOf(o1) === A.prototype
o1 instanceof A
// false, because B.prototype is nowhere in o1's prototype chain
o1 instanceof B

const o2 = new B();
// true, because Object.getPrototypeOf(Object.getPrototypeOf(o2)) === A.prototype
o2 instanceof A
// true, because Object.getPrototypeOf(o2) === B.prototype
o2 instanceof B

If constructor has a Symbol.hasInstance method, the method will be called in priority, with object as its only argument and constructor as this.

// This class allows plain objects to be disguised as this class's instance,
// as long as the object has a particular flag as its property.
class Forgeable {
  static isInstanceFlag = Symbol("isInstanceFlag");

  static [Symbol.hasInstance](obj) {
    return Forgeable.isInstanceFlag in obj;
  }
}

const obj = { [Forgeable.isInstanceFlag]: true };
console.log(obj instanceof Forgeable); // true

`instanceof` and multiple contexts (e.g. frames or windows)

Different scopes have different execution environments. This means that they have different built-ins (different global object, different constructors, etc.). This may result in unexpected results. For instance, [] instanceof window.frames[0].Array will return false, because Array.prototype !== window.frames[0].Array.prototype and arrays inherit from the former.

This may not make sense at first, but for scripts dealing with multiple frames or windows, and passing objects from one context to another via functions, this will be a valid and strong issue. For instance, you can securely check if a given object is, in fact, an Array using Array.isArray(myObj)

For example, checking if a Node is a SVGElement in a different context, you can use myNode instanceof myNode.ownerDocument.defaultView.SVGElement.

Examples

Using instanceof with String

The following example shows the behavior of instanceof with String objects.

const literalString = 'This is a literal string';
const stringObject = new String('String created with constructor');

literalString instanceof String;  // false, string primitive is not a String
stringObject  instanceof String;  // true

literalString instanceof Object;  // false, string primitive is not an Object
stringObject  instanceof Object;  // true

stringObject  instanceof Date;    // false

Using instanceof with Date

The following example shows the behavior of instanceof with Date objects.

const myDate = new Date();

myDate instanceof Date;      // true
myDate instanceof Object;    // true
myDate instanceof String;    // false

Objects created using Object.create()

The following example shows the behavior of instanceof with objects created using Object.create()

function Shape() {
}

function Rectangle() {
  Shape.call(this); // call super constructor.
}

Rectangle.prototype = Object.create(Shape.prototype);

Rectangle.prototype.constructor = Rectangle;

const rect = new Rectangle();

rect instanceof Object; // true
rect instanceof Shape;  // true
rect instanceof Rectangle; // true
rect instanceof String; // false

const literalObject = {};
const nullObject = Object.create(null);
nullObject.name = "My object";

literalObject instanceof Object; // true, every object literal has Object.prototype as prototype
({}) instanceof Object; // true, same case as above
nullObject instanceof Object; // false, prototype is end of prototype chain (null)

Demonstrating that `mycar` is of type `Car` and type `Object`

The following code creates an object type Car and an instance of that object type, mycar. The instanceof operator demonstrates that the mycar object is of type Car and of type Object.

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const mycar = new Car('Honda', 'Accord', 1998);
const a = mycar instanceof Car; // returns true
const b = mycar instanceof Object; // returns true

Not an instanceof

To test if an object is not an instanceof a specific constructor, you can do

if (!(mycar instanceof Car)) {
  // Do something, like:
  // mycar = new Car(mycar)
}

This is really different from:

if (!mycar instanceof Car) {
  // unreachable code
}

This will always be false. (!mycar will be evaluated before instanceof, so you always try to know if a boolean is an instance of Car).

Specifications

Specification
ECMAScript Language Specification # sec-relational-operators

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	Deno	Node.js
`instanceof`	1	12	1	5	4	1	4.4	18	4	10.1	1	1.0	1.0	0.10.0