The instanceof operator 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.
Syntax
object instanceof constructor
Parameters
object- : The object to test.
constructor- : Constructor to test against.
Exceptions
- TypeError
- : Thrown if
constructoris not an object. Ifconstructordoesn't have a@@hasInstancemethod, it must also be a function.
- : Thrown if
Description
The instanceof operator tests the presence of constructor.prototype in object's prototype chain. This usually (though not always) 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 <span class="createlink">Prototype</span> 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;
For bound functions, instanceof looks up for the prototype property on the target function, since bound functions don't have prototype.
class Base {}
const BoundBase = Base.bind(null, 1, 2);
console.log(new Base() instanceof BoundBase); // true
instanceof and @@hasInstance
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
Because all functions inherit from Function.prototype by default, most of the time, the Function.prototype[@@hasInstance] method specifies the behavior of instanceof when the right-hand side is a function. See the Symbol.hasInstance page for the exact algorithm of instanceof.
instanceof and multiple realms
JavaScript execution environments (windows, frames, etc.) are each in their own realm. 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 in the current realm 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, neglecting which realm it comes from.
For example, to check if a Node is an 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).
Overriding the behavior of instanceof
A common pitfall of using instanceof is believing that, if x instanceof C, then x was created using C as constructor. This is not true, because x could be directly assigned with C.prototype as its prototype. In this case, if your code reads private fields of C from x, it would still fail:
class C {
#value = "foo";
static getValue(x) {
return x.#value;
}
}
const x = { __proto__: C.prototype };
if (x instanceof C) {
console.log(C.getValue(x)); // TypeError: Cannot read private member #value from an object whose class did not declare it
}
To avoid this, you can override the behavior of instanceof by adding a Symbol.hasInstance method to C, so that it does a branded check with in:
class C {
#value = "foo";
static [Symbol.hasInstance](x) {
return #value in x;
}
static getValue(x) {
return x.#value;
}
}
const x = { __proto__: C.prototype };
if (x instanceof C) {
// Doesn't run, because x is not a C
console.log(C.getValue(x));
}
Note that you may want to limit this behavior to the current class; otherwise, it could lead to false positives for subclasses:
class D extends C {}
console.log(new C() instanceof D); // true; because D inherits @@hasInstance from C
You could do this by checking that this is the current constructor:
class C {
#value = "foo";
static [Symbol.hasInstance](x) {
return this === C && #value in x;
}
}
class D extends C {}
console.log(new C() instanceof D); // false
console.log(new C() instanceof C); // true
console.log({ __proto__: C.prototype } instanceof C); // false