instanceof Keyword
| Site: | Saylor Academy |
| Course: | PRDV401: Introduction to JavaScript I |
| Book: |
instanceof Keyword
|
| Printed by: | Guest user |
| Date: | Wednesday, May 14, 2025, 10:54 AM |
instanceof Keyword
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.
Try It
Click here to try an example using Code Runner.
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).