`instanceof` Keyword

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).

instanceof Keyword