JavaScript instanceof operator

The JavaScript instanceof operator is used to check the type of an object at the run time. The return value is a boolean value. The instanceof operator tests the presence of constructor.prototype in object‘s prototype chain.

var myVar = objectName instanceof objectType

One thing worth mentioning is instanceof evaluates to true if the object inherits from the class’s prototype:

var p = new Person("Jon");
p instanceof Person

That is p instanceof Person is true since p inherits from Person.prototype.

A simple example code checks the current object and returns true if the object is of the specified object type.

<!DOCTYPE html>
<html>
<body>

  <script>

   var color1 = new String("green");
   console.log(color1 instanceof String); 

   var color2 = "coral"; 
   console.log(color2 instanceof String); 

 </script>

</body>
</html>

Output:

The Instanceof operator also takes inheritance into account. It returns true if the object inherits from the classes’ prototype.

Example code returns true if obj belongs to the Class or a class inheriting from it.

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const auto = new Car('Honda', 'Accord', 1998);

console.log(auto instanceof Car);
// true

console.log(auto instanceof Object);
// true

Another example

class Person {
  constructor(name) {
    this.name = name;
  }
}

class Student extends Person {
  constructor(name, grade) {
    super(name);
    this.grade = grade;
  }
}

const john = new Person('John');
const jane = new Student('Jane', 10);

console.log(john instanceof Person);   // true
console.log(john instanceof Student);  // false

console.log(jane instanceof Person);   // true
console.log(jane instanceof Student);  // true

In the example above, we have two classes, Person and Student, where Student is a subclass of Person. By using the instanceof operator, we can determine whether an object is an instance of a particular class or its subclass.

Comment if you have any doubts or suggestions on this JS Operator example.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version

JavaScript instanceof operator | Check type of object