The JavaScript instanceof operator is used to find out the type of objects or values you have in your code.
The instanceof syntax is as follows:
object instanceof Object;
// returns a boolean: true or false
The instanceof will test if the Object constructor is present in the object’s prototype chain.
The example code below shows how you can use the instanceof method to test the Person object instance:
class Person {}
let person = new Person();
console.log(person instanceof Person); // true
console.log(person instanceof Object); // true
console.log(person instanceof Boolean); // false
Because the person variable’s value is an instance of both Person and Object classes, the instanceof operator returns true.
But the person object is not an instance of the Boolean class, so the operator returns false.
The instanceof operator also works when you create an object from a function as shown below:
function Dog() {}
let dog = new Dog();
console.log(dog instanceof Dog); // true
As you can see, the dog variable is an instance of Dog, so the instanceof operator returns true.
JavaScript instanceof vs typeof
JavaScript also has the typeof operator that can be used to check the type of values you define in your code.
The typeof operator returns a string representing the type of the value. It is commonly used for built-in JavaScript types as shown below:
console.log(typeof "Hello"); // "string"
console.log(typeof 123); // "number"
The instanceof and typeof operators work in different ways.
The instanceof returns either true or false while the typeof returns the actual type name of the value.
In general, the instanceof is used for testing advanced and custom types, while typeof is used for common JavaScript types.
The code below shows the difference between the two operators:
class Person {}
let person = new Person();
console.log(person instanceof Person); // true
console.log(typeof person); // "object"
console.log("Hello" instanceof String); // false
console.log(typeof "Hello"); // "string"
console.log(typeof "Hello" == "string"); // true
console.log(true instanceof Boolean); // false
console.log(typeof true); // "boolean"
console.log(true instanceof "boolean"); // ERROR
Now you’ve learned how the instanceof operator works in JavaScript, as well as the difference between the instanceof and typeof operators.
I hope this tutorial has been useful for you. 🙏