Learn React with TypeScript 3

Using the instanceof keyword

The instanceof keyword is another JavaScript keyword. It checks whether an object has a particular constructor function. It is typically used to determine whether an object is an instance of a class.

Let's go through an example:

1. We have two classes representing Person and Company:

class Person {  id: number;  firstName: string;  surname: string;}class Company {  id: number;  name: string;}

2. We also have a union type combining both of these classes:

type PersonOrCompany = Person | Company;

3. We now need to write a function that takes in a Person or Company and outputs their name to the console:

function logName(personOrCompany: PersonOrCompany) {  if (personOrCompany instanceof Person) { console.log(`${personOrCompany.firstName} ...