TypeScript

Type defintion (User with name and age)

type User = {
	name: string,
	age: number
}

Interface definition (Admin with name and role)

interface Admin {
	name: string;
	age: number;
}

Union of two types/interfaces (Person from Admin and User)

type Person = Admin | User

Checking if it is a certain type? Come back to this

A few options:

1. Check for a unique property

   1. example:

      if ('role' in Person)

2. Use instanceOf - this only works with classes

   1. example:

      const userAccount = login(); 
      if (userAccount instanceof FreeAccount) {
        console.log('Welcome to the free account!');

3. Use typeof

   1. example:

Using type predicates

To define a user-defined type guard, we simply need to define a function whose return type is a type predicate

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

pet is Fish is the type predicate.