How to extend types in Typescript?

Typescript Faysal Shuvo

Problem: 

I have created a type in my TypeScript project. Now I want to add more to this type.

type Book = {
   name: string;
   releaseDate: Date;
   genre: string;
}

 I want to extend the type like this: 

type BookPrice extends Book = {
   price: number; 
}

But this is giving me an error.

In this article, we are going to learn how to extend types in Typescript?


Solution 1: 

The first mistake you are doing is using extends with type. The extends keyword only works with interfaces and classes. First, you can define the type like this: 

type Book = {
    name: string;
    releaseDate: Date;
    genre: string;
}

 Now you can extend the Book type using the interface.

interface BookPrice extends Book {
   price: number; 
}

It will not work the other way round BookPrice must be declared as interface if you want to use extends syntax.


Solution 2:

You can also extend your type without using extends keyword. To do this you can use intersection type:

type LastName = {
    lastName: string;
};
type FirstName = {
    firstName: string;
};
type FullName = LastName & FirstName;

Now, you can use this extended  type anywhere you want in your code.

const authorName: FullName = {
    lastName: 'Stephen',
    firstName: 'King',
};

Thank you for reading the article. If you face any problems please comment below.