README.md

Object types (and interfaces)

Often in JavaScript, you have something like this:

// JavaScript
const person = {
  name: 'Alice',
  age: 25,
  isStudent: true,
};

This is how we can type it in TypeScript:

// TypeScript

// Person is an object type
type Person = {
  name: string;
  age: number;
  isStudent: boolean;
};

const person: Person = {
  name: 'Alice',
  age: 25,
  isStudent: true,
};

The syntax for object types is similar to JavaScript object literals, except that each key maps to a type instead of a value.

Notice the similarity between the two:

// Notice that the object type and the object literal look similar
type User = {
  name: string;
  age: number;
}

const user: User = {
  name: 'Alice',
  age: 25,
}

Interfaces

TypeScript also allows you to define an interface. This would be familiar for those coming from object-oriented languages like Java or C#.

Interfaces are essentially object types, but with a different syntax:

// Interface
interface User {
  name: string;
  age: number;
}

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

The two are mostly equivalent and essentially interchangable. You may use the syntax that you prefer.

Note: There is a small difference between the two, which we will cover in an advanced topic. Only interfaces can be used for declaration merging and module augmentation. Until then, you may ignore this.

Extending types/interfaces

You can extend interfaces by using the extends keyword:

type User {
  name: string;
  age: number;
}

interface UserWithRole extends User {
  role: string;
}
// ^ UserWithRole: { name: string, age: number, role: string }

You can also extend types using &:

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

type UserWithRole = User & {
  role: string;
}
// ^ UserWithRole: { name: string, age: number, role: string }