TypeScript helps us keep a contract with our vales by allowing us to define fine-grained type checking for them. Interfaces is the primary mechanism in TypeScript to accomplish that.

Let’s say for example that we have the following code where we define two users:

const user1 = {
  id: 1,
  firstName: 'John',
  lastName: 'Doe',
  proUser: false,
  email: 'john.doe@foobar.com'
}

const user2 = {
  firstName: 'Jane',
  lastName: 'Doe',
  proUser: true,
  email: 'jane.doe@foobar.com'
}

Let’s establish a contract so that our user objects respect a specified type definition:

interface User {
  id: number;
  firstName: string;
  lastName: string;
  proUser: boolean;
  email: string;
}

const user1: User = {
  id: 1,
  firstName: 'John',
  lastName: 'Doe',
  proUser: false,
  email: 'john.doe@foobar.com'
}

const user2: User = {
  id: 2,
  firstName: 'Jane',
  lastName: 'Doe',
  proUser: true,
  email: 'jane.doe@foobar.com'
}

Now if we try to define User objects with missing properties, additional properties or with properties of the wrong type, the TypeScript compiler will complain and tell us exactly what’s wrong with our user definition.

You can define optional properties in your interfaces with the Elvis operator (?):

interface User {
  id: number;
  firstName: string;
  lastName: string;
  proUser: boolean;
  email: string;
  avatar?: string;
}

const user1: User = {
  id: 1,
  firstName: 'John',
  lastName: 'Doe',
  proUser: false,
  email: 'john.doe@foobar.com'
}

const user2: User = {
  id: 2,
  firstName: 'Jane',
  lastName: 'Doe',
  proUser: true,
  email: 'jane.doe@foobar.com',
  avatar: 'https://something/my-face.jpg'
}

Notice how we didn’t have to specify any avatar value for user1, and TypeScript won’t complain.

Your interfaces can be defined in their own files, and then imported with the ES6 module syntax:

import { User } from '../interfaces/user';

const user1: User = {
  id: 1,
  firstName: 'John',
  lastName: 'Doe',
  proUser: false,
  email: 'john.doe@foobar.com'
}

// ...