Typescript: Interface
Tags: typescript, webdev, beginners, tutorial
Published: 2023-05-10 17:10:52

In this article, you'll learn about what interface are and how to use them, along with the difference between type and interface. This article gives you a basic understanding of the interface.

> This is going to be a full series of typescript where you will learn from basic topics like string, boolean to more complex like Type Aliases, enums, Interface, generics, and etc.

### Table of Contents
{%- # TOC start -%}
- [Interface](#interface)
- [Interface vs Type](#interface-vs-type)
  * [Adding new fields](#adding-new-fields)
  * [Extends](#extends)
  * [Union](#union)
{%- # TOC end -%}

## Interface

An *interface declaration* is another way to name an object type. You can create it by using the `interface` keyword:

```ts
interface User {
	name: string,
	age: number,
}

// ✅ CORRECT
let newUser : User = {name: "John", age: 28};
// ❌ ERROR: property 'age' is missing
let newUser : User = {name: "John"};
// ❌ ERROR: missing the following properties from type 'User': name, age 
let newUser : User = {};
```

You can also use `readonly` and optional approach in `interface`:

```ts
interface User {
	readonly id: number 	// readonly variable
	name: string,
	age: number,
	specialKey? : string, 	// optional 
}
```

You can also pass functions to the `interface`, there are two ways to do that:

```ts
// Method-1
interface User {
	getDiscount(coupon: string): number    	
}

// For Both you need to call this like this:
const newUser: User = {
	getDiscount: (coupon: "KIJ298DD9J") => {
		return 10;
	}
}

// Method-2
interface User {
	getDiscount: (coupon: string) => number
}

// 👉 You see I have changed the 'coupon' to 'couponName'
// You don't need to match the name of the parameter here
// It will take care of it
const newUser: User = {
	getDiscount: (couponName: "KIJ298DD9J") => {
		return 10;
	}
}
```

In **Method 1** you can simply use the `()` to say it functions like: `getDiscount(): number` and string are the return types, and it takes no arguments.

In **Method 2** we use an arrow function like `getDiscount: () => number`.

## Interface vs Type

Type aliases and interfaces are very similar, and in many cases, you can choose between them freely. Almost all features of an `interface` are available in `type` The key distinction is that a type cannot be reopened to add new properties vs. an interface that is always extendable.

Let’s Differentiate them with a few examples:

### Adding new fields

In `interface` you can add new fields to the existing interface, but you cannot add new fields to an existing type. It will throw an error.

**Interface**

```typescript
interface User {
    id: string;    
    email: string;
}

interface User {
    name: string;
}

// Now you can add all the three values to the User interface
const user: User = {
    id: "2323232",
    email: "foo@email.com",
    name: "Foo";
}
```

**Type**

```typescript
type User = {
    id: string;
}

type User = {
    email: string;
}

// ❌ ERROR: Duplicate identifier "User"
```

In `interface` you can add new fields and change them however you want, but in `type` you can't do that. Once a `type` is created, it can't be changed.

### Extends

To extend the already defined `interface` or `type` both have a different approach. `interface` uses `extends` keyword, while `type` uses intersection.

**interface**

```typescript
interface Car {
    model: string;
    color: string;
}

// 👇 You can extend an interface using 'extends' keywords
interface Tesla extends Car {
  autoPilotModelName: string;
};

// ✅ Use Case
const newCar: Tesla = {
    model: "S",
    color: "red",
    autoPilotModelName: "xyz"
}
```

**type**

```typescript
type Car = {
    model: string;
    color: string;
}
// 👇 In type you need to use Intersection
type Tesla = Car & {
  autoPilotModelName: string;
};

const newCar: Tesla = {
    model: "S",
    color: "red",
    autoPilotModelName: "xyz"
}
```

### Union

In `interface` you cannot create a union type, but you can do that if you are using `type`. Let's take an example:

**interface**

```typescript
interface User  {
    email: string;
}

interface Admin  {
    email: string;
    adminKey: string;
}

// ❌ ERROR: '{' expected.
interface Person = User | Admin;

// ✅ CORRECT: you can create union type like this
type Person = User | Admin;

// ✅ CORRECT: However you can use union type inside the interface
interface Person {
    person: User | Admin;
}
```

**type**

```typescript
type User = {
    email: string;
}

type Admin = {
    email: string;
    adminKey: string;
}

// ✅ You can do that.
type Person = User | Admin;
```

In the above example, you might have noticed that when I tried to assign a union type to the interface (`interface Person = User | Admin`), it threw an error. It's because you cannot assign anything to `interface`. Its deceleration syntax is similar to `class`. But both have different working styles.

```ts
class MyClass {}

interface Hello {}
```

### Wrapping up

In this article, I have explained what `interface` is and how to use it, along with the difference between `type` and `interface`. This article gives you a basic understanding of the interface.

This is a series of Typescript lessons that will help you learn Typescript from scratch. If you enjoyed this article, then don't forget to give ❤️ and bookmark 🏷️for later use and if you have any questions or feedback then don't hesitate to drop them in the comments below. I'll see you in the next one.

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