# TypeScript Readonly Utility type

This is the last of the commonly known interface modifying utility types.
Do note there are a few more, but we'll get to those in a later stage as they are a bit more advanced.

I want to go over the `Readonly` utility type in this article.

Using the read-only type, you can transform a type to be read-only, making it impossible to change after the initial assignment.

## Using the Readonly Utility type

Let's retake this user interface.

```js
interface User {
  id?: number;
  firstname: string;
  lastname: string;
  age?: number;
}
```

If we would now assign some information to this object, we could always re-assign it later in our code.

```js
const user: User = {
  firstname: 'Chris',
  lastname: 'Bongers',
};
user.id = 123;
```

We can now modify any of the existing properties to be a new value.

And we don't always want that.

So in order to prevent this from happening you can wrap the type used in a `Readonly` type like so:

```js
const user: Readonly<User> = {
  firstname: 'Chris',
  lastname: 'Bongers',
};
```

Which will give us the following TypeScript error.

![TypeScript readonly type](https://cdn.hashnode.com/res/hashnode/image/upload/v1644730309446/Ep9udmmle.png)

This Readonly type can be super helpful to represent frozen objects.
Or objects that should not mutate on their own.

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