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.

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.

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:

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

Which will give us the following TypeScript error.

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

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