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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