Why is the 'readonly' property in typescript not working?

Question

interface ReadExample {
    readonly book_id: number,
    book_name: string
}

class Book implements ReadExample{
    book_id= 3;
    book_name = 'A Thousand Stars';
    constructor(book_id:number, book_name:string){
        this.book_id = book_id;
        this.book_name = book_name;
    }

}
let book: Book = new Book(2, 'Sis');
console.log(book);
book.book_name = 'Sister';
book.book_id = 3;
console.log(book);

Why is this not throwing me any error. You see the property book_id is readonly. So why does it not throw an error here when I try to assign, book.book_id = 3? Does it not violate readonly?

BenMcLean981 · Accepted Answer · 2021-06-20 16:56:36Z

4

Because readonly in Typescript is not actually readonly.

I don't fully understand it but you can find a discussion on it in this Github issue: https://github.com/microsoft/TypeScript/issues/13002

The more I learn and grow as a programmer the more I feel theres a lot of crap out there. Readonly is just crappy, it doesn't "just work" the way you'd expect it to.

The best you could do is not do what you're doing here.

answered Jun 20, 2021 at 16:56

BenMcLean981

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1 Comment

Bidisha Das Over a year ago

Haha, yes. Thoroughly enjoyed reading the comments in that GitHub issue. Thank you @BigBadBeny. :)

MayThrow · Accepted Answer · 2023-11-09 16:27:41Z

1

I was able to work around this by using Readonly<T>

class Foo {
   public bar: Readonly<string>
}

fooInstance.bar = 'test'; // <- compile-time error

answered Nov 9, 2023 at 16:27

MayThrow

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