Generics T extends U but Readonly does not

Question

IMHO, this is not a duplicate of this question. Here is a minimal example:

class Indexer {
  private readonly someIndex = 200;
  public getIndex() { return this.someIndex; }
}

class Person extends Indexer {}
class Something<T extends Indexer> {
  s = new Map<number, T>();
  insert(t: T){ /* ... */ }
}

const c1 = new Something<         Person >();
const c2 = new Something<Readonly<Person>>();

On one hand my class needs to subclass Indexer
On the other hand it needs to be Readonly

I can not achieve the two properties together:

$ npx tsc --target es2020 main.ts
main.ts:13:26 - error TS2344: Type 'Readonly<Person>' does not satisfy the constraint 'Indexer'.
  Property 'someIndex' is missing in type 'Readonly<Person>' but required in type 'Indexer'.

13 const c2 = new Something<Readonly<Person>>();
                            ~~~~~~~~~~~~~~~~

  main.ts:2:20
    2   private readonly someIndex = 200;
                         ~~~~~~~~~
    'someIndex' is declared here.

Does just making `Something` like `class Something>` work for you? — wonderflame, Aug 25 '23 at 12:48

score 1 · Accepted Answer · answered Aug 25 '23 at 12:54

That doesn't work because Readonly is a mapped type and mapped types are dropping the private keys.

Thus the error you get :

Type 'Readonly<Person>' does not satisfy the constraint 'Indexer'.
  Property 'someIndex' is missing in type 'Readonly<Person>' but required in type 'Indexer'.

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Generics T extends U but Readonly does not

1 Answers1