CODEWITHSUNDEEP
genericstypescripttype-safety
Balazs Nemeth
Balazs Nemeth

Reputation: 2332

Typescript generic constraint extends other generic type

I would like to create a reusable network service component, which would be responsible for CRUD requests of an Item.

Let"s say my CatService want to request a list of cats, then it can have a restService instance and it can use it for list, create, update, delete...:

private restService:RestListService<Cat[]> = RestListService();
...
restService.list(urlToGetCats).then(cats => console.log(listdata));
...
restService.update(urlToUpdateACat, updatedCat);

I implemented this generic components, but it not safe enough. The class declaration looks like:

export class RestListService<T extends Array<Identifiable>> {

    private dataSubject$: BehaviorSubject<T> = null;

    list(url: string): Promise<T> { }

    // PROBLEM: I cannot specify the `savingInstance` & the returned Promise type: 
    update(updateURL: string, savingInstance: Identifiable): Promise<Identifiable> { }

}

Ideally, I would do something like introduce a generic V as the type of the items in the array to make the array (and the whole class) more typesafe:

export class RestListService<T extends Array<V extends Identifiable>> {

    //Here the Promise is type safe:
    update(updateURL: string, savingInstance: Identifiable): Promise<V> { }

}

But it's not permitted at the moment (as I see).

Can I somehow solve the type safety in this situation?

Thanks for your help!

Upvotes: 3

Views: 978

Answers (1)

Murat Karag&#246;z
Murat Karag&#246;z

Reputation: 37604

Do you mean like this?

export class RestListService<V extends Identifiable, T extends Array<V>> {

    //Here the Promise is type safe:
    update(updateURL: string, savingInstance: Identifiable): Promise<V> { }

}

Upvotes: 3

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