Typescript resolves type differently when passed as generic

Question

I have the following piece of code

// Address and User are two classes, bothe of which have an id
type Q = Address | User

// This works just fine 
async function c<EntityType extends Q>(ids: Q["id"][]) {
}

// This gives me the error described bellow
async function c<EntityType extends Q>(ids: EntityType["id"][]) {
}

When I in vscode hover over ids in the first function I get (parameter) ids: number[] (Which is expected as Q.id is a number) but if I hover over ids in the second function I get ids: EntityType["id"][]

This is leading to errors downstream... Why is this happening and how can I gix this?

Edit: Adding the aforementioned errors:

In the function I am using the line

  const e = await em.findBy(entityClass, { id: In(ids) });

And typescript complains:

Argument of type '{ id: FindOperator<any>; }' is not assignable to parameter of type 'FindOptionsWhere<EntityType> | FindOptionsWhere<EntityType>[]'.
  Types of property 'id' are incompatible.
    Type 'FindOperator<any>' is not assignable to type 'FindOptionsWhereProperty<NonNullable<EntityType["id"]>>'

On {id: In(ids)} (entityClass is of type EntityType)

I don't see a problem with this tsplay.dev/ND2yjW What exactly is the error you have? — VLAZ
– VLAZ, Commented Oct 11, 2022 at 20:19

Dylan Dang · Accepted Answer · 2022-10-11 21:05:15Z

0

Alright, after searching around, I found that you were using typeorm. Instead of having Q be a union of the classes you define. You should make it an interface in which classes can implement, which I've renamed EntityType based on your generic. The resulting code would be:

interface EntityType {
    id: number;
}

class Address implements EntityType {
    id: number;
    // your code
}

class User implements EntityType {
    id: number;
    // your code
}

async function b<T extends EntityType>(entityClass: EntityTarget<T>, em: EntityManager, ids: number[]): Promise<T[]> {
    const e = await em.findBy(entityClass as EntityTarget<EntityType>, { id: In(ids) });
    return e as T[];
}

edited Oct 11, 2022 at 21:05

answered Oct 11, 2022 at 20:46

Dylan Dang

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3 Comments

sev Over a year ago

Will this also work if I do: async function b<T extends EntityType>(entityClass: EntityTarget<T>, em: EntityManager, ids: number[]): Promise<T[]> {

Dylan Dang Over a year ago

hmm not sure why typeorms type declarations seem to complain when the input is generic. However, it should be safe to cast it within the function so typescript can stop complaining. I'll update my answer accordingly

sev Over a year ago

When I do this it does unfortunately not remove the error I am getting...

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