TypeScript does not infer type

Question

I have the following function:

type DefaultEntity = {
  id: string;
  createdBy: string;
  [fieldName: string]: unknown;
};

abstract find<T, Schema extends string | (new () => T)>(
    schema: Schema,
    id: string,
): Promise<(Schema extends string ? DefaultEntity : T) | null>;

When I call it with a string as schema the return type is DefaultEntity. However if I use MyClass for the schema argument the return type is unknown not MyClass. It should be MyClass because T becomes MyClass. Did I miss something or is this something TypeScript just cannot do?

Answer 1

To infer the type of Schema you don't need an additional T type parameter. You just have to infer the instance-type of the class either by using the infer T keyword or using the built in helper InstanceType<Class>. Like this:

type DefaultEntity = {
  id: string;
  createdBy: string;
  [fieldName: string]: unknown;
};

abstract class Base {
    // The first way
    abstract find<Schema extends string | (new () => any)>(
        schema: Schema,
        id: string,
    ): Promise<(Schema extends (new () => infer T) ? T : DefaultEntity) | null>;

    // this also works
    abstract find2<Schema extends string | (new () => any)>(
        schema: Schema,
        id: string,
    ): Promise<(Schema extends (new () => any) ? InstanceType<Schema> : DefaultEntity) | null>;
}

class UserEntity {
    id: string = ''
    createdBy: string = ''
    name: string = ''
}

class BaseImpl extends Base {
    async a () {
        (await this.find('asdfqer' , 'asdfasd')) satisfies DefaultEntity | null;
        (await this.find(UserEntity, 'asdf') ) satisfies UserEntity | null;
    }
}

Note: it's better to use overloads or even separate methods, i.e findByName(string, string) and findByEntity(Schema, string), it would be much easier to maintain and read in the future.