What is wrong with Inheritance in Typescript?

Question

What is wrong in inheritance in Typescript?

i create this class:

export class ExtendedService<T extends ExtendedEntity> {
    protected repository: Repository<T>;

    constructor(
        repository?: Repository<T>
    ){
        this.repository = repository;
    }

    public async create(data: DeepPartial<T>): Promise<T> {
        const entity: T = this.repository.create(data);
        return entity.save();
    }
}

and extends this class in this one:

@Injectable()
export class UserService extends ExtendedService<UserEntity> {

    constructor(
        protected readonly repository: Repository<UserEntity>
    ){
        super();
    }

    public async register(data: DeepPartial<UserEntity>): Promise<UserEntity> {
        const user = new UserEntity();
        user.createdAt = new Date();
        user.firstName = data.firstName;
        user.lastName = data.lastName;
        user.email = data.email;
        user.role = UserRole.USER;
        user.isDeleted = false;
        
        const salt = await genSalt(10);
        user.password = await hash(data.password, salt);

        return await this.create(user);
    }
}

but after launch this function, my console throw me this:

[Nest] 19980   - 2020-06-29 17:30:56   [ExceptionsHandler] Cannot read property 'create' of undefined +1378ms
TypeError: Cannot read property 'create' of undefined
    at Repository.create (C:\Users\Piotruś\Desktop\auth\node_modules\typeorm\repository\Repository.js:49:29)
    at UserService.create (C:\Users\Piotruś\Desktop\auth\dist\helpers\service\extended-service.js:27:40)
    at UserService.register (C:\Users\Piotruś\Desktop\auth\dist\user\user.service.js:34:27)
    at async UserController.register (C:\Users\Piotruś\Desktop\auth\dist\user\user.controller.js:25:9)
    at async C:\Users\Piotruś\Desktop\auth\node_modules\@nestjs\core\router\router-execution-context.js:46:28
    at async C:\Users\Piotruś\Desktop\auth\node_modules\@nestjs\core\router\router-proxy.js:9:17

so, the problem is in ts inheritance, can anybody tell me what is wrong with my inheritance?

Answer 1

You're not passing the repository parameter down to your base class. You must supply all base class arguments when you call super() in your constructor.

@Injectable()
export class UserService extends ExtendedService<UserEntity> {

    constructor(
        protected readonly repository: Repository<UserEntity>
    ){
        super(repository);
    }
...
}

As you can see in this recreation , making the baseclass repository argument mandatory, causes Typescript to throw a compilation error:

Expected 1 arguments, but got 0.

I would recommend you to avoid making arguments optional unless it's necessary, and if you do, take precautions to make the necessary null checks.