Assigning child class to a parent class typed property

问题

I do have the following 2 base clases:

class BaseModel {}

class BaseService{
  protected model:BaseModel;
}

Now I want to implement BaseHelper and BaseService for a specific use case and assign a derived class to my property.

class MyModel extends BaseModel{
  constructor(param:string){
    super();
  }
}

class MyService extends BaseService {
  model = MyModel;  
}

However, this gives me the error Type 'typeof MyModel' is not assignable to type 'BaseModel'.

Important: I want to attach the class MyModel, not an instance of the class MyModel!

回答1:

You need to instantiate MyModel using the new keyword (new MyModel()).
You assigned the actual class (model = MyModel) instead of an instance of it.

Also, you might want to make BaseService generic:

class BaseModel {}

class BaseService<T extends BaseModel> {
    protected model: T;

    constructor(model: T) {
        this.model = model;
    }
}

class MyModel extends BaseModel{}

class MyService extends BaseService<MyModel> {
    constructor() {
        super(new MyModel());
    }
}

(code in playground)

Edit

If you need the class and not the instance, then something like:

class BaseModel {}

type BaseModelConstructor = { new(): BaseModel };

class BaseService {
    protected modelCtor: BaseModelConstructor;
}

class MyModel extends BaseModel {}

class MyService extends BaseService {
    modelCtor = MyModel;  
}

(code in playground)

Or you can use generics here as well:

class BaseModel {}

type BaseModelConstructor<T extends BaseModel> = { new(): T };

class BaseService<T extends BaseModel> {
    protected modelCtor: T;
}

class MyModel extends BaseModel {}

class MyService extends BaseService<BaseModel> {
    modelCtor = MyModel;  
}

(code in playground)

If your derived classes have different ctor signatures then you can either deal with it in the base ctor type:

type BaseModelConstructor<T extends BaseModel> = { new(...args: any[]): T };

Here you can pass any count and kind of parameters, but you can also supply different signatures:

type BaseModelConstructor<T extends BaseModel> = { 
    new(): T;
    new(str: string): T;
    new(num: number, bool: boolean): T;
};

But you can also use a different type per derived class:

type MyModelConstructor = { new(param: string): MyModel };

回答2:

model should be an instance of MyModel:

class MyModel extends BaseModel{}

class MyService extends BaseService {
  model = new MyModel()
}

回答3:

Use instance of BaseModel

class MyService extends BaseService {
  model = new MyModel();  
}

来源：https://stackoverflow.com/questions/38540956/assigning-child-class-to-a-parent-class-typed-property

标签

javascript

inheritance

typescript