There is a non-public library I haven't worked with in a while, and recently I tried using it in a TypeScript project that is targeting es2022. Several classes that used to transpile file are now throwing TypeScript error:

TS2612: Property goods will overwrite the base property in RecBase. If this is intentional, add an initializer. Otherwise, add a declare modifier or remove the redundant declaration.

I understand the suggested solutions, but neither are valid for my situation; I need to be able to instantiate the base class as well as the subclass which rules out abstract. From my limited TypeScript knowledge, declare will prevent the compiler from generating JS code for the subclass property.

I'm stuck. Any ideas on how I can solve this (other than targeting something before es2022)?

Here's a complete example that illustrates my predicament:

//  RecBase and ListBase cannot be abstract classes (i.e., I need to be able to create instances of them)
class RecBase {
  id: string
  goods: ListBase
}

class ListBase {
  lineNumber: number
}

//  This pair of subclasses extends the base classes with additional, domain specific properties
class RecOrder extends RecBase {
  orderID: string

  //  Note: I want to overwrite the base class goods, I don't want 'goods2' or 'goodsEx'
  goods: OrderList
}

class OrderList extends ListBase {
  price: number
}

There is a non-public library I haven't worked with in a while, and recently I tried using it in a TypeScript project that is targeting es2022. Several classes that used to transpile file are now throwing TypeScript error:

TS2612: Property goods will overwrite the base property in RecBase. If this is intentional, add an initializer. Otherwise, add a declare modifier or remove the redundant declaration.

I understand the suggested solutions, but neither are valid for my situation; I need to be able to instantiate the base class as well as the subclass which rules out abstract. From my limited TypeScript knowledge, declare will prevent the compiler from generating JS code for the subclass property.

I'm stuck. Any ideas on how I can solve this (other than targeting something before es2022)?

Here's a complete example that illustrates my predicament:

//  RecBase and ListBase cannot be abstract classes (i.e., I need to be able to create instances of them)
class RecBase {
  id: string
  goods: ListBase
}

class ListBase {
  lineNumber: number
}

//  This pair of subclasses extends the base classes with additional, domain specific properties
class RecOrder extends RecBase {
  orderID: string

  //  Note: I want to overwrite the base class goods, I don't want 'goods2' or 'goodsEx'
  goods: OrderList
}

class OrderList extends ListBase {
  price: number
}

• typescript

• I think that would violate the Liskov Substitution principle. You will not be able to substitute any RecBase with a RecOrder because your RecOrder requires more specialized arguments – apokryfos Commented Mar 8 at 8:21

1 Answer 1

You can use declare to set more narrow type than in the base class, so your solution could look like:

Playground

//  RecBase and ListBase cannot be abstract classes (i.e., I need to be able to create instances of them)
class RecBase {
  declare id: string
  declare goods: ListBase;
  constructor(props: Partial<RecBase> = {}){
    const {id = '1', goods = new ListBase} = props;
    Object.assign(this, {id, goods});
  }
}

class ListBase {
  lineNumber: number = 0
}

//  This pair of subclasses extends the base classes with additional, domain specific properties
class RecOrder extends RecBase {
  declare orderID: string
  declare goods: OrderList;
  constructor(props: Partial<RecOrder> = {}){
    const {orderID = '1', goods = new OrderList} = props;
    super(props);
    Object.assign(this, {orderID, goods});
  }
}

class OrderList extends ListBase {
  price: number = 0;
}

const order = new RecOrder;
console.log(order.goods.price); // number;