Dependency Inversion Principle

• Top-level modules should not depend on lower-level modules. Both types of modules should depend on abstractions.
• Abstractions should not depend on parts. Parts should depend on abstractions.

interface ITodo {}

abstract class DB {
    data: ITodo[];
    save: (data: ITodo[]) => void;
}

class LocalDB {
    saveTodos(items: ITodo[]) {
        // ...logic
    }
}

class ExternalDB {
    saveTodos(items: ITodo[]) {
        // ...logic
    }
}

class TodoListDB extends DB {
    data: ITodo[];

    save(db) {
        db.saveTodos(this.items);
    }
}

Example Solution

abstract class DB {
    save: <T>(data: T[]) => void;
}

class LocalDB extends DB {
    save(data) {
        // logic
    }
}

class ExternalDB extends DB {
    save(data) {
        // logic
    }
}

class TodoList {
    saveTodos(db: DB) {
        db.save(this.items);
    }
}