I am studying backend development using NodeJS and TypeScript. To start, I am checking this sample inventory. While reading the models, I got confused because there is an interface that is both extended (but inside the Model wrapper) and implemented.

The interface:

interface SaleAttributes {
    id: string;
    quantity: number;

    // foreign key
    productId?: string;
}

Base Class:

export class Sale extends Model<SaleAttributes, SaleCreationAttributes>
    implements SaleAttributes {
.
.
.
}

This is the complete code.