Adding Data to Firebase Cloud Firestore and Angular

I used typescript spread operator

  add(wizard: Wizard): Promise<DocumentReference> {
    return this.wizardCollection.add({...wizard});
  }

You can try this way, hope you are using latest typescript

product.ts

export interface Product {
   code: string;
   desc: string;
   stock: number;
   price: number;
   off: number;
   details?: string
}

in your product.component.ts

export class AdminProductComponent {

model:Product;
successMsg = 'Data successfully saved.';

productsRef: AngularFirestoreCollection<Product>;
product: Observable<Product[]>;

constructor(private afs: AngularFirestore) {
    this.productsRef = this.afs.collection<Product>('productos');

    this.model = {
      code:'',
      desc:'',
      stock:0,
      price:0,
      off:0
    }
}

save() {
    this.productsRef.add(this.model).then( _ => alert(this.successMsg));
}

I think by doing this model = new Product('', '', 0, 0, 0); you get an instance of the class not the object.

I used

save() {
    const param = JSON.parse(JSON.stringify(this.model));
    this.productsRef.add(param).then( _ => alert(this.successMsg));
 }