sagaInClass.ts

// ISaga.ts
export default interface ISaga {
    register(): Iterable<any>;
}

export default class ProductUpdatePageSaga implements ISaga {
    private readonly backend: IBackendApi;
    private readonly history: History;

    constructor(backend: IBackendApi, history: History) {
        this.backend = backend;
        this.history = history;
    }

    *register(): Iterable<any> {
        yield takeEvery(SAVE_PRODUCT, this.saveProduct());
    }
     
    saveProduct = () => {
        const { backend, history } = this;  // it's a little bit tricky with TS and no-invalid-this
        return function*(action: ISaveProductAction) {
          const productResult = backendApi.saveProduct(action.productId, action.productData);
          yield put(saveUserDetails(userDetails));
          history.push("/products");
          return true;
        };
    };  
}
      
      
// test
      
it("saves product", () => {
    const push = jest.fn();
    const history = ({ push } as unknown) as History;
    const saveProduct = jest.fn().mockReturnValue({success: true});
    const backendApi = ({ saveProduct } as unknown) as IBackendApi;
    
    const productData = {... };
    const action: ISaveProduct = {
        type: SAVE_PRODUCT,
        productData,
    };
    const saga = new Saga(backendApi, history);   // inject   
      
    const gen = saga.saveProduct()(action);
    const next = gen.next();
    expect(saveProduct).toHaveBeenCalledWith({success: true});
    expect(push).toHaveBeenCalledWith("/products");
    expect(next.done).toBeTruthy();  
});