我使用async await进行REST API调用，返回一个单项数组。saveProduct返回一个数组。当我在调用saveProduct时尝试从数组中提取第一项时，我返回undefined。

saveProduct(product: Product): Promise<Product[]> {
    return firstValueFrom(
        this._httpService.requestCall(this._url, ApiMethod.POST, product)
    );
}

const newProduct: Product = await this._productService.saveProduct(product)[0];

console.log(newProduct) => undefined

const newProduct: Product = await this._productService.saveProduct(product);

console.log(newProduct[0]) => properly prints the product object