vue3 pinia typescript initialize declare variable ref array using interface typescript.

vue_pinia_typescript

export interface orderCart {
  id_product: number;
  total_product: number;
}

  state: () => {
    return {
      allProduct: [],
      totalAll: "",
      checkoutItem: ref<Array<orderCart>>([]),
    };
  }

addToCart(total_product: number, id_product: number) {
      const obj = {
        id_product: id_product,
        total_product: total_product,
      };
      this.checkoutItem?.push(obj);

      console.log(obj);
    },