web-v1-old | Static Ecommerce site built using GatsbyJs | Ecommerce library

I'm working on a dummy fullstack ecommerce app using Postgres, Express and React whilst going through a fullstack course. This question is pretty specific to React.

On login to the app I can successfully create or retrieve a cart from the db and save it to state. It's being saved as a normal object:

Cart: {id: 2, user_id: 159, product_count: 0, price: '£0.00'}

-From Chrome Dev Tools: Extensions React Developer Tools.

I'm then using React useEffect hooks to persist this state in sessionStorage:

App.js

 useEffect(() => {
    if (sessionStorage["cart"]) {
      setCart(window.sessionStorage.getItem("cart"));
    }
  }, []);

  useEffect(() => {
    window.sessionStorage.setItem("cart", cart);
  }, [cart]);

Whenever I refresh the page or go to another route the normal object seems to be turning into the string: "[object Object]".

I know it has something to do with the way I'm saving the cart state variable into the localStorage but I'm not familiar with it enough to know exactly where this is happening or how to fix it so any help would be appreciated.

I've tried changing the assignment variables within the useEffect calls to include trying to save the item as an object, but I'm not familiar enough with sessionStorage to know what's happening behind the scenes.