Question: Proper responds status code from API

[microsoft]

Question: Proper responds status code from API

[darrelmiller]

I wouldn't recommend using 404 to indicate an empty shopping cart. If there is an explicit process to create a cart and delete a cart then a 404 would indicate the need to create a cart. A client that requests to see the contents of the cart when the cart is empty, is not a "client error". At least that is quite a stretch.

Assuming that carts are implicitly created when you add items to them, there are two options for indicating an empty cart. You can return a 200 and a empty object/array, or return a 204 that has no response body at all.

[zaverden]

See here: https://github.com/Microsoft/api-guidelines/blob/vNext/Guidelines.md#51-errors

4xx indicates that client has sent an invalid request (wrong path, bad format, unsupported method, etc.). And if I get you description right, then empty card is not a problem, it is a valid state of application => 404 does not suet here.

200 vs 204 is like empty array vs null. Decision here depends on you style and preferences.
Personally, I prefer to have 200 and be sure that I always receive array (empty or not) in a response body.