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I understand that the Shopify Rest Api does not have the capability to return shopping cart items. I also understand that the Shopify Ajax Api has the /cart.js endpoint which allows you to retrieve the current user shopping cart. It uses a session cookie to identify the current user. But this means that the Ajax Api /cart.js could be used only from the context of a Shopify front-end.
Still I can see various Shopify apps or services that seem to be able to get the shopping cart for a given customer outside the context of a Shopify front-end. Does anyone know how I can call the Ajax Api /cart.js or any other endpoint in the Rest or Ajax api, and give it a user id perhaps or a shopping cart id and get the cart items for it?
Thanks