carabiner | Tony Dewan for original library

From my experience, if you hit Shopify with multiple cart changes at the same time, only one will go through. My guess is that Shopify handles the cart something like this:

At time 0 we have our initial cart - call it cart0

At time t, we submit n simultaneous requests to add/update/change the cart

Request 0: take cart0 and apply request, return result Request 1: take cart0 and apply request, return result ... Request n: take cart0 and apply request, return result

This seems consistent with the cart return values that I got when I tested multiple requests at the same time - each callback function would trigger with a cart that had a single change applied.

I've found two ways to deal with this limitation. The first is to make sure the requests are chained so that we wait for one to finish before making the next - effectively turning the asynchronous requests into synchronous ones. Not the best for speed.

The other method only works if we don't have line item properties to worry about: combining all the additions/changes into one call to /cart/update.js, passing the variant IDs and desired quantities all at once - see https://help.shopify.com/themes/development/getting-started/using-ajax-api#update-cart for examples. If you don't have line properties to worry about, changing your single-line additions into one big update will be a significant speed and reliability increase.