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Fix network requests to cart.js
Fix network requests to cart.js

Learn why network requests to cart.js may fail when using Discount Ninja on specific themes and how you can fix this.

Bart Coppens avatar
Written by Bart Coppens
Updated over a week ago


How do cart line item keys work in Shopify?

When Shopify applies a discount to an item in the cart, it also changes the "key" of that cart line item. The key of a line item is a unique identifier that consists of the variant id, followed by a colon, followed by a hashcode that changes when:

  • line item properties change

  • the selected selling plan (subscription) changes

  • discounts are applied

The line item key changes when discounts are applied in one of the following ways:

  • run an automatic discount

  • use Shopify Plus scripts

  • use native discount functions

Discount Ninja, when configured to use the Shopify functions checkout mode, leverages native discount functions.

The sequence that causes the issue

Some themes do not handle line item keys properly, a problem that only becomes apparent when discounts are applied after the page has loaded:

  • the cart page is rendered

  • the cart line item keys are included in the HTML rendered by Shopify

  • an interaction (for example a change of quantity) triggers a discount

  • Shopify changes the key in the backend

  • the theme does not update the key in the HTML rendered

Some themes rely on the cart line item present in the HTML to perform updates of the cart. For example, when the cart quantity is changed or a line is removed, the theme calls Shopify's Cart.js endpoints using the cart line item key as the unique identifier for the cart line item to update. When the keys are out of synch, this causes those update calls to fail.

So, is this an issue in Discount Ninja?

No, it's a limitation that is inherent to how the Shopify platform works and how themes are built.

To demonstrate this (and test if your theme handles this correctly), run the following test:

  • Activate this discount.

  • Preview the copy of your theme.

  • Ensure the cart is empty.

  • Next, add 98 items of the selected product to the cart.
    Inspect the HTML generated in the cart page. Note the cart line item key. At this point, this key is correct.

  • Then, without refreshing the page, change the quantity of the line from 98 to 99.
    The network request to change the quantity will succeed.
    The automatic discount will now be applied to the cart and, crucially, Shopify will change the cart line item key. This can be observed in the network requests tab of your browser by inspecting the payload of the request and the response received.

  • Finally, try to change the quantity of the line from 99 to 100.
    If this fails with HTTP status code 400, your theme has failed to update the key of the cart line item. The theme has sent a request to Shopify that uses the old key (which no longer exists) as the unique identifier for the line.


Discount Ninja addresses this shortcoming in the Shopify platform by keeping track of situations where the key of a cart line item changes. This helps the app to avoid many situations where adding a discount would break your theme's ability to update the quantity of the cart.

However, there are situations where your theme's JavaScript may still fail when keys are updated by Shopify. If your theme is in that case, we suggest the following approach:

  1. Work with the theme developer to have this fixed: theme developers may be able to avoid this issue by relying on the line number instead of the key (cf.

  2. Alternatively, programmatically ensure the cart line item keys are updated by leveraging our client-side JavaScript API, documented at

Our client-side JavaScript API is included in the App embed and can help you implement the second approach. Specifically:

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