Build a Discount Function that rejects invalid codes

Anchor to TargetsTargets

The [[extensions.targeting]] sections define the Function targets. Each section includes a target, input_query, and export property.

The target property defines where the Function is applied.

The input_query property is the path to the Function's input query file. This file defines the Function's input parameters, including the enteredDiscountCodes field that you'll use for validation.

The export property specifies the build entrypoint that will run for a specified target. This will usually be the function name of your main export.

Anchor to Update your input query to include entered discount codesUpdate your input query to include entered discount codes

Update your cart_lines_discounts_generate_run.graphql file to query the enteredDiscountCodes field.

enteredDiscountCodes returns an array of active discount codes. Each code includes a rejectable boolean indicating whether your Function can reject this discount code.

Anchor to Implement discount rejection logicImplement discount rejection logic

Update your cart_lines_discounts_generate_run Function to implement custom validation logic and return rejection operations.

This example keeps only the last entered discount code with the INF- prefix.

Anchor to Review the rejection operationReview the rejection operation

The enteredDiscountCodesReject operation has the following structure:

To reject multiple discount codes with different messages, you can return multiple rejection operations.

Automatic discount Functions can only reject discount codes that are part of the enteredDiscountCodes input and are rejectable.

Anchor to Open the GraphiQL interface for the GraphQL Admin APIOpen the GraphiQL interface for the GraphQL Admin API

1. Open the GraphiQL interface by pressing g in the terminal window where you started your app.
2. On the GraphiQL interface, for API Version, select the latest stable release.

Anchor to Get the handle of your FunctionGet the handle of your Function

1. Use your function handle in GraphQL mutations instead of querying for the function ID.

   Your function handle is defined in shopify.extension.toml as handle:

   shopify.extension.toml

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   [[extensions]]

   name = "discount-rejection-function"

   handle = "discount-rejection-function-rs"

   type = "function"

   uid = "3d664979-ccd6-e9dd-4497-41289ece62373715032a"
   Note

   If you're upgrading to API version 2025-10 or later, you no longer need to query for function IDs. Use your stable function handle instead, which remains consistent across environments.

Anchor to Create the discountCreate the discount

To create a discount in your store, use the discountAutomaticAppCreate mutation. Discount rejection operations are only supported for automatic discounts.

1. For functionHandle, provide the Function handle from the previous step.
2. Add PRODUCT, ORDER or both discountClasses to your discount input.

You should receive a GraphQL response that includes the ID of the created discount. If the response includes any userErrors, then review them, check that your mutation and functionHandle are correct, and try the request again.

Anchor to Create codes that can be rejectedCreate codes that can be rejected

To test your function's rejection logic, you'll need to create two discount codes.

Create two product or order discount codes (for example, INF-ALICE and INF-BOB). Both codes must:

• Use the prefix INF-
• Be set to combine with all other discounts

Anchor to Reject discount codes in checkoutReject discount codes in checkout

1. Create a cart and navigate to checkout.
2. Enter one of the two discount codes so it applies to the cart.
3. Enter the other discount code. This new code applies and the former code is rejected. When a discount code is rejected, a message displays as a warning banner in checkout.

Anchor to Review the Function executionReview the Function execution

1. In the terminal where shopify app dev is running, review your Function executions.

   When testing Functions on development stores, the dev output shows Function executions, debug logs you've added, and a link to a local file containing full execution details.

2. In a new terminal window, use the Shopify CLI command app function replay to replay a Function execution locally. This lets you debug your Function without triggering it again on Shopify.

   Terminal

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   shopify app function replay

3. Select the Function execution from the top of the list. Press q to quit when you are finished debugging.

In the Function run logs, you'll see:

• The entered discount codes that were queried
• The rejection operations returned with the custom message