You can use cart and checkout validation functions to ensure that purchases meet certain criteria before checking out, or completing an order. In this tutorial, you’ll use [Shopify Functions](/docs/apps/build/functions) to enforce an order maximum for buyers with insufficient order history, preventing them from placing their order.

> Note:
> Errors from validation functions are exposed to the Storefront API's [`Cart`](/docs/storefronts/headless/building-with-the-storefront-api/cart/manage) object, in themes using the [`cart` template](/docs/storefronts/themes/architecture/templates/cart) and during checkout.

## What you'll learn

In this tutorial, you’ll learn how to do the following tasks:

- Generate starter code for Shopify Functions.
- Use GraphQL to define the input of your function.
- Deploy functions to the Shopify platform.
- Review logs for your function.


![Screenshot that shows a checkout validation error](/assets/apps/checkout/validation/cart-checkout-validation-screenshot.png)

<p>
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</div>
</div>

<script data-option="filename" data-value="Storefront API example response"></script>
<script data-option="nocopy" data-value="true"></script>

<script type="text/plain" data-language="json">
RAW_MD_CONTENT{
 "data": {
  "cartLinesAdd": {
   "cart": null,
   "userErrors": [
   {
    "code": "VALIDATION_CUSTOM",
    "field": [
     "cartId"
    ],
    "message": "There is an order maximum of $1,000 for customers without established order history"
   }
   ]
  }
 }
}
END_RAW_MD_CONTENT</script>

</div>
</p>


## Requirements


> Tip:
> Shopify defaults to Rust as the most performant and recommended language choice to stay within the platform limits. For more information, refer to [language considerations](/docs/apps/build/functions/programming-languages).



- You've created a [Partner account](https://www.shopify.com/partners).
- You've created a [development store](/docs/api/development-stores#create-a-development-store-to-test-your-app)
        with the [Checkout and Customer Accounts Extensibility developer preview enabled](/docs/api/developer-previews#enable-a-developer-preview).
- You've [created an app that uses Shopify CLI 3.49.5 or higher](/docs/apps/build/scaffold-app). If you previously installed Shopify CLI, then make sure that you're using the [latest version](/docs/api/shopify-cli#upgrade).
  
    If you plan to create a UI for your extension, then start with the [Remix app template](/docs/api#app-templates).
  
  
- You've installed [Node.js](https://nodejs.org/en/download) 16 or higher.
- You've [installed your app](/docs/apps/build/scaffold-app#step-3-install-your-app-on-your-development-store) on the development store
     with the [Checkout and Customer Accounts Extensibility developer preview enabled](/docs/api/developer-previews#enable-a-developer-preview).




### Rust-specific requirements

The following requirements are specific to Rust-based development with Shopify Functions.

- You've installed [Rust](https://www.rust-lang.org/tools/install).

    On Windows, Rust requires the [Microsoft C++ Build Tools](https://docs.microsoft.com/en-us/windows/dev-environment/rust/setup). Make sure to select the **Desktop development with C++** workload when installing the tools.

- You've installed the [`wasm32-wasip1` target](https://doc.rust-lang.org/rustc/platform-support/wasm32-wasip1.html):

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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTrustup target add wasm32-wasip1
    END_RAW_MD_CONTENT</script>

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    </p>




## Step 1: Create the validation function

To create your validation function, you can use Shopify CLI to [generate](/docs/api/shopify-cli/app/app-generate-extension) a starter function, specify the inputs for your function using an [input query](/docs/apps/build/functions/input-output/metafields-for-input-queries), and implement your function logic using Javascript or Rust.

1. Navigate to your app directory:

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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTcd <directory>
    END_RAW_MD_CONTENT</script>

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1. Run the following command to create a new validation function:

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        <script type="text/plain" data-language="bash">
        RAW_MD_CONTENTshopify app generate extension --template cart_checkout_validation --name cart-checkout-validation
        END_RAW_MD_CONTENT</script>

        </div>
        </p>


    > Tip:
> Shopify Functions support any language that compiles to WebAssembly (Wasm), such as Rust, AssemblyScript, or TinyGo. You specify the Wasm template option when you're using a language other than Rust and can conform to the Wasm API. [Learn more about the Wasm API](/docs/apps/build/functions/programming-languages/webassembly-for-functions).

1.  Choose the language that you want to use. For this tutorial, you should select either **Rust** or **JavaScript**.

    Shopify defaults to Rust as the most performant and recommended language choice to stay within the platform limits. For more information, refer to [language considerations](/docs/apps/build/functions/programming-languages).


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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENT?  What would you like to work in?
    > (1) Rust
      (2) JavaScript
      (3) TypeScript
      (4) Wasm
    END_RAW_MD_CONTENT</script>

    </div>
    </p>



1. Navigate to `extensions/cart-checkout-validation`:

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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTcd extensions/cart-checkout-validation
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


1. Replace the contents of `src/run.graphql` file with the following code.

    `run.graphql` defines the input for the function. You need the customer's order count and current cart subtotal.

    The query differs slightly in Rust and JavaScript due to code generation requirements.

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    </div>
    </div>

    <script data-option="filename" data-value="run.graphql"></script>

    <script type="text/plain" data-language="graphql" data-title="Rust input query" data-filename="src/run.graphql">
    RAW_MD_CONTENTquery Input {
      cart {
        buyerIdentity {
          customer {
            numberOfOrders
          }
        }
        cost {
          subtotalAmount {
            amount
          }
        }
      }
    }
    END_RAW_MD_CONTENT</script>
<script type="text/plain" data-language="graphql" data-title="JavaScript input query" data-filename="src/run.graphql">
    RAW_MD_CONTENTquery RunInput {
      cart {
        buyerIdentity {
          customer {
            numberOfOrders
          }
        }
        cost {
          subtotalAmount {
            amount
          }
        }
      }
    }
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


1. If you're using JavaScript, then run the following command to regenerate types based on your input query:

    <p>
    <div class="react-code-block" data-preset="terminal">
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    <div class="react-code-block-preload-bar "></div>
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    <div class="react-code-block-preload-codeline"></div>
    </div>

    </div>
    </div>


    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTshopify app function typegen
    END_RAW_MD_CONTENT</script>

    </div>
    </p>



1. Replace the `src/run.rs` or `src/run.js` file with the following code.

    This function logic checks for order subtotals greater than a set value and errors when a new customer is detected. You can adjust the subtotal limit or new customer detection logic as needed.

    > Tip:
    > You can associate a validation error with a specific checkout UI field by specifying the `target` property. The `target` property follows the pattern that's provided in the [Validation API reference](/docs/api/functions/reference/cart-checkout-validation/graphql#supported-checkout-field-targets). For example, `$.cart.deliveryGroups[0].deliveryAddress.postalCode`.

    <p>
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    <script type="text/plain" data-language="rust" data-title="Rust" data-filename="src/run.rs">
    RAW_MD_CONTENTuse shopify_function::prelude::*;
    use shopify_function::Result;
    
    // The configured entrypoint for the 'purchase.validation.run' extension target
    #[shopify_function_target(query_path = "src/run.graphql", schema_path = "schema.graphql")]
    fn run(input: input::ResponseData) -> Result<output::FunctionRunResult> {
        let mut errors = Vec::new();
        let error = output::FunctionError {
            localized_message:
                "There is an order maximum of $1,000 for customers without established order history"
                    .to_owned(),
            target: "cart".to_owned(),
        };
    
        // Parse the decimal (serialized as a string) into a float.
        let order_subtotal: f64 = input.cart.cost.subtotal_amount.amount.into();
    
        // Orders with subtotals greater than $1,000 are available only to established customers.
        if order_subtotal > 1000.0 {
            if let Some(buyer_identity) = input.cart.buyer_identity {
                if let Some(customer) = buyer_identity.customer {
                    // If the customer has ordered less than 5 times in the past,
                    // then treat them as a new customer.
                    if customer.number_of_orders < 5_i64 {
                        errors.push(error);
                    }
                } else {
                    errors.push(error);
                }
            // If there's no customer data, then treat them as a new customer.
            } else {
                errors.push(error);
            }
        }
    
        Ok(output::FunctionRunResult { errors })
    }
    END_RAW_MD_CONTENT</script>
<script type="text/plain" data-language="javascript" data-title="JavaScript" data-filename="src/run.js">
    RAW_MD_CONTENT// @ts-check
    
    // Use JSDoc annotations for type safety
    /**
    * @typedef {import("../generated/api").RunInput} RunInput
    * @typedef {import("../generated/api").FunctionRunResult} FunctionRunResult
    */
    
    // The configured entrypoint for the 'purchase.validation.run' extension target
    /**
    * @param {RunInput} input
    * @returns {FunctionRunResult}
    */
    export function run(input) {
      // The error
      const error = {
        localizedMessage:
            "There is an order maximum of $1,000 for customers without established order history",
        target: "cart"
      };
      // Parse the decimal (serialized as a string) into a float.
      const orderSubtotal = parseFloat(input.cart.cost.subtotalAmount.amount);
      const errors = [];
    
      // Orders with subtotals greater than $1,000 are available only to established customers.
      if (orderSubtotal > 1000.0) {
        // If the customer has ordered less than 5 times in the past,
        // then treat them as a new customer.
        const numberOfOrders = input.cart.buyerIdentity?.customer?.numberOfOrders ?? 0;
    
        if (numberOfOrders < 5) {
          errors.push(error);
        }
      }
    
      return { errors };
    };
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


## Step 2: Preview the function on a development store

To test your function, you need to make it available to your development store.

1. If you're developing a function in a language other than JavaScript or TypeScript, ensure you have configured `build.watch` in your [function extension configuration](/docs/api/functions/configuration#properties).




1. Navigate back to your app root:

    <p>
    <div class="react-code-block" data-preset="terminal">
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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTcd ../..
    END_RAW_MD_CONTENT</script>

    </div>
    </p>



1. Use the Shopify CLI [`dev` command](/docs/api/shopify-cli/app/app-dev) to start app preview:

    <p>
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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTshopify app dev 
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


    You can keep the preview running as you work on your function. When you make changes to a watched file, Shopify CLI rebuilds your function and updates the function extension's drafts, so you can immediately test your changes.

1. Follow the CLI prompts to preview your app, and install it on your development store.


## Step 3: Activate the validation

1. From the Shopify admin, go to **Settings** > **Checkout**.
1. In the **Checkout Rules** section of the page click **Add rule**.

    A dialog opens and shows the `cart-checkout-validation` function that you just deployed.
1. To add a validation, click **Add rule** and select the validation.

1. Click **Activate** to activate the validation.

1. Click on **Save**.

1. Optional: Control how checkout behaves when encountering runtime exceptions by clicking on the validation and selecting or deselecting **Allow all customers to submit checkout**.

## Step 4: Test the validation

1. From your online store, without logging in, create a cart with more then $1,000 in merchandise.
1. Proceed to **Checkout** and verify that a warning message displays.
1. Verify that checkout progress is blocked. Clicking the **Continue to shipping** button shouldn't redirect the user.
1. Using the Storefront API [`cartLinesAdd` mutation](/docs/api/storefront/2024-01/mutations/cartLinesAdd), confirm that the mutation's `userErrors` field contains the function's error message, and that executing the mutation was unsuccessful.
1. Open your terminal where `shopify app dev` is running, and review your function executions.

    When [testing functions on development stores](/docs/apps/build/functions/test-debug-functions#test-your-function-on-a-development-store), the output of `dev` includes executions of your functions, any debug logging you have added to them, and a link to a local file with the full function execution details.
1. In a new terminal window, use the Shopify CLI [`app function replay`](/docs/api/shopify-cli/app/app-function-replay) command to [replay a function execution locally](/docs/apps/build/functions/test-debug-functions#execute-the-function-locally-using-shopify-cli), and debug your function without the need to re-trigger the function execution on Shopify.

    <p>
    <div class="react-code-block" data-preset="terminal">
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    <div class="react-code-block-preload-codeline"></div>
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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTshopify app function replay
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


1. Select the function execution from the top of the list. Press `q` to quit when you are finished debugging.


## Next steps


- Learn more about how [Shopify Functions](/docs/apps/build/functions) work and the benefits of using Shopify Functions.
- Consult the [API references for Shopify Functions](/docs/api/functions).
- Learn how to use [variables](/docs/apps/build/functions/input-output/use-variables-input-queries) in your input query.