How to Create Shopify Cart Validation Function

Step 1: Set Up Your Environment

Ensure you have Node.js installed and set up your project environment. You’ll also need the Shopify CLI installed.

Step 2: Install Required Packages

Install the necessary packages:

npm install @shopify/shopify-api dotenv

Step 3: Create and Configure checkout_validation.js

Create a new file named checkout_validation.js and add the following code:

// Import necessary modules
const { extend } = require('@shopify/shopify-api');
const { error } = require('@shopify/shopify-api/adapters/node');
require('dotenv').config();

/**
 * Run function to validate the cart
 * @param {RunInput} input
 * @returns {FunctionRunResult}
 */
export function run(input) {
  const errors = [];
  input.cart.lines.forEach((lineItem) => {
    const { quantity, merchandise } = lineItem;
    const max = Number.parseInt(merchandise?.product?.max_orders?.value);

    if (max && quantity > max) {
      errors.push({
        localizedMessage: `Can't order more than ${max} of ${merchandise?.product?.handle}`,
        target: 'cart',
      });
    }
  });

  return { errors };
}

Step 4: Set Up Your Shopify App

Use the Shopify CLI to create a new app or use an existing one:

shopify app create node

Navigate to your app directory:

cd your-app-name

Step 5: Generate a Checkout Validation Function

Generate a new extension for your app:

shopify app generate extension

Select Discount as the type of extension. This will scaffold a new extension directory with the necessary files.

Step 6: Deploy the Function

Deploy your app to Shopify:

shopify app deploy

Step 7: Implement the Validation Logic

In the generated extension directory, replace the existing index.js with your custom validation logic from checkout_validation.js. Ensure the structure matches the Shopify Functions’ requirements.

Example index.js

// Import necessary modules
const { extend } = require('@shopify/shopify-api');
const { error } = require('@shopify/shopify-api/adapters/node');
require('dotenv').config();

/**
 * Run function to validate the cart
 * @param {RunInput} input
 * @returns {FunctionRunResult}
 */
export function run(input) {
  const errors = [];
  input.cart.lines.forEach((lineItem) => {
    const { quantity, merchandise } = lineItem;
    const max = Number.parseInt(merchandise?.product?.max_orders?.value);

    if (max && quantity > max) {
      errors.push({
        localizedMessage: `Can't order more than ${max} of ${merchandise?.product?.handle}`,
        target: 'cart',
      });
    }
  });

  return { errors };
}

Step 8: Configure Environment Variables

Ensure your environment variables are set up correctly in a .env file:

SHOPIFY_API_KEY=your-api-key
SHOPIFY_API_SECRET=your-api-secret
SHOPIFY_API_VERSION=2022-01
SHOPIFY_SHOP_NAME=your-shop-name

Step 9: Test the Function

Test your function by adding items to your cart and ensuring the validation logic is applied correctly. If the cart does not meet the criteria, appropriate error messages should be displayed.

Summary

By following these steps, you will have created a custom Shopify cart validation function. This function checks the quantity of items in the cart and ensures they do not exceed a specified maximum quantity per product. This setup provides a robust foundation for implementing further custom cart validation logic as needed.