Introduction

Customizing your Shopify cart with cart attributes can elevate the shopping experience by enabling personalized options, such as gift messages or special instructions. However, this customization can sometimes trigger unexpected errors, like the message: "You can only add this item in increments of undefined." This issue is a common pain point among Shopify merchants, as seen in numerous discussions on the Shopify Community Forums. Fortunately, the fix is straightforward, involving a minor adjustment to your theme’s JavaScript code. In this post, we’ll guide you through the process to resolve this error and ensure your cart operates seamlessly.

Understanding the Issue

Cart attributes are a powerful Shopify feature that allows merchants to collect additional customer information during checkout. Examples include adding gift messages, specifying delivery dates, or providing special instructions. However, implementing these attributes requires careful integration with the cart’s JavaScript code to avoid conflicts.

The root cause of the error "you can only add this item in increments of undefined" often lies in the incorrect placement or implementation of customization code, which disrupts the cart’s ability to handle quantity changes. By updating the onChange function, you can address this issue and restore full functionality.

Solution

To fix the error, you need to update the onChange function in the assets/cart.js file of your Shopify theme. Replacing the existing function with a corrected version ensures that quantity increments are handled properly, eliminating the error.

Note: Always duplicate your theme and work on the copy when making code changes. This allows you to test modifications without affecting your live store. You can duplicate your theme in the Shopify admin under Online Store > Themes > Actions > Duplicate.

Steps to Implement the Fix

Follow these steps to resolve the error:

1. Access Your Theme’s Code Editor:

   • Log in to your Shopify admin dashboard.

   • Navigate to Online Store > Themes.

   • Select your active theme and click Actions > Edit code.

2. Locate the cart.js File:

   • In the left-hand panel, find the Assets folder.

   • Click on cart.js to open the file.

3. Identify the onChange Function:

   • Search for the onChange function within the cart.js file. This function is typically responsible for handling updates when cart quantities are changed.

4. Replace the onChange Function:

   • Replace the entire onChange function with the updated code provided below.

  onChange(event) {
     // console.log('event.target.type', event.target.type);
    if(event.target.type == 'number')
    {
      this.validateQuantity(event);
    }
    
  }Copy

5. Save Changes and Test:

   • After inserting the new code, click Save to apply the changes.

   • Test your cart by adding products and adjusting quantities to confirm that the error is resolved.

Final Thoughts

There you go—your Shopify cart should now be free of that pesky "increments of undefined" error. A quick tweak to the cart.js file can make all the difference in keeping your checkout process smooth and customer-friendly. Need more help or a tailored solution for your store? Drop us a message in the comments below—we’d love to assist!