Worked on the labd/commercetools-node-mock repository to deliver support for the 'key' field when adding line items to both carts and shopping lists. This involved updating the CartUpdateHandler and ShoppingListUpdateHandler to persist the 'key' alongside each line item, ensuring that data integrity and interoperability with commercetools usage patterns were maintained. Expanded the automated test suite to validate the correct handling of the 'key' field in both cart and shopping list services. The work was implemented using TypeScript and Node.js, with a focus on robust API development and comprehensive testing to ensure reliable feature integration.