X4 BPMS developers can use the Web App Control Project component to extend X4 Web Apps by custom JavaScript functions based on web technologies and the X4 ESB with its web service interface. This approach allows to access the data and features of the web app via JavaScript API functions. You should not only have a detailed knowledge of the data, workflows, and common web technologies such as HTML, CSS, and JavaScript, but you should also be well grounded in XML, XSLT, and XPath.