After developing few custom Jira gadgets that relied on data retrieved by JavaScript from Jira REST API and external services, I asked myself: "why should I develop and package new gadget every time". Since JavaScript is dynamically interpreted, we can have some wrapper Jira gadget plugin and give ability to add custom JavaScript code. The custom code will be loaded into the gadget wrapper and executed there.
So whenever I need to show a table or a chart with some data that are retrieved through Jira REST API, I just write a JavaScript that fetches and displays the data. No need to create and package new gadget every time.