JIRA provides a lot of options to manage the various fields on an issue. Field configuration schemes, screen schemes, and so on, help the JIRA admins to show or hide fields, mark them as mandatory, and so on, differently for different issue types and projects.
Irrespective of how configurable these schemes are, there are still areas where we need to perform some custom development. For example, if we need to show or hide fields based on the values of another field, then JIRA doesn't have any built-in option to do so.
In that case, what is the best way to deal with this? It is always possible to create a new composite custom field that can have multiple fields driven by each other's behavior. But probably an easier way—that doesn't need developing a plugin—is to drive this using JavaScript. To make things better, JIRA offers a jQuery library that can be used to write neat JavaScript code!
However, using JavaScript to handle field behavior can create problems...