Documentation for JIRA 4.4. Documentation for other versions of JIRA is available too.
Please note that adding Javascript to custom fields is a customisation and not maintained as a supported part of JIRA.
In Atlassian's support JIRA, when a user creates an issue with "Critical" priority, it will display the "Priority Explanation" field.
This can be achieved by performing the following steps:
<script type="text/javascript"> priority = document.getElementById('priority'); if (priority) { target = document.getElementById('customfield_10000'); // Hide the target field if priority isn't critical if (priority.value != 2) target.style.display='none'; priority.onchange=function() { if (this.value == 2) { target.style.display = ''; target.value="enter message here"; } else { target.style.display='none'; } } } </script>