Displaying a Field Based on Another Field Selection

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:

1. Create a "Free Text Field (unlimited text)" custom field type (Administration -> Issue Fields -> Custom Fields)
2. Fill in the following text into the "Description" field:

   <script type="text/javascript">
     priority = document.getElementById('priority');
     if (priority) {
   	  target = document.getElementById('customfield_10420FieldArea');
   	  // 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 = '';		  
                     } else {
   	             target.style.display='none';
   		  }
   	  }
     }
    </script>
   

3. Make sure to change the custom field id and priority id.