Use case

Return the display name of the previous assignee.

Expression

Create and configure a Smart field using this expression in Jira expression parsing mode:

let assignees= issue.changelogs.map(i=>i.items).flatten().filter(i=>i.field=="assignee");
assignees.length>0 ? 
	assignees.map(a=>a.fromString)[0] == null ?
    	""
        :assignees.map(a=>a.fromString)[0]
    :  ""