We need to create validators for each status. The validator will check if transition from current state is allowed or not.

Script
var validators = {
  "open": {
    "closed": function(issue, user) {
      return issue.hasAssignee(user, Issue.OPEN)
    }
  },
  "closed": {
    // can be reopened by anyone
    "open": function(issue, user) {
      return true
    }
  }
}