title: How to make day field optional in toLocalDateString
body: <p>I want to make day parameter optional in toLocalDateString as following:</p>
  
  <pre><code>if (item.series.xaxis.tickSize[1] == 'day') {
      var day_exist = 'day:"2-digit",';
  }
  else {
     day_exist = '';
  }
  
  dateString = d.toLocaleDateString(
      navigator.language, {day_exist month:'short', year:'2-digit'});
  </code></pre>
  
  <p>But this is not working. Any idea?</p>