title: Why substring does not handle negative indices?
body: <p><code>substr()</code> handles negative indices perfectly but <code>substring()</code> only accepts nonnegative indices.</p>
  
  <p>Is there a reason of not using <code>substr</code> in favor of <code>substring</code>? The usage of negative indices are so useful in a lot of cases by viewing the space of indices as cyclic group. Why <code>substr</code> is  indicated "deprecated" by MDN?</p>