Which is the equivalent of php drupal format_interval for javascript?

Drupal doesn't implement a JS version of format_interval(); this is a rough (minimally tested) port:

Drupal.formatInterval = function(interval, granularity, langcode) {
  granularity = typeof granularity !== 'undefined' ? granularity : 2;
  langcode = typeof langcode !== 'undefined' ? langcode : null;

  var units = {
    '1 year|@count years': 31536000,
    '1 month|@count months': 2592000,
    '1 week|@count weeks': 604800,
    '1 day|@count days': 86400,
    '1 hour|@count hours': 3600,
    '1 min|@count min': 60,
    '1 sec|@count sec': 1
  },
  output = '';

  for (var key in units) {
    var keys = key.split('|'); 
    var value = units[key];
    if (interval >= value) {
      output += (output.length ? ' ' : '') + Drupal.formatPlural(Math.floor(interval / value), keys[0], keys[1], {}, { langcode: langcode });
      interval %= value;
      granularity--;
    }

    if (granularity == 0) {
      break;
    }
  }

  return output.length ? output : Drupal.t('0 sec', {}, { langcode: langcode });
}

Some random results using the above (they seem to match up to the PHP function as expected):

• 3643 => 1 hour 43 sec
• 92900 => 1 day 1 hour
• 2592000 => 1 month
• 9331200 => 3 months 2 weeks
• 297605232 => 9 years 5 months