myTheme.js

(function ($) {
  /**
   * Register the myTheme behavior.
   */
  Drupal.behaviors.myTheme = {
    attach: function (context, settings) {
      Drupal.myTheme.DatePicker(context, settings);
    }
  };

  /**
   * Class containing functionality for myTheme.
   */
  Drupal.myTheme = {};
  
  /**
   * Class containing functionality for DatePicker.
   */
  Drupal.myTheme.DatePicker = function (context, settings) {
    $('#datepicker', context).datepicker();
  };

})(jQuery);