adaptivetheme-8.x-3.x-dev/at_core/scripts/at.windowSize.js

(function ($) {

  "use strict";

  /**
   * Renders a widget for displaying the current width of the browser.
   */
  Drupal.behaviors.atWS = {
    attach: function (context, settings) {

      var indicator = $('<div class="window-size-indicator wsi" />').appendTo(document.body),
          indicator_px = $('<div class="wsi__px" />').appendTo(indicator),
          indicator_em = $('<div class="wsi__em" />').appendTo(indicator),
          indicator_bp = $('<div class="wsi__bp" />').appendTo(indicator);

      // Bind to the window.resize event to continuously update the width.
      $(window).bind('resize.window-size-indicator', function () {
        indicator_px.html($(this).width() + 'px');
        indicator_em.html($(this).width() /16 + 'em');
      }).trigger('resize.window-size-indicator');
    }
  };
})(jQuery);