SUPER an existing javascript function.

super.js

// Existing function is T2.page.popup.open(number)


T2.page.popup.open = (function(_super) {
  
    return function(popupNumber) {

      // 1. Add in your custom code here
      // e.g someting.completePage()
      
      // 2. Then this line executes the old code
        return _super.apply(this,[popupNumber]);
    };
              
  })(T2.page.popup.open);

})