When building a DevExtreme application, you specify a layoutSet. The layouts from this set are used to display views. The DevExtreme framework comes with ready-to-use layout sets that contain predefined layouts. Most of these layouts include a toolbar as a common element that is usually added to application views. Toolbars in predefined layouts are command containers. This means that you can define a command within a view and it will be displayed as a toolbar button in the place that you denote using a command mapping. In this tutorial, you will learn how to define a command and how to add it to the application's command mapping.