Writing listener in Ext JS

Writing listener in Ext JS

<!DOCTYPE html>
<html>
   <head>
      <link href = "https://cdnjs.cloudflare.com/ajax/libs/extjs/6.0.0/classic/theme-neptune/resources/theme-neptune-all.css"
         rel = "stylesheet" />
      <script type = "text/javascript"
         src = "https://cdnjs.cloudflare.com/ajax/libs/extjs/6.0.0/ext-all.js"></script>

      <script type = "text/javascript">
         Ext.onReady(function() {
            Ext.create('Ext.Button', {
               renderTo: Ext.getElementById('helloWorldPanel'),
               text: 'My Button',

               listeners: {
                  click: function() {
                     Ext.MessageBox.alert('Alert box', 'Button is clicked');
                  }
               }
            });
         });
      </script>
   </head>

   <body>
      <p> Please click the button to see event listener </p>
      <div id = 'helloWorldPanel' />   <!--  panel will be rendered here-- >
   </body>
</html>