Ext.ComponentManager

// After defining a class such as Ext.FormPanel
// it can be registered with an xtype
// Ext manages this for its own classes
// Ext.reg('form', Ext.FormPanel);
// Ext.reg('textfield', Ext.form.TextField);

// Example:
var a = new Ext.Window({
  id: 'formWin',
  width: 300,
  height: 120,
  layout: 'fit',
  items: [{
    xtype: 'form',
    defaults: {anchor: '100%'},
    items: [{
      xtype: 'textfield',
      fieldLabel: 'Username'
    },{
      xtype: 'textfield',
      inputType: 'password',
      fieldLabel: 'Password'
    }]
  }],
  buttons: [{text: 'Login'}]
});
a.show();