如何将xtype: 'selectfield‘放到var定义的对象里

var selectItem=new Ext.form.Select({ //xtype: 'selectfield',id:'chooseIndexChart',                    name: 'chartType',                    options: [                        {text: 'Line',  value: 'line'},                        {text: 'Bar', value: 'bar'},{text: 'Pie',value:'pie'},{text: 'Scatter',value:'scatter'},{text:'Area',value:'area'}                    ],listeners: {                            change: { fn: function(){ //lineChart.hide();chageIndexChart(this.getValue());}                            }}});