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());
}
}
}
});