关于ext的grid问题？

Question

  var cm = new Ext.grid.ColumnModel([
        {header:'编号',dataIndex:'id'},
        {header:'名称',dataIndex:'name'},
        {header:'描述',dataIndex:'descn'}
    ]);    var data = [
        ['1','name1','descn1'],
        ['2','name2','descn2'],
        ['3','name3','descn3'],
        ['4','name4','descn4'],
        ['5','name5','descn5']
    ];    var ds = new Ext.data.Store({
        proxy: new Ext.data.MemoryProxy(data),
        reader: new Ext.data.ArrayReader({}, [
            {name: 'id'},
            {name: 'name'},
            {name: 'descn'}
        ])
    });
    ds.load();    var grid = new Ext.grid.Grid('grid', {
        ds: ds,
        cm: cm
    });
    grid.render();
为什么说缺少对象？ 大虾指点指点......