我这里有个树形对象var i1={
id:1,
name:'root',
is:[{
id:'2',
name:'2'
},{
id:'3',
name:'3',
is:[{
id:'4',
name:'4',
is:[{
id:'6',
name:'6'
},{
id:'7',
name:'7'
}]
},{
id:'5',
name:'5',
is:[{
id:'8',
name:'8'
},{
id:'9',
name:'9'
}]

}]
}]

}
我想用一个方法fnGetFloor(i1)来取得这个树形对象的深度,这个函数该怎么写?