数组根据某个属性值组成新的数组

const data = [
  {
    "name": "name",
    "file": "file",
    "cityname":'a',
  },{
    "name": "name",
    "file": "file",
    "cityname":'a',
  },
  {
    "name": "name",
    "file": "file",
    "cityname":'a',
  },{
    "name": "name",
    "file": "file",
    "cityname":'b',
  },
  {
    "name": "name",
    "file": "file",
    "cityname":'c',
  },{
    "name": "name",
    "file": "b",
    "cityname":'a',
  }
];

resetData(data) {
  let dataInfo = {}
  data.forEach((item, index) => {
    let { cityname } = item
    if (!dataInfo[cityname ]) {
      dataInfo[cityname] = {
        cityname ,
        children: [],
      }
    }
    dataInfo[cityname].children.push(item)
  })
  let list = Object.values(dataInfo)
  return list
},