现有一堆数据，我需要按时间进行分组，以便前端视图呈现

[
  {"date":"2017-12-22","start_time":"10:00:00","end_time":"10:00:00","status":"Performance Time"},
  {"date":"2017-12-22","start_time":"10:40:00","end_time":"10:40:00","status":"Performance Time"},
  {"date":"2017-12-23","start_time":"10:00:00","end_time":"10:00:00","status":"Performance Time"},
  {"date":"2017-12-23","start_time":"10:40:00","end_time":"10:40:00","status":"Performance Time"}
]

需转化为如下

[
  {
    date: '2017-12-22',
    data: [
      {
        date: '2017-12-22',
        start_time: '10:00:00',
        end_time: '10:00:00',
        status: 'Performance Time'
      },
      {
        date: '2017-12-22',
        start_time: '10:40:00',
        end_time: '10:40:00',
        status: 'Performance Time'
      }
    ]
  },
  {
    date: '2017-12-23',
    data: [
      {
        date: '2017-12-23',
        start_time: '10:00:00',
        end_time: '10:00:00',
        status: 'Performance Time'
      },
      {
        date: '2017-12-23',
        start_time: '10:40:00',
        end_time: '10:40:00',
        status: 'Performance Time'
      }
    ]
  }
]

1.原始方法，网络一大堆

  var map = {},
    nList = []
  //遍历原始数组
  for (var i = 0; i < arr.length; i++) {
    var item = arr[i]
    //如果map没有则在新nList中添加
    if (!map[item.date]) {
      nList.push({
        date: item.date,
        data: [item]
      })
      map[item.date] = item
    } else {
      //遍历nList
      for (var j = 0; j < nList.length; j++) {
        var nItem = nList[j]、
        //如查找到date符合则添加
        if (nItem.date == item.date) {
          nItem.data.push(item)
          //跳出循环
          break
        }
      }
    }
  }

运行效率：遍历1000个约3ms，总觉得不优雅，而且没用到ES5的特性，于是决定自己优化一下！

2.使用ES5特性

将for替换为forEach和every

  let map = {},
    nList = []
  arr.forEach((item) => {
    if (!map[item.date]) {
      nList.push({
        date: item.date,
        data: [item]
      })
      map[item.date] = item
    } else {
      //因为forEach不支持break，所以使用every实现
      nList.every((nItem) => {
        if (nItem.date === item.date) {
          nItem.data.push(item)
          return false
        }
        return true
      })
    }
  })

性能优化50%，约1.5ms！

3.性能优化实践

因为我的数组中的date是按顺序排列，而且没有重复，这样可以考虑去除第二个循环

  let map = {},
    nList = []
  //设置初始key为0
  let _nkey = 0
  arr.forEach((item, index) => {
    if (index === 0) {
      nList.push({
        date: item.date,
        data: [item]
      })
    } else {
      let oItem = arr[index - 1]
      //和前一个date一致则在当前添加，否则添加至nList
      if (item.date === oItem.date) {
        nList[_nkey]['data'].push(item)
      } else {
        nList.push({
          date: item.date,
          data: [item]
        })
        _nkey ++
      }
    }
  })

效率再次优化50%，约1ms！

项目最终效果：