js 转树结构

    toTree (data) {
      var map = {}
      var val = []
      data.forEach(function (item) {
        map[item.id] = item
      })
      data.forEach(function (item) {
        var parent = map[item.parentId]
        if (parent) {
            (parent.children || (parent.children = [])).push(item)
        } else {
            val.push(item)
        }
      })
      return val
    }