题主你最后的那个结构怪怪的

var data = [{obj0:"2016-12-01",obj1:"交通事故",obj2:"218"},{obj0:"2016-12-01",obj1:"医疗事故",obj2:"1"},{obj0:"2016-12-02",obj1:"交通事故",obj2:"200"},{obj0:"2016-12-02",obj1:"医疗事故",obj2:"5"}];
var res = {};//字典
data.forEach(function(item){
  if(!res[item.obj0]){//如果不存在对应的键值对，则创建一个
    res[item.obj0]=[];
   }
   var obj = {}; 
   obj[item.obj1] = item.obj2;
   res[item.obj0].push(obj);
});

这样就能得到一个对象，跟你要的结构差不多了，后面转化我就不写了。

let data = [{obj0:"2016-12-01",obj1:"交通事故",obj2:"218"},{obj0:"2016-12-01",obj1:"医疗事故",obj2:"1"},{obj0:"2016-12-02",obj1:"交通事故",obj2:"200"},{obj0:"2016-12-02",obj1:"医疗事故",obj2:"5"}],
    res = {};
for (let item of data) {
    let obj = res[item.obj0] || {};
    obj[item.obj1] = item.obj2;
    res[item.obj0] = obj;
}
console.log(JSON.stringify(res))
// {"2016-12-01":{"交通事故":"218","医疗事故":"1"},"2016-12-02":{"交通事故":"200","医疗事故":"5"}}

现在的问题咋越来越水了