javascript for group object properties based on another property

1var list = [
2     {   date: "2017-01-01",
3         type: "type1",
4         amount: 100
5     },
6     {   date: "2017-01-01",
7         type: "type2",
8         amount: 150
9     },
10     {   date: "2017-01-02",
11         type: "type1",
12         amount: 200
13     }]
14
15var dateArrKeyHolder = [];
16var dateArr = [];
17list.forEach(function(item){
18    dateArrKeyHolder[item.date] = dateArrKeyHolder[item.date]||{};
19    var obj = dateArrKeyHolder[item.date];
20    if(Object.keys(obj).length == 0)
21    dateArr.push(obj);
22    
23    obj.date = item.date;
24    obj.activities  = obj.activities || [];
25    
26    obj.activities.push({type:item.type, amount: item.amount });
27});
28
29
30console.log(JSON.stringify(dateArr));
31
32/* Result:
33 [{"date":"2017-01-01","activities":[{"type":"type1","amount":100},{"type":"type2","amount":150}]},
34  {"date":"2017-01-02","activities":[{"type":"type1","amount":200}]}] 
35*/