1//Repositories
2https://github.com/trekhleb/javascript-algorithms

1var item,
2    i = 0,
3    groups = {},
4    year, day;
5while (item = eventsArray[i++]) {
6    item = new Date(item.date);
7    year = item.getFullYear();
8    day = item.getDate();
9    groups[year] || (groups[year] = {}); // exists OR create {}
10    groups[year][day] || (groups[year][day] = []);  // exists OR create []
11    groups[year][day].push(item);
12}
13