Array Of JS Dates How To Group By Days

I'm trying to figure out the most optimal and with as minimum amount of loops way to group my array of js dates objects from this: (Take a note this is browser console output it's actully real JS dates like new Date())

[Sat Aug 08 2015 08:30:00 GMT+0200 (Central Europe Daylight Time), Sat Aug 08 2015 09:30:00 GMT+0200 (Central Europe Daylight Time), Sun Aug 09 2015 08:30:00 GMT+0200 (Central Europe Daylight Time), Sun Aug 09 2015 09:30:00 GMT+0200 (Central Europe Daylight Time), Mon Aug 10 2015 18:00:00 GMT+0200 (Central Europe Daylight Time), Mon Aug 10 2015 23:00:00 GMT+0200 (Central Europe Daylight Time), Tue Aug 11 2015 18:00:00 GMT+0200 (Central Europe Daylight Time), Tue Aug 11 2015 23:00:00 GMT+0200 (Central Europe Daylight Time), Wed Aug 12 2015 18:00:00 GMT+0200 (Central Europe Daylight Time), Wed Aug 12 2015 23:00:00 GMT+0200 (Central Europe Daylight Time)]

to oragnized array with each date of the same day inside a "chunk" so I can display it on the UI "Aug 08" and show 2 or how many dates inside that day.

for example:

[{day: 'Aug 08', times:[Sat Aug 08 2015 08:30:00 GMT+0200 (Central Europe Daylight Time), Sat Aug 08 2015 09:30:00 GMT+0200 (Central Europe Daylight Time)]}]

My current way I thought about doing it was

var startDays = _.map(occurences, function (date) {
  return moment(date).startOf('day').format();
});

After that to get unique days:

_.uniq(startDays, true)

and after I got the unique days another loop to add the same day to this group as you can see by now you might see why I don't like it and this is why I would love to get some smart help because nothing gets to my head with this. Thank you.

Solution

Underscore has the _.groupBy function which should do exactly what you want:

var groups = _.groupBy(occurences, function (date) {
  return moment(date).startOf('day').format();
});

This will return an object where each key is a day and the value an array containing all the occurrences for that day.

To transform the object into an array of the same form as in the question you could use map:

var result = _.map(groups, function(group, day){
    return {
        day: day,
        times: group
    }
});

To group, map and sort you could do something like:

var occurrenceDay = function(occurrence){
    return moment(occurrence).startOf('day').format();
};

var groupToDay = function(group, day){
    return {
        day: day,
        times: group
    }
};

var result = _.chain(occurences)
    .groupBy(occurrenceDay)
    .map(groupToDay)
    .sortBy('day')
    .value();