Array Of JS Dates How To Group By Days

Question

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 outp

Answer 1

Presuming your data is actually strings, I don't know why you think you need either of those libraries. You are just grouping strings based on substrings.

ES5 introduced reduce, which is great for accumulating things:

A helper to create an array of dates:

// Generate a dates array given a start date and how many to create:
function genDates(startDate, count) {
  var d = new Date(+startDate),
      dates = [d];
  for (var i=0; i<count; i++) {
    d = new Date(+d);
    d.setHours(d.getHours() + 10);
    dates.push(d);
  }
  return dates;
}

This answer originally dealt with strings, modified to work with Dates:

// Generate date key 'MMM dd'
// Replaces use of moment.js
function getDateKey(date) {
  var d = date.getDate();
  var m = ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'];
  return m[date.getMonth()] + ' ' + ((d<10?'0':'') + d);
}

// Generate an array in format [{day:'MMM dd', times:[d0, d1, ...]}, ...]
// Replaces use of underscore.js
var obj = dates.reduce(function(acc, d) {
            var p = getDateKey(d)
            if (!acc[0].hasOwnProperty(p)) acc[0][p] = [];
            acc[0][p].push(d);
            return acc;
          },[{}])
          .reduce(function(acc, v){
            Object.keys(v).forEach(function(k){acc.push({day:k, times:v[k]})});
            return acc;
          },[]);

console.log(JSON.stringify(obj));

If optimal performance is the key, the above is 20 times faster than the underscore + Moment solution for an array of 5 to 100 dates. To make it faster, remove all use of iterators and libraries and use a single function with for loops. Note that the above is only one line of code longer than the solution using Moment.js and underscore.js.

Answer 2

If you need to grouping also by year or (and) month with day - I recommend to use my solution.

In answers above if you'll get different month or year with the same day - your grouping will be incorrect.

Look at the good solution:

_.groupBy(arrayOfDates, function (el) {
      return (el.getFullYear() + '|y|') + (el.getMonth() + '|m|') + (el.getDate() + '|d|');
    });

What I do here? Just create an unique keys for each date, which includes: year, month and day. And then I group an array by this unique key.

result

Answer 3

Why do need this optimization? If your array is not large enoguh than you probably don't need to optimize your algorithm.

I am not familiar with the given js libraries, but you can group your array by days with one loop. But you need to somehow determine the current day in the array and then create corresponding js object with a day-field and a times-array field and then add this object to your resulting array. It will be much faster if you presort your array before implementing this algorithm.

Answer 4

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();