Split array of objects into new arrays based on year of object's date

Question

I have an array of objects called objarray. Each object looks like this:

var object = {
    age: "45"
    coords: "-37.807997 144.705784"
    date: Sun Jul 28 2002 00:00:00 GMT+1000 (EST)
}

(date is a Date object)

I need to push each object into a new array based on the date. I want the end result to look like this:

var dateGroups = [[object, object, object],[object, object], [object, object, object]];

Each array within dateGroups contains objects with the same date.

Is this possible to do with arrays? Previously I generated a new object which contained all the objarray objects grouped by date (dates generated from the data):

var alldates = {
  "1991" : [object, object, object],
  "1992" : [object, object],
  //etc...
}

The above seems like a weird solution in practice though, I only need to be able to access the objects by year: i.e. dateGroups[0] = array of objects from the first year

How would I get the data into something like the dateGroups array? Is there a better way to store this type of data?

Platter

better to choose JSON for this. – Anagram

What kind of JSON structure would be suitable to group by date? – Platter

Answer 1

Consider using the Underscore.js groupBy function, followed by sortBy.

groupBy will produce a structure like alldates, if you call it like so:

var alldates = _.groupBy(objarray, function(obj) {
    return obj.date.getFullYear();
});

sortBy can be used to simply sort by key, like this:

var dateGroups = _.sortBy(alldates, function(v, k) { return k; });

You can also combine the two like this:

var dateGroups = _.chain(objarray)
                  .groupBy(function(obj) { return obj.date.getFullYear(); })
                  .sortBy(function(v, k) { return k; })
                  .value();

Depending on your use case, I can see reasons for using an array of arrays, or an object map of arrays. If you're looking up on index, definitely use the later.

Gaudette

This worked fantastically. Thanks! I'll definitely need to use the indexes later on. – Platter

Pure javascript version for this? We are not allowed to use this library (loadsh) – Dario

Without lodash or a similar library, a solution based on reduce like naortor's is the best place to start. – Gaudette

Answer 2

An answer using reduce.

var ans = objects.reduce(function(prev,curr) {
    if (!prev[curr.date.getFullYear()]) prev[curr.date.getFullYear()] = [];
    prev[curr.date.getFullYear()] = curr;
    return prev;
},[]).reduce(function(prev,curr) {
    prev.push(curr);
    return prev;
},[]);

the first reduce is for grouping the objects by dates, and the second is for making the key of the array run from 0 to the number of different years - instead of the key being the year itself.

Brnaby

A bit late, but I think it should be if(!prev[...]) {prev[...] = [curr];} else {prev[...].push(curr)} – Scrubby

Answer 3

Once you have the dates in a structure that looks like this:

var alldates = {
  "1991" : [object, object, object],
  "1992" : [object, object],
  //etc...
}

You can just use this code to convert them to the structure you want:

var dateGroups = [];

for(var year in allDates){
   dateGroups[dateGroups.length] = allDates[year];
}

Zachery