How can I group an array of objects by month?

Question

I'm using JavaScript. I have an array that contains data in this format:

[
        {"USER_NAME":"User1","LAST_SUCCESSFUL_CONNECT":"1373978337642"},
        {"USER_NAME":"User2","LAST_SUCCESSFUL_CONNECT":"1374515704026"},
        {"USER_NAME":"User3","LAST_SUCCESSFUL_CONNECT":"1374749782479"}
]

(the numbers above represent UTC date/time in milliseconds.

I would like to group (count) the data by month. Something like this:

[
    {"Month":"January, 2014","User_Count": 2},
    {"Month":"February, 2014","User_Count": 1},
]

I could use jQuery if it simplifies matters.

Answer 1

This looks like a map reduce problem. The high-level solution is as follows:

1. Re-organize the members of the list.
2. Count them.

Here is a step-by-step how-to for achieving this:

Map

1. Iterate through the list of dictionaries
2. Convert datetime string to javascript datetime object.
3. Use month-year as key and list of dictionaries as value.

These are now grouped by month-year.

Example:

var l = [...];
var o = {};
var f = function(x){
    var dt_object = Date(x["LAST_SUCCESSFUL_CONNECT"]); // convert to datetime object
    var key = dt_object.year + '-' + dt_object.month;

    if (o[key] === undefined) {
        var o[key] = [];
    };

    o[key].push(x)
}

_.map(l, f(x)) //apply f to each member of l

Reduce

1. Iterate through the new object containing dictionaries of lists.
2. Calculate length of each dictionary's list.
3. Use count as key and length of list as its value.

Example:

var g = function(member_count){
    //extra logic may go here
    return member_count
}

for member in o {
    count = _.reduce(l, g(member))
    member['count'] = count
}

Resulting API

o['month-year'] //for list of dictionaries in that month
o['month-year']['count'] //for the count of list of dictionaries in that month.

References:

For map and reduce functions in javascript see underscore.js:
http://underscorejs.org/#map
http://underscorejs.org/#reduce

Javascript Date object:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date

For more information on Date and DateTime objects:
https://en.wikipedia.org/wiki/ISO_8601

For more information on map reduce:
https://en.wikipedia.org/wiki/MapReduce