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.

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

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

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

Map

Iterate through the list of dictionaries
Convert datetime string to javascript datetime object.
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

Iterate through the new object containing dictionaries of lists.
Calculate length of each dictionary's list.
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

Use Map-reduce. Here is one example using underscore.js. It is very simple though it is a bit verbose.

var data = [{
    "USER_NAME": "User1",
        "LAST_SUCCESSFUL_CONNECT": "1373978337642"
}, {
    "USER_NAME": "User2",
        "LAST_SUCCESSFUL_CONNECT": "1374515704026"
}, {
    "USER_NAME": "User3",
        "LAST_SUCCESSFUL_CONNECT": "1374749782479"
}, {
    "USER_NAME": "User4",
        "LAST_SUCCESSFUL_CONNECT": "1274749702479"
}];

var monthNames = ["January", "February", "March", "April", "May", "June",
    "July", "August", "September", "October", "November", "December"];

var map_result = _.map(data, function (item) {
    var d = new Date(new Number(item.LAST_SUCCESSFUL_CONNECT));
    var month = monthNames[d.getMonth()] + ", " + d.getFullYear();
    return {
        "Month": month,
        "User_Count": 1
    };
});

var result_temp = _.reduce(map_result, function (memo, item) {
    if (memo[item.Month] === undefined) {
        memo[item.Month] = item.User_Count;
    }else{
        memo[item.Month] += item.User_Count;
    }
    return memo;
},{});

//then wrap the result to the format you expected.
var result = _.map(result_temp, function(value, key){
    return {
        "Month": key,
        "User_Count": value
    };
});

console.log(result);

maybe this will be useful

LINQ for JavaScript
http://linqjs.codeplex.com/

来源：https://stackoverflow.com/questions/17860382/how-can-i-group-an-array-of-objects-by-month

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