Sorting Dates Does Not Return Sorted Dates

Question

For some strange reason the following function I wrote for converting an array of date strings to javascript date objects, sorting them, then returning an array of sorted date strings does not sort properly:

sortdates: function(dates, separator) {
    var sorteddates = [],
        datestr =[];
    sorteddates = dates.map(function(val) {
        return new Date(val.replace("/"+separator+"/g", " "));
    }).sort();
    for ( i=0; i<sorteddates.length; i++ ) {
        datestr.push((sorteddates[i].getMonth()+1) + "-" + sorteddates[i].getDate() + "-" + sorteddates[i].getFullYear());
    }
    return datestr;
}

If I make a test array of date strings and apply this function:

var testarray = ["2013-8-1", "2013-8-8", "2013-8-15", "2013-8-22", "2013-9-5", "2013-9-12", "2013-8-2", "2013-8-3", "2013-8-4", "2013-8-7", "2013-8-11", "2013-8-14", "2013-8-18", "2013-8-25"];
console.log(sortdates(testarray, "-"));

I get the following logged to the console:

["8-2-2013", "8-3-2013", "8-4-2013", "8-11-2013", "8-18-2013", "8-25-2013", "8-1-2013", "8-8-2013", "8-15-2013", "8-22-2013", "9-5-2013", "9-12-2013", "8-7-2013", "8-14-2013"]

Clearly not sorted.

nnnnnn · Accepted Answer · 2013-08-04 05:45:20Z

0

.sort() will sort in "dictionary" order, not numeric or date/time order. If the values aren't strings they'll get a default string conversion which for dates does not give you something useful to sort on.

But you can supply a custom sort callback that knows how to sort dates:

.sort(function(a,b) { return a.getTime() - b.getTime(); });

Demo: http://jsfiddle.net/TUAz9/

answered Aug 4, 2013 at 5:45

nnnnnn

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Comments

jcsanyi · Accepted Answer · 2013-08-04 07:01:12Z

sort() will sort based on the string representation of the entries in the array. In this case, the string representation of a Date is something like Thu Aug 01 2013 00:00:00 GMT-0700 (PDT), so the primary sort will be by the day of the week, alphabetically.

Fri Aug 02 2013 00:00:00 GMT-0700 (PDT)
Sat Aug 03 2013 00:00:00 GMT-0700 (PDT)
Sun Aug 04 2013 00:00:00 GMT-0700 (PDT)
Sun Aug 11 2013 00:00:00 GMT-0700 (PDT)
Sun Aug 18 2013 00:00:00 GMT-0700 (PDT)
Sun Aug 25 2013 00:00:00 GMT-0700 (PDT)
Thu Aug 01 2013 00:00:00 GMT-0700 (PDT)
Thu Aug 08 2013 00:00:00 GMT-0700 (PDT)
Thu Aug 15 2013 00:00:00 GMT-0700 (PDT)
Thu Aug 22 2013 00:00:00 GMT-0700 (PDT)
Thu Sep 05 2013 00:00:00 GMT-0700 (PDT)
Thu Sep 12 2013 00:00:00 GMT-0700 (PDT)
Wed Aug 07 2013 00:00:00 GMT-0700 (PDT)
Wed Aug 14 2013 00:00:00 GMT-0700 (PDT)

You probably want to look at using a custom sort callback function, or some other representation of the date - perhaps start by mapping the array to a timestamp value instead of a Date object?

return new Date(/* stuff */).getTime();

...along with the corresponding changes to the formatting code later in your function.

Putting this all together, a function like this would work like you're expecting:

sortdates: function(dates, separator) {
    return dates.map(function(val) {
        return new Date(val.replace("/"+separator+"/g", " ")).getTime();
    }).sort().map(function(val) {
        var d = new Date(val);
        return (d.getMonth()+1) + "-" + d.getDate() + "-" + d.getFullYear();
    });
}

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