Javascript function that takes an array of ISO string formatted dates related to a users signin activity and determines they have signed in a minimum 1x per week over the past four weeks. Used to determine if they are "Embedded" in cohort terms.

checkEmbedded.js

// bad
dates = ["2013-04-02T08:27:01.209Z", "2013-04-26T08:27:01.209Z", "2013-05-16T08:27:01.209Z", "2013-05-23T08:27:01.209Z", "2013-05-24T08:27:01.209Z"];

// good
dates = ["2013-04-25T08:27:01.209Z", "2013-04-31T08:27:01.209Z", "2013-05-06T08:27:01.209Z", "2013-05-13T08:27:01.209Z", "2013-05-20T08:27:01.209Z", "2013-05-24T08:27:01.209Z"];

function checkEmbedded(dates) {
    // takes an array of ISO string formatted dates and determines 
    // they have signed in a minimum 1x per week over the past four weeks
    var today = new Date();
    var week = 7*24*60*60*1000;
    dates.sort();
    dates.reverse();
    if (dates.length < 4){
        // hasn't visited more than 4 times yet
        return false;
    } else {
        // been 4 times so validate no less than 7 days between each visit
        var forth = new Date(dates[3]); // Check 4th date not over 4 weeks
        var last = new Date(dates[dates.length-1]); // Check last date no less than 4 weeks
        if (((today-forth)/(week*4))>1 || (today-last) < (week*4)){
            // the 4th date is older than 4 weeks old might as well stop now
            return false;
        } else {
            var last_date = new Date(dates[0]); // skip the first
            for (var i=1;i<dates.length;i++){
                var this_date = new Date(dates[i]);
                if (((today-this_date)/(week*4))<1){
                    // still under four weeks    
                    if ((last_date-this_date)>week) {
                        // Gap is greater than 7 days
                        return false;
                    } else {
                        // Gap is less, get ready for next loop
                        last_date = this_date;
                    }
                } else {
                    // we're now over four weeks without issues from today so stop
                    return true;
                }
            }
            return true; // In case there's just four good weeks 
        }
    }
}