For cohort analysis on logins

checkCohort.js

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 
        }
    }
}

function checkCohort(signup, dates) {
    // Checks for the following scenarios and returns one of the four states:
    // 1. Embedded (Sign in minimum 1x per week over the past 4 weeks)
    // 2. Active (Sign in a minimum of 1x per month in the past 4 weeks)
    // 3. Deactive (Have been part of the platform for more than 4 weeks and yet have not
    // achieved Active or Embedded status)
    // 4. Initial (have signed in but have not been part of the platform for a full month yet)
    // signup should be timestamp of signup, dates an array of ISO string formatted dates
    var today = new Date();
    var week = 7*24*60*60*1000;
    var sign_up = new Date(signup);
    if (((today-sign_up)/(week*4))<1){
        // less than four weeks since signup
        return "initial";
    } else {
        // more than four weeks since signup
        if (dates.length === 0){
            return "deactive";
        } else {
            if (checkEmbedded(dates)) {
                return "embedded";
            } else {
                // check if any of the dates are in the last month
                dates.sort();
                dates.reverse();
                for (var i=1;i<dates.length;i++){
                    var this_date = new Date(dates[i]);
                    if (((today-this_date)/(week*4))<1){
                        return "active";
                    }
                }
                // drop back to deactive
                return "deactive";
            }
        }
    }
}