Calculate available time windows

scheduleEvent.js

// Designed for Azure functions
module.exports = function (context, req) {
    context.log('JavaScript HTTP trigger function processed a request.');
    const scheduledEvents = [];

    //Add all scheduled events into an array
    req.body.forEach(function (calendar) {
        calendar.items.forEach(function (items) {
            scheduledEvents.push({
                start: new Date(items['start']).toLocaleTimeString('en-US', { hour12: false }),
                end: new Date(items['end']).toLocaleTimeString('en-US', { hour12: false })
            });
            context.log(items['start']);
            context.log(new Date(items['start']).getUTCHours());
        })
    });

    // Sort all events chronologically
    scheduledEvents.sort(compare);

    const schedule = findSlotInDay(scheduledEvents);

    // Find first slot 
    context.log(JSON.stringify(schedule));

    context.res = {
        body: {
            availableSlots: schedule.length,
            availableTime: schedule
        }
    };

    context.done();
};

// Data comparison function
function compare(a, b) {
    if (a['start'] < b['start']) {
        return -1;
    }
    if (a['start'] > b['start']) {
        return 1;
    }
    return 0;
}

// Courtesy of StackOverflow 
// https://stackoverflow.com/questions/19277348/how-to-calculate-free-available-time-based-on-array-values
function findSlotInDay(scheduledEvents) {
    const schedule = [];
    let start = '08:00:00';
    let end = '17:00:00';
    const finalEnd = '17:00:00'
    for(let i = 0, l = scheduledEvents.length; i < l; i++) {
        end = subtractMinute(scheduledEvents[i].start);
      
        if (i)
            start = addMinute(scheduledEvents[i-1].end);
      
        if ((end && !i) || (end && i && scheduledEvents[i-1].end < scheduledEvents[i].start))
            schedule.push({start: start, end: end});
      
        if (i+1 === l) {
            start = addMinute(scheduledEvents[i].end);
      
            if (start) {
                schedule.push({start: start, end: finalEnd});
            }
        }
    }
    return schedule;
}

function subtractMinute(time) {
    let h = + time.substr(0, 2);
    let m = + time.substr(3, 2);
  
    if (m > 0) {
      m -= 1;
    } else {
      if (h > 0) {
        h -= 1;
      } else {
        return false;
      }
      m = 59;
    }
  
    if (h < 10)
      h = '0' + h;
  
    if (m < 10)
      m = '0' + m;
  
    return h + ':' + m + ':00';
}
  
function addMinute(time) {
    let h = + time.substr(0, 2);
    let m = + time.substr(3, 2);
  
    if (m < 59) {
      m += 1;
    } else {
      if (h < 22) {
        h += 1;
      } else {
        return false;
      }
      m = 0;
    }
  
    if (h < 10)
      h = '0' + h;
  
    if (m < 10)
      m = '0' + m;
  
    return h + ':' + m + ':00';
}