Solution to the problem of merging overlapping calendar appointments.

merge_overlapping_calendar_appointme nts.js

'use strict';

/******************************************************************************/

const overlaps = (previous, current) => current.startTime <= previous.endTime;

/******************************************************************************/

const merge_meetings = (most_recently_merged) => (next) =>
  ({ startTime : most_recently_merged.startTime, endTime : next.endTime })

/******************************************************************************/

const tap = (val, fn) => (arg) => (fn(arg), val)

// @impure
const rpush = (list) => (element) => tap(list, (element) => list.push(element))

/******************************************************************************/

const head = (list) => list[0]

const tail = (list) => list[list.length - 1]

const push_if_meetings_overlap = (merged) => (current) => {
  const most_recently_merged = tail(merged),
    length = merged.length;

  return tap(merged, () => {
    // @impure
    if (overlaps(most_recently_merged, current))
      merged[length - 1] = merge_meetings(most_recently_merged)(current)
    else
      merged.push(current)
  })();
}

/******************************************************************************/

const merge_overlapping_meetings = (list_of_meetings) => {
  list_of_meetings.sort((a, b) => a.startTime < b.startTime ? -1 : 1);

// @impure
  return list_of_meetings.slice(1).reduce((merged, current, index) => {
    return push_if_meetings_overlap(merged)(current);
  }, [head(list_of_meetings)])
}

/******************************************************************************/

module.exports = merge_overlapping_meetings;

var a =   [
    {startTime: 0,  endTime: 1},
    {startTime: 3,  endTime: 5},
    {startTime: 4,  endTime: 8},
    {startTime: 10, endTime: 12},
    {startTime: 9,  endTime: 10},
]

console.log(merge_overlapping_meetings(a));

I should update nomenclature and curry overlaps for consistency, but you get the idea