const localeFunc = (number: number, index: number, totalSec: number): [string, string] => {
  // number: the timeago / timein number;
  // index: the index of array below;
  // totalSec: total seconds between date to be formatted and today's date;
  return [
    ['just now', 'right now'],
    ['%s seconds ago', 'in %s seconds'],
    ['1 minute ago', 'in 1 minute'],
    ['%s minutes ago', 'in %s minutes'],
    ['1 hour ago', 'in 1 hour'],
    ['%s hours ago', 'in %s hours'],
    ['1 day ago', 'in 1 day'],
    ['%s days ago', 'in %s days'],
    ['1 week ago', 'in 1 week'],
    ['%s weeks ago', 'in %s weeks'],
    ['1 month ago', 'in 1 month'],
    ['%s months ago', 'in %s months'],
    ['1 year ago', 'in 1 year'],
    ['%s years ago', 'in %s years']
  ][index];
};
// register your locale with timeago
register('my-locale', localeFunc);

// use it
format('2016-06-12', 'my-locale');