RelativeTime formatter inspired by DayJs

index.ts

import {relativeTime} from './RelativeTime.ts'

console.log(relativeTime(new Date(), new Date(Date.now()-(15*1000))));
// -> "a few seconds ago" 

RelativeTime.ts

/**
 * Locale strings
 * You can get it from https://github.com/iamkun/dayjs/blob/dev/src/locale/
 */
const loc = {
  future: 'in %s',
  past: '%s ago',
  s: 'a few seconds',
  m: 'a minute',
  mm: '%d minutes',
  h: 'an hour',
  hh: '%d hours',
  d: 'a day',
  dd: '%d days',
  M: 'a month',
  MM: '%d months',
  y: 'a year',
  yy: '%d years'
} as const

/**
 * Thresholds
 * l: locale string
 * r: max value displayed with current locale string
 * d: divisor to convert from the previous unit to the current one
 *
 * @example :
 * We start with milliseconds. Thresholds[0]:
 * then we convert it to seconds dividing it by d=1000 and up to r=44 seconds we show the string of the locale l="s" (a few seconds)
 * next: Thresholds[1]:
 * we do not convert or div by d=1, and up to r=89 seconds we show the locale l="m" (a minute)
 * etc...
 */
const Thresholds = [
  { l: 's', r: 44, d: 1000 },
  { l: 'm', r: 89, d: 1 },
  { l: 'mm', r: 44, d: 60 },
  { l: 'h', r: 89, d: 1 },
  { l: 'hh', r: 21, d: 60 },
  { l: 'd', r: 35, d: 1 },
  { l: 'dd', r: 25, d: 24 },
  { l: 'M', r: 45, d: 1 },
  { l: 'MM', r: 10, d: 365/12 },
  { l: 'y', r: 17, d: 1 },
  { l: 'yy', r: Number.MAX_VALUE, d: 12 }
] as const;

export function relativeTime(from = new Date(), to = new Date()): string {
  let result = to.getTime() - from.getTime();
  for (const { l: localeString, r: max, d: divider } of Thresholds) {
    result /= divider;
    let abs = Math.round(Math.abs(result));
    if (abs <= max) {
      return (result > 0 ? loc.future : loc.past).replace('%s', loc[localeString].replace('%d', abs.toString()));
    }
  }
  throw new Error('Invalid Threshold object');
}