A utility helper to calculate "time ago" or "from now " in a readable format using Intl API

fromNow.js

// Order does matter
const units = [
  ['second', 1],
  ['minute', 60],
  ['hour', 60 * 60],
  ['day', 24 * 60 * 60],
  ['month', 30 * 24 * 60 * 60],
  ['year', 12 * 30 * 24 * 60 * 60],
];

export function diffUnitFor(from) {
  const delta = Math.round((from - Date.now()) / 1000); // Rounded delta in seconds

  for (let i = 0; i < units.length; i += 1) {
    if (Math.abs(delta) < units[i][1]) {
      const [unit, value] = units[i - 1];

      // Exit the function once a unit will be found
      return { unit, delta: Math.round(delta / value) };
    }
  }

  return {
    delta: Math.round(delta / (12 * 30 * 24 * 60 * 60)),
    unit: 'year'
  };
}

/**
 * Calculate "time ago" or "from now" in a readable format using Intl API
 *
 * @param {Date|String} date
 * @param {String} lang
 * @returns {String}
 */
export default function fromNow(date, lang = 'en') {
  let from = date;

  if (!date) {
    return '';
  }

  if (!(date instanceof Date)) {
    from = new Date(date);
    
    if (isNaN(from.getTime())) {
      return '';
    }
  }

  const rtf1 = new Intl.RelativeTimeFormat(lang, { style: 'long' });
  const { delta, unit } = diffUnitFor(from);

  return rtf1.format(delta, unit);
}


// assertions
const originalNowUtil = Date.now;

describe('utils/fromNow', () => {
  beforeAll(() => {
    /* eslint-disable global-require */
    const RelativeTimeFormat = require('relative-time-format').default;

    RelativeTimeFormat
      .addLocale(require('relative-time-format/locale/en.json'));

    RelativeTimeFormat
      .addLocale(require('relative-time-format/locale/fa.json'));

    global.Intl.RelativeTimeFormat = RelativeTimeFormat;

    global.Date.now = jest
      .fn(() => new Date('2020-04-20T10:20:30Z').getTime());
  });

  describe('#fromNow', () => {
    it('should return time ago for given time string', () => {
      expect(fromNow('2017-07-10T12:24:01Z')).toBe('3 years ago');

      expect(fromNow('2020-04-20T10:20:00Z')).toBe('30 seconds ago');

      expect(fromNow('2020-03-20T10:20:00Z')).toBe('1 month ago');

      expect(fromNow('2020-04-20T10:19:00Z')).toBe('1 minute ago');

      expect(fromNow('2020-04-19T10:19:00Z')).toBe('1 day ago');

      expect(fromNow('2020-04-20T10:15:00Z')).toBe('5 minutes ago');
    });

    it('should return time ago for given time string and lang', () => {
      expect(fromNow('2017-07-10T12:24:01Z', 'fa')).toBe('3 سال پیش');

      expect(fromNow('2020-04-20T10:15:00Z', 'fa')).toBe('5 دقیقه پیش');
    });

    it('should return empty string if date is not provided', () => {
      expect(fromNow('')).toBe('');
    });

    it('should return empty string if date is invalid', () => {
      expect(fromNow('dummy input')).toBe('');
    });
  });

  afterAll(() => {
    global.Date.now = originalNowUtil;
  });
});