Find date format from date string

dateUtil.ts

import { addDays, getHours, isAfter, isBefore, parse, setHours, startOfDay, subDays } from 'date-fns';

export class dateUtil {
  /**
   * Find the correct date format, by comparing two consecutive days. 
   *
   * @param date The date string to find the format for
   * @param refDate The date string to compare. This should be either the next or previous day.
   *                This is only needed to determine if the day is before or after the month in the format.
   */
  findDateFormat(date: string, refDate: string): string | null {
    const [d1, t1] = date.split(' ');
    const [d2, t2] = refDate.split(' ');
    // Generate all possible date formats
    const formats = [
      ['d', 'dd'],
      ['M', 'MM'],
      ['yy', 'yyyy'],
    ];
    const delimiters = ['.', '/', '-', ''].filter(d => d1.includes(d)); // Only apply delimiters that are present in the given date string
    let dateFormats = delimiters
      .flatMap(delimiter =>
        formats[0].flatMap(day =>
          formats[1].flatMap(month =>
            formats[2].flatMap(year =>
              [
                [day, month, year],
                [month, day, year],
                [year, month, day],
              ].map(order => order.join(delimiter)),
            ),
          ),
        ),
      )
      .filter(
        (format, i, arr) =>
          arr.indexOf(format) === i && !format.includes('d.d') && !format.includes('M.M') && !format.includes('y.y'),
      );
    // Support those formats which does not separate date and time with a space
    if (delimiters.length === 1 && t1 == null) {
      dateFormats = dateFormats.reduce((acc, f) => {
        return [...acc, f, `${f}H`, `${f}HH`];
      }, [] as string[]);
    }
    // Loop through all formats and try to parse the date and reference date
    for (const format of dateFormats) {
      const date = parse(d1, format, new Date());
      const referenceDate = parse(d2, format, new Date());
      if (!isNaN(date.getTime()) && !isNaN(referenceDate.getTime())) {
        // If the date is one day after the reference date or one day before, this is our format.
        const nextDay = setHours(addDays(new Date(referenceDate), 1), getHours(date));
        const previousDay = setHours(subDays(new Date(referenceDate), 1), getHours(date));
        if (
          (isAfter(date, referenceDate) && date.getTime() === nextDay.getTime()) ||
          (isBefore(date, referenceDate) && date.getTime() === previousDay.getTime())
        ) {
          if (t1 != null) {
            const [hour, minute] = t1.split(':');
            return `${format} ${
              minute != null
                ? `H${hour.length > 1 ? 'H' : ''}:m${minute.length > 1 ? 'm' : ''}`
                : `H${hour.length > 1 ? 'H' : ''}`
            }`;
          }
          return format;
        }
      }
    }
    return null;
  }
}