String to 12hr Time in TypeScript

/**
 * Given a string time string convert to 12hr format (HH:MM:am|pm)
 *
 * @param {string} timeString Time string
 * @returns {string | null} valid 12hr time string (or null for unparsable times)
 *
 * @example
 * convertTo12HourFormat("1575") -> "4:15pm"
 * convertTo12HourFormat("2530pm") -> "1:30pm"
 * convertTo12HourFormat("2500") -> "1:00am"
 * convertTo12HourFormat("0090") -> "1:30am"
 * convertTo12HourFormat("3500") -> "11:00am"
 * convertTo12HourFormat("1pm") -> "1:00pm"
 * convertTo12HourFormat("1:30pm") -> "1:30pm"
 * convertTo12HourFormat("1445") -> "2:45pm"
 * convertTo12HourFormat("1445pm") -> "2:45am"
 * convertTo12HourFormat("14") -> "2:00pm"
 * convertTo12HourFormat("2400") -> "12:00am"
 * convertTo12HourFormat("3500") -> "11:00am"
 */
export function convertTo12HourFormat(timeString: string): string | null {
  // Regular expression patterns to match different time formats
  const validTimeStringPattern = /^(\d{1,2})(:)?(\d{2})?$/i;
  const isAmOrPm = /(?:pm|am)$/i;
 
  // Match the input time string with different patterns
  let validTimeStringMatch = timeString.match(validTimeStringPattern);
  let isAmOrPmMatch = timeString.match(isAmOrPm);
 
  let isPm = isAmOrPmMatch && isAmOrPmMatch[0].toLowerCase() === 'pm';
 
  if (isAmOrPmMatch) {
    validTimeStringMatch = timeString.replace('am', '').replace('pm', '').match(validTimeStringPattern);
  }
 
  // valid date string match
  if (validTimeStringMatch) {
    // get hour and minute parts
    let hour = parseInt(validTimeStringMatch[1]);
    let minute = validTimeStringMatch[3] ? parseInt(validTimeStringMatch[3]) : 0;
 
    // check minute is within valid range (0-59)
    while (minute > 60) {
      minute -= 60;
 
      // for each 60mins, add another hour to the total
      hour += 1;
    }
 
    // Handle hours exceeding 24 (special cases for 2400 and 2500)
    if (hour >= 24) {
      // Wrap around for other values
      hour %= 24;
 
      if (hour === 2400) {
        hour = 0; // Set to 00:00 for 2400
      } else {
        // Wrap around for other values
        hour %= 24;
      }
    }
 
    // convert to 12hr time
    let hr12Time = hour;
    let meridiem: 'pm' | 'am' = isPm ? 'pm' : 'am';
    // check minute is within valid range (0-59)
    while (hr12Time > 12) {
      hr12Time -= 12;
 
      // switch between meridiem
      meridiem = meridiem === 'pm' ? 'am' : 'pm';
    }
 
    // Format and return the time in 24-hour format
    return `${(hr12Time === 0 ? hr12Time + 12 : hr12Time).toString()}:${minute.toString().padStart(2, '0')}${meridiem}`;
  }
 
  // Return null for invalid input
  return null;
}

convertTo12HourFormat("1575") // "4:15pm"
convertTo12HourFormat("2530pm") // "1:30pm"
convertTo12HourFormat("2500") // "1:00am"
convertTo12HourFormat("0090") // "1:30am"
convertTo12HourFormat("3500") // "11:00am"
convertTo12HourFormat("1pm") // "1:00pm"
convertTo12HourFormat("1:30pm") // "1:30pm"
convertTo12HourFormat("1445") // "2:45pm"
convertTo12HourFormat("1445pm") // "2:45am"
convertTo12HourFormat("14") // "2:00pm"
convertTo12HourFormat("2400") // "12:00am"
convertTo12HourFormat("3500") // "11:00am"