https://www.hackerrank.com/challenges/time-conversion/problem

timeConversion.js

function timeConversion(s) {
  let [hour, minute, seconds] = s.split(':');
  hour = parseInt(hour);
  const secs = seconds.substring(0, 2);

  if (hour === 12 && minute <= 59 && seconds.endsWith('AM')) {
    hour = parseInt(hour) - 12 === 0 ? '00' : parseInt(hour) - 12;
    return `${hour}:${minute}:${secs}`;
  } else if (seconds.endsWith('PM') && hour >= 1 && hour <= 11 && parseInt(minute) <= 59) {
    hour = parseInt(hour) === 12 ? '12' : parseInt(hour) + 12;
    hour = hour < 10 ? `0${hour}` : hour;
    return `${hour}:${minute}:${secs}`;
  } else {
    hour = hour < 10 ? `0${hour}` : hour;
    return `${hour}:${minute}:${secs}`;
  }
}

console.log(timeConversion('12:40:22AM'));