I think there is another bug 🐞:

When the hour part of the sum is more than two digits, then only the last two digits of the hour value is returned-

For example,

addTimes("99:00:00","01:0:0") // returns '00:00:00' instead of '100:00:00'

This can be fixed by not using slice(-2) on hour value in the return expression return ('0' + hours).slice(-2) + ':' + ('0' + minutes).slice(-2) + ':' + ('0' + seconds).slice(-2)

I fixed this by replacing the return expression to this-

 return Number('0' + hours) + ':' + ('0' + minutes).slice(-2) + ':' + ('0' + seconds).slice(-2)

✅ Here is the complete code with this minor tweak:

/**
 * Add two string time values (HH:mm:ss) with javascript
 *
 * Usage:
 *  > addTimes('04:20:10', '21:15:10');
 *  > "25:35:20"
 *  > addTimes('04:35:10', '21:35:10');
 *  > "26:10:20"
 *  > addTimes('30:59', '17:10');
 *  > "48:09:00"
 *  > addTimes('19:30:00', '00:30:00');
 *  > "20:00:00"
 *
 * @param {String} startTime  String time format
 * @param {String} endTime  String time format
 * @returns {String}
 */
function addTimes(startTime, endTime) {
  var times = [0, 0, 0];
  var max = times.length;

  var a = (startTime || '').split(':');
  var b = (endTime || '').split(':');

  // normalize time values
  for (var i = 0; i < max; i++) {
    a[i] = isNaN(parseInt(a[i])) ? 0 : parseInt(a[i]);
    b[i] = isNaN(parseInt(b[i])) ? 0 : parseInt(b[i]);
  }

  // store time values
  for (var i = 0; i < max; i++) {
    times[i] = a[i] + b[i];
  }

  var hours = times[0];
  var minutes = times[1];
  var seconds = times[2];

  if (seconds >= 60) {
    var m = (seconds / 60) << 0;
    minutes += m;
    seconds -= 60 * m;
  }

  if (minutes >= 60) {
    var h = (minutes / 60) << 0;
    hours += h;
    minutes -= 60 * h;
  }

  return (
    Number('0' + hours) +
    ':' +
    ('0' + minutes).slice(-2) +
    ':' +
    ('0' + seconds).slice(-2)
  );
}

After the update:

addTimes("99:00:00","01:0:0") // returns  the correct output '100:00:00'

I really liked your solution, and I appreciate it so much, it saved me lots of time.
If you may allow me to inform you a bug I found.

My scenario was that I have two inputs one for the checkIn time and another one for checkOut time,
users select the checkIn time so that the checkOut time - which will be 45mins after the checkIn time- can be displayed at the other input checkout input.
I found a bug when the users chose a AM time value, as your function returns an invalid value which won't be displayed at the checkout

After investigation I found that:
when I pass the startTime which is 07:00 for example and the endTime 00:45 your function returns 7:45 which is invalid for inputs 07:45 is the valid one

so I've modified the returned value in my code to look something like this
const formattedHours = Number("0" + hours).toString().length === 2 ? Number("0" + hours) :0${Number("0" + hours)}return formattedHours + ":" + ("0" + minutes).slice(-2);

And it solved my problem, and the checkout value displayed properly at the checkOut input.

@iShubhamPrakash

Your code looks nice, here's the TS version:

function addTimes(startTime: string, endTime: string): string {
  const times = [0, 0, 0]
  const max = times.length

  const a = (startTime || "").split(":")
  const b = (endTime || "").split(":")

  // normalize time values
  for (let i = 0; i < max; i++) {
    const si = i.toString()
    a[si] = Number.isNaN(Number.parseInt(a[i])) ? 0 : Number.parseInt(a[i])
    b[si] = Number.isNaN(Number.parseInt(b[i])) ? 0 : Number.parseInt(b[i])
  }

  // store time values
  for (let i = 0; i < max; i++) {
    const si = i.toString()
    times[si] = a[i] + b[i]
  }

  let hours = times[0]
  let minutes = times[1]
  let seconds = times[2]

  if (seconds >= 60) {
    const m = (seconds / 60) << 0
    minutes += m
    seconds -= 60 * m
  }

  if (minutes >= 60) {
    const h = (minutes / 60) << 0
    hours += h
    minutes -= 60 * h
  }

  return `${Number(`0${hours}`)}:${(`0${minutes}`).slice(-2)}:${(`0${seconds}`).slice(-2)}`
}