formatSeconds

Returns the ISO format of the given number of seconds.

• Divide s with the appropriate values to obtain the appropriate values for hour, minute and second.
• Store the sign in a variable to prepend it to the result.
• Use Array.prototype.map() in combination with Array.prototype.floor() and String.prototype.padStart() to stringify and format each segment.
• Use String.prototype.join(':') to combine the values into a string.

const formatSeconds = s => {
  const [hour, minute, second, sign] =
    s > 0
      ? [s / 3600, (s / 60) % 60, s % 60, '']
      : [-s / 3600, (-s / 60) % 60, -s % 60, '-'];

  return (
    sign +
    [hour, minute, second]
      .map(v => `${Math.floor(v)}`.padStart(2, '0'))
      .join(':')
  );
};

formatSeconds(200); // '00:03:20'
formatSeconds(-200); // '-00:03:20'
formatSeconds(99999); // '27:46:39'