formatDuration

Returns the human-readable format of the given number of milliseconds.

• Divide ms with the appropriate values to obtain the appropriate values for day, hour, minute, second and millisecond.
• Use Object.entries() with Array.prototype.filter() to keep only non-zero values.
• Use Array.prototype.map() to create the string for each value, pluralizing appropriately.
• Use String.prototype.join(', ') to combine the values into a string.

const formatDuration = ms => {
  if (ms < 0) ms = -ms;
  const time = {
    day: Math.floor(ms / 86400000),
    hour: Math.floor(ms / 3600000) % 24,
    minute: Math.floor(ms / 60000) % 60,
    second: Math.floor(ms / 1000) % 60,
    millisecond: Math.floor(ms) % 1000
  };
  return Object.entries(time)
    .filter(val => val[1] !== 0)
    .map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
    .join(', ');
};

formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574);
// '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'