Convert milliseconds to a formatted duration string [Javascript].

msToDuration.js

/**
 * Creates a duration string from a number of milliseconds.
 * 
 * Example:
 * ```js
 * const dateA = new Date("2022-01-01T12:55:02");
 * const dateB = new Date("2022-01-08T15:00:00");
 * const msDuration = nowDate - someOlderDate;
 * const str = msToDuration(msDuration);
 * console.log(str); // "7 d 02 h 04 m 58 s"
 * const otherStr = msToDuration(3000);
 * console.log(otherStr); // "03 s"
 * ```
 *
 * @param ms Number of milliseconds
 * @returns Formatted date string
 */
function msToDuration(ms) {
  // Returns an array with the integer division and modulo of (a,b).
  // Used bellow for readability when computing days/hours/etc.
  const divMod = (a, b) => [Math.floor(a / b), a % b];

  // Get the days, hours, minutes and seconds as integers.
  const [days,    ms1] = divMod(ms,  24 * 60 * 60 * 1000);
  const [hours,   ms2] = divMod(ms1,      60 * 60 * 1000);
  const [minutes, ms3] = divMod(ms2,           60 * 1000);
  const [seconds, _]   = divMod(ms3,                1000);

  // Boolean checks (if false then don't display them).
  const hasDays    = days > 0;
  const hasHours   = hours > 0   || hasDays;
  const hasMinutes = minutes > 0 || hasHours;
  const hasSeconds = seconds > 0 || hasMinutes;

  // Construction of the formatted string.
  const str =
    (hasDays    ? `${days} d ` : "") +
    (hasHours   ? `${hours.toString().padStart(2, "0")} h ` : "") +
    (hasMinutes ? `${minutes.toString().padStart(2, "0")} m ` : "") +
    (hasSeconds ? `${seconds.toString().padStart(2, "0")} s` : "");

  return str;
}