25Implement a countdown timer with pause, resume and reset

Implement two functions:

• formatTime(ms): format a remaining time as "mm:ss", rounding up to the next whole second, so the display shows 00:00 only when the time is really over.
• createCountdown(durationMs, { interval = 1000, onTick, onComplete }): return a timer with start(), pause(), resume(), reset(), and read-only remaining (milliseconds left) and running (boolean) properties.

formatTime(61000); // "01:01"
formatTime(59001); // "01:00"

const timer = createCountdown(5 * 60 * 1000, {
  onTick: (ms) => (display.textContent = formatTime(ms)),
  onComplete: () => alert("Time is up"),
});
timer.start();
timer.pause();
timer.resume();
timer.reset();

Requirements

• onTick(remaining) runs every interval while running. onComplete runs exactly once, when the time reaches 0.
• Do not drift: compute the remaining time from the clock, not by subtracting interval on every tick. Browsers delay timers, especially in background tabs.
• pause() freezes remaining; resume() continues from there.
• reset() stops the timer and restores the full duration.
• Calling start() twice must not create a second interval.