simple delayed timer with onExpire callback

timer.js

const SEC = 1000
const MIN = SEC * 60

/** @param time, Δ ms */
function getMinSec(time) {
  return {
    sec: Math.floor((time % MIN) / SEC),
    min: Math.floor(time / MIN)
  }
}

/** @param time, Δ ms */
function formatTime(time) {
  var time = getMinSec(time)
  return pad2(time.min) + ':' + pad2(time.sec)
}

function pad2 (number) {
  return number > 9 ? number : ('0' + number)
}

 /**
  * @param interval, Δ ms
  * @param delayShow, Δ ms
  * @param onExpire — callback
  **/
  function startTimer(interval, delayShow, onExpire) {
    var initialTime = new Date().valueOf()

    /** always positive */
    function calcElapsed() {
      var currentTime = new Date().valueOf()
      var elapsed = interval - (currentTime - initialTime)
      return elapsed > 0 ? elapsed : 0
    }
    function handleExpire() {
      hide()
      onExpire()
    }
    function tick() {
      var elapsed = calcElapsed()
      if (!elapsed) handleExpire()
      return renderTimer({ 'time-remained': formatTime(elapsed) })
    }
    function start() {
      tickId = window.setInterval(tick, SEC / 2)
      return showTimer()
    }
    function hide() {
      if (tickId) window.clearInterval(tickId)
      tickId = null
      return hideTimer()
    }
    function renderTimer(data) {
      // ...
    }
    function showTimer() {
      // ...
    }
    function hideTimer() {
      // ...
    }

    return window.setTimeout(start, delayShow)
  }