Simple Countdown Timer in Seconds

simple-seconds-countdown.html

<span id="seconds">10</span>

<script>
  timeLeft = 10;

  function countdown() {
    timeLeft--;
    document.getElementById("seconds").innerHTML = String(timeLeft);
    if (timeLeft > 0) {
      setTimeout(countdown, 1000);
    }
  }

  setTimeout(countdown, 1000);
</script>

simple-seconds-countup.html

<!-- See a test example here: https://codepen.io/cmoen89/pen/yLKoYxJ?editors=1010 -->

<span id="seconds">0</span>

<script>
  numStart = 0;
  numEnd = 10;

  function countup() {
    numStart++;
    document.getElementById("seconds").innerHTML = String(numStart);
    if (numStart < numEnd) {
      setTimeout(countup, 1000);
    }
  }

  setTimeout(countup, 1000);
</script>