<video id="videoPlayer" controls>
  <source src="sample-video.mp4" type="video/mp4">
</video>

<script>
  var videoPlayer = document.getElementById("videoPlayer");

  videoPlayer.ontimeupdate = function() {
    if (Math.floor(videoPlayer.currentTime) === 10) {
      alert("10 seconds have passed!");
    }
  };
</script>