HTMLMediaElement: loadstart イベント

<div class="example">

    <button type="button">Load video</button>
    <video controls width="250"></video>

    <div class="event-log">
        <label>Event log:</label>
        <textarea readonly class="event-log-contents"></textarea>
    </div>

</div>

.event-log-contents {
  width: 18rem;
  height: 5rem;
  border: 1px solid black;
  margin: .2rem;
  padding: .2rem;
}

.example {
  display: grid;
  grid-template-areas:
              "button log"
              "video  log";
}

button {
  grid-area: button;
  width: 10rem;
  margin: .5rem 0;
}

video {
  grid-area: video;
}

.event-log {
  grid-area: log;
}

.event-log>label {
  display: block;
}

const loadVideo = document.querySelector('button');
const video = document.querySelector('video');
const eventLog = document.querySelector('.event-log-contents');
let source = null;

function handleEvent(event) {
    eventLog.textContent = eventLog.textContent + `${event.type}\n`;
}

video.addEventListener('loadstart', handleEvent);
video.addEventListener('progress', handleEvent);
video.addEventListener('canplay', handleEvent);
video.addEventListener('canplaythrough', handleEvent);

loadVideo.addEventListener('click', () => {

    if (source) {
        document.location.reload();
    } else {
        loadVideo.textContent = "Reset example";
        source = document.createElement('source');
        source.setAttribute('src', 'https://interactive-examples.mdn.mozilla.net/media/examples/flower.webm');
        source.setAttribute('type', 'video/webm');

        video.appendChild(source);
    }
});