canvas获取视频第一帧设置为视频封面

function vSetImg(obj) {
  const canvas = document.createElement("canvas");
  canvas.width = obj.videoWidth;
  canvas.height = obj.videoHeight;
  canvas.getContext('2d').drawImage(obj, 0, 0, canvas.width, canvas.height);
  obj.setAttribute('poster', canvas.toDataURL("image/png"));
  //插入img标签查看封面图（调试用）
  const img = document.createElement("img");
  img.src = canvas.toDataURL("image/png");
  document.body.appendChild(img);
}


<video preload="auto" onloadeddata="vSetImg(this)" controls>
    <source src="wel.mp4">
</video>