js:页面加载时显示视频的第一帧

<video id="myVideo" src="video.mp4" poster="poster.jpg"></video>

var video = document.getElementById("myVideo");
video.addEventListener("loadedmetadata", function() {
  // 在此处执行显示第一帧的操作
});

var canvas = document.createElement("canvas");
var context = canvas.getContext("2d");
canvas.width = video.videoWidth;
canvas.height = video.videoHeight;
context.drawImage(video, 0, 0, canvas.width, canvas.height);

var img = document.createElement("img");
img.src = canvas.toDataURL();
document.body.appendChild(img);