<p>
 <button onclick="openMedia()">打开</button>
 <button onclick="closeMedia()">关闭</button>
 <button onclick="drawMedia()">截取</button>
</p>
<video id="video" class="bg"></video>
<canvas id="qr-canvas"></canvas>

<script type="text/javascript">
 var video = document.querySelector('video');
 var text = document.getElementById('text');
 var canvas1 = document.getElementById('qr-canvas');
 var context1 = canvas1.getContext('2d');
 var mediaStreamTrack;

 // 一堆兼容代码
 window.URL = (window.URL || window.webkitURL || window.mozURL || window.msURL);
 if (navigator.mediaDevices === undefined) {
 navigator.mediaDevices = {};
 }
 if (navigator.mediaDevices.getUserMedia === undefined) {
 navigator.mediaDevices.getUserMedia = function(constraints) {
  var getUserMedia = navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia;
  if (!getUserMedia) {
  return Promise.reject(new Error('getUserMedia is not implemented in this browser'));
  }
  return new Promise(function(resolve, reject) {
  getUserMedia.call(navigator, constraints, resolve, reject);
  });
 }
 } 

 //摄像头调用配置
 var mediaOpts = {
 audio: false,
 video: true,
 video: { facingMode: "environment"} // 或者 "user"
 // video: { width: 1280, height: 720 }
 // video: { facingMode: { exact: "environment" } }// 或者 "user"
 }

 // 回调
 function successFunc(stream) {
 mediaStreamTrack = stream;
 video = document.querySelector('video');
 if ("srcObject" in video) {
  video.srcObject = stream
 } else {
  video.src = window.URL && window.URL.createObjectURL(stream) || stream
 }
 video.play();
 }
 function errorFunc(err) {
 alert(err.name);
 }

 // 正式启动摄像头
 function openMedia(){
 navigator.mediaDevices.getUserMedia(mediaOpts).then(successFunc).catch(errorFunc);
 }

 //关闭摄像头
 function closeMedia(){
 mediaStreamTrack.getVideoTracks().forEach(function (track) {
  track.stop();
  context1.clearRect(0, 0,context1.width, context1.height);//清除画布
 });
 }

 //截取视频
 function drawMedia(){
 canvas1.setAttribute("width", video.videoWidth);
 canvas1.setAttribute("height", video.videoHeight);
 context1.drawImage(video, 0, 0, video.videoWidth, video.videoHeight);
 }

</script>