const constrains = {
   video: true,
   audio: true
  }
 navigator.mediaDevices.getUserMedia(constrains)
  .then(stream => {
   console.log('得到stream的类型是MediaStream')
  })
  // 在一些老的浏览器上的话：
  // navigator.webkitGetUserMedia
  // navigator.mozGetUserMedia

 const video = document.createElement('video');
 //document.body.appendChild(video)
 const constrains = {
   video: true,
   audio: true
  }
 navigator.mediaDevices.getUserMedia(constrains)
  .then(stream => {
  video.srcObject = stream;
  video.play();
  })

 // 展示相片的image标签
 const image = document.createElement('image')
 // canvas会读取video中的内容，然后输出（有点类似于给video截图）
 const canvas = document.createElement('canvas')
 const video = document.getElementById('video');
 const context = canvas.getContext('2d')
 context.drawImage(video, 0, 0, canvas.height, canvas.width)
 // 此时将canvas生成的图片转换成一个data url
 const url = canvas.toDataURL()
 image.src = url

 const constrains = {
   video: true,
   audio: true
  }
 let mediaRecorder
 navigator.mediaDevices.getUserMedia(constrains)
  .then(stream => {
  mediaRecorder = new MediaRecorder(stream)
  })
 
 startRecord() {
  mediaRecorder.start()
  mediaRecorder.ondataavailable = e => {
   chunks.push(e.data)
  }
 }

 stopRecord() {
  mediaRecorder.stop()
  return new Promise(resolve => {
   mediaRecorder.onstop = e => {
    const blob = new Blob(chunks, { type: 'video/ogg; codecs=opus' })
    const audioURL = window.URL.createObjectURL(blob)
    resolve(audioURL)
   }
  })
 }

 // video标签用来展示录制下来的内容
 const video = document.createElement('video');
 //document.body.appendChild(video)

 // 开始录像
 startRecord()

 // do something....，比方说这里搞两个button、一个开始录影，一个结束录影

 // 结束录影
 stopRecord().then(videoUrl => {
  video.src = videoUrl
 })
 // 此时video标签的内容就是你录制下来的内容。