axure.js

<button onclick="start()">Start!</button><br />
<video id="v" autoplay playsinline></video>

<script type="text/javascript">
var config = {
  video: {
    width: 1280,
    facingMode: {
      exact: "environment"
    }
  }
};

var start = () => navigator.mediaDevices.getUserMedia(config)
  .then(stream => {
    currentStream = stream;
    v.srcObject = stream;
  })
  .then(() => new Promise(resolve => v.onloadedmetadata = resolve))
  .catch(log);

</script>