<video id="video" controls autoplay playsInline></video>
<div>
  <button id="btn-front">Front</button>
  <button id="btn-back">Back</button>
</div>

* {
  padding: 0;
  margin: 0;
}

video {
  max-width: 100vw;
}

button {
  width: 60px;
  margin-right: 20px;
}

(() => {
  const videoElm = document.querySelector('#video');
  const btnFront = document.querySelector('#btn-front');
  const btnBack = document.querySelector('#btn-back');

  const supports = navigator.mediaDevices.getSupportedConstraints();
  if (!supports['facingMode']) {
    alert('Browser Not supported!');
    return;
  }

  let stream;

  const capture = async facingMode => {
    const options = {
      audio: false,
      video: {
        facingMode,
      },
    };

    try {
      if (stream) {
        const tracks = stream.getTracks();
        tracks.forEach(track => track.stop());
      }
      stream = await navigator.mediaDevices.getUserMedia(options);
    } catch (e) {
      alert(e);
      return;
    }
    videoElm.srcObject = null;
    videoElm.srcObject = stream;
    videoElm.play();
  }

  btnBack.addEventListener('click', () => {
    capture('environment');
  });

  btnFront.addEventListener('click', () => {
    capture('user');
  });
})();