Html

<p>
   <button onclick="openMedia()">開啟</button>
    <button onclick="closeMedia()">關閉</button>
    <button onclick="drawMedia()">擷取</button>
</p>
<video id="video" class="bg"></video>
<canvas id="qr-canvas"></canvas>

Javascript

<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;

    // 一堆相容程式碼
    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);
            });
        }
    }    

    //攝像頭呼叫配置
    var mediaOpts = {
        audio: false,
        video: true,
        video: { facingMode: "environment"} // 或者 "user"
        // video: { width: 1280, height: 720 }
        // video: { facingMode: { exact: "environment" } }// 或者 "user"
    }

    // 回撥
    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);
    }

    // 正式啟動攝像頭
    function openMedia(){
        navigator.mediaDevices.getUserMedia(mediaOpts).then(successFunc).catch(errorFunc);
    }

    //關閉攝像頭
    function closeMedia(){
        mediaStreamTrack.getVideoTracks().forEach(function (track) {
            track.stop();
            context1.clearRect(0, 0,context1.width, context1.height);//清除畫布
        });
    }

    //擷取視訊
    function drawMedia(){
        canvas1.setAttribute("width", video.videoWidth);
        canvas1.setAttribute("height", video.videoHeight);
        context1.drawImage(video, 0, 0, video.videoWidth, video.videoHeight);
    }

</script>