注：只在微信QQ内置浏览器（水果机暂不支持）、Chrome浏览器（安卓机）、Safari浏览器下测试过

参考自 jeromeetienne/AR.js

// 判断Android还是IOS
var u = navigator.userAgent;
var isAndroid = u.indexOf('Android') > -1 || u.indexOf('Adr') > -1;
var isiOS = !!u.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/);

// 摄像头调起失败
var onError = function(error){
    alert('Webcam Error\nName: '+error.name + '\nMessage: '+error.message);
}
// 摄像头调起成功
var onSuccess = function(stream){
    var video = document.createElement('video');
    video.style.width = document.documentElement.clientWidth + 'px';
    video.style.height = document.documentElement.clientHeight + 'px';

    video.srcObject = stream;
    
    // to start the video, when it is possible to start it only on userevent. like in android
    video.body.addEventListener('click', function(){
        video.play();
    });

    // wait until the video stream is ready
    var interval = setInterval(function(){
        if(!video.videoWidth){
            return;
        }
        document.body.appendChild(video);
        
        clearInterval(interval);
    }, 1000/50);
}

// check API is available
if(navigator.mediaDevices === undefined || navigator.mediaDevices.enumerateDevices === undefined || navigator.mediaDevices.getUserMedia === undefined){
    if( navigator.mediaDevices === undefined ){
        var fctName = 'navigator.mediaDevices';
    }
    else if( navigator.mediaDevices.enumerateDevices === undefined ){
        var fctName = 'navigator.mediaDevices.enumerateDevices';
    }
    else if( navigator.mediaDevices.getUserMedia === undefined ){
        var fctName = 'navigator.mediaDevices.getUserMedia';
    }
    else{
        console.assert(false);
    }
    onError({
        name: '',
        message: 'WebRTC issue-! '+fctName+' not present in your browser'
    })
}

// get available devices
navigator.mediaDevices.enumerateDevices().then(function(devices){
    // 如果是水果机
    if(isiOS){
        navigator.mediaDevices.getUserMedia({
            audio: false,
            video: {
                facingMode: 'environment'
            }
        }).then(onSuccess).catch(onError);
    }
    // 如果是安卓机
    else{
        var videoSourceId;
        var exArray = [];
        for(var i = 0; i < devices.length; i++){
            var deviceInfo = devices[i];
            // 判断是否是相机设备
            if(deviceInfo.kind == "videoinput"){
                exArray.push(deviceInfo.deviceId);
                // 判断是否是后置摄像头
                if(deviceInfo.label.split(', ')[1] == "facing back") {
                    videoSourceId = deviceInfo.deviceId;
                }
            }
        }
        // deviceInfo.label为空
        if (!videoSourceId) {
            switch (exArray.length) {
                // 单摄像头
                case 1:
                    videoSourceId = exArray[0];
                    break;
                // 多摄像头
                case 2:
                    videoSourceId = exArray[1];
                    break;
                default:
                    break;
            }
        }
        navigator.mediaDevices.getUserMedia({
            audio: false,
            video: {
                optional: [{sourceId: videoSourceId}]
            }
        }).then(onSuccess).catch(onError);
    }
}).catch(onError);

网上大多数的demo都是调用 Navigator.getUserMedia()，但实际上支持iOS11调用摄像头的是 MediaDevices.getUserMedia()。
经测试，发现facingMode: "environment"属性对安卓系统无效，于是只能用类似
MediaStreamTrack.getSources 函数的方法，感觉不是很严谨呢 _(:3」∠)_

经测试，安卓机已不再支持 MediaStreamTrack.getSources 函数方法，并且从
mediaDevices.enumerateDevices 函数获取的所有 devices[i].label 值为空，不再包含 "facing back" 字段。