从前置摄像头切换到后置摄像头（后置/环境）？

Question

我为我糟糕的英语道歉。我有一个二维码扫描仪FOUND，我想用于一个智能手机网站，但它只是打开前置摄像头，因为我自动切换后置摄像头（环境）我试过，但它没有工作

 if(n.getUserMedia)
    n.getUserMedia({video:  { width: { facingMode: { exact: "environment" } } }}, success, error);
else
if(n.webkitGetUserMedia)
{
    webkit=true;
    n.webkitGetUserMedia({video: { width: { facingMode: { exact: "environment" } } }}, success, error);
}
else
if(n.mozGetUserMedia)
{
    moz=true;
    n.mozGetUserMedia({video:  { width: { facingMode: { exact: "environment"     } } }}, success, error);

它不起作用我不记得更多我想要的解决方案之后的恶劣时间，但我不知道如何解决这个问题

hir down是原始代码

// QRCODE reader Copyright 2011 Lazar Laszlo
// http://www.webqr.com

var gCtx = null;
var gCanvas = null;
var c=0;
var stype=0;
var gUM=false;
var webkit=false;
var moz=false;
var v=null;

var imghtml='<div id="qrfile"><canvas id="out-canvas" width="320" height="240"></canvas>'+
'<div id="imghelp">drag and drop a QRCode here'+
'<br>or select a file'+
'<input type="file" onchange="handleFiles(this.files)"/>'+
'</div>'+
'</div>';

var vidhtml = '<video id="v" autoplay></video>';

function dragenter(e) {
  e.stopPropagation();
  e.preventDefault();
}

function dragover(e) {
  e.stopPropagation();
  e.preventDefault();
}
function drop(e) {
  e.stopPropagation();
  e.preventDefault();

  var dt = e.dataTransfer;
  var files = dt.files;
  if(files.length>0)
  {
    handleFiles(files);
  }
  else
  if(dt.getData('URL'))
  {
    qrcode.decode(dt.getData('URL'));
  }
}

function handleFiles(f)
{
var o=[];

for(var i =0;i<f.length;i++)
{
    var reader = new FileReader();
    reader.onload = (function(theFile) {
    return function(e) {
        gCtx.clearRect(0, 0, gCanvas.width, gCanvas.height);

        qrcode.decode(e.target.result);
    };
    })(f[i]);
    reader.readAsDataURL(f[i]); 
    }
}

function initCanvas(w,h)
{
gCanvas = document.getElementById("qr-canvas");
gCanvas.style.width = w + "px";
gCanvas.style.height = h + "px";
gCanvas.width = w;
gCanvas.height = h;
gCtx = gCanvas.getContext("2d");
gCtx.clearRect(0, 0, w, h);
}


function captureToCanvas() {
if(stype!=1)
    return;
if(gUM)
{
    try{
        gCtx.drawImage(v,0,0);
        try{
            qrcode.decode();
        }
        catch(e){       
            console.log(e);
            setTimeout(captureToCanvas, 500);
        };
    }
    catch(e){       
            console.log(e);
            setTimeout(captureToCanvas, 500);
        };
    }
}

function htmlEntities(str) {
    return String(str).replace(/&/g, '&amp;').replace(/</g,                 '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
}

function read(a)
{
var html="<br>";
if(a.indexOf("http://") === 0 || a.indexOf("https://") === 0)
    html+="<a target='_blank' href='"+a+"'>"+a+"</a><br>";
html+="<b>"+htmlEntities(a)+"</b><br><br>";
document.getElementById("result").innerHTML=html;
}   

function isCanvasSupported(){
  var elem = document.createElement('canvas');
return !!(elem.getContext && elem.getContext('2d'));
}
function success(stream) {
if(webkit)
    v.src = window.webkitURL.createObjectURL(stream);
else
if(moz)
{
    v.mozSrcObject = stream;
    v.play();
}
else
    v.src = stream;
gUM=true;
setTimeout(captureToCanvas, 500);
}

function error(error) {
gUM=false;
return;
}

function load()
{
if(isCanvasSupported() && window.File && window.FileReader)
{
    initCanvas(800, 600);
    qrcode.callback = read;
    document.getElementById("mainbody").style.display="inline";
    setwebcam();
}
else
{
    document.getElementById("mainbody").style.display="inline";
    document.getElementById("mainbody").innerHTML='<p id="mp1">QR code     scanner for HTML5 capable browsers</p><br>'+
    '<br><p id="mp2">sorry your browser is not supported</p><br><br>'+
    '<p id="mp1">try <a href="http://www.mozilla.com/firefox"><img     src="./firefox.png"/></a> or <a href="http://chrome.google.com"><img     src="./chrome_logo.gif"/></a> or <a href="http://www.opera.com"><img     src="./Opera-logo.png"/></a></p>';
}
}

function setwebcam()
{
document.getElementById("result").innerHTML="- scanning -";
if(stype==1)
{
    setTimeout(captureToCanvas, 500);    
    return;
}
var n=navigator;
document.getElementById("outdiv").innerHTML = vidhtml;
v=document.getElementById("v");

if(n.getUserMedia)
    n.getUserMedia({video: true, audio: false}, success, error);
else
if(n.webkitGetUserMedia)
{
    webkit=true;
    n.webkitGetUserMedia({video:true, audio: false}, success, error);
}
else
if(n.mozGetUserMedia)
{
    moz=true;
    n.mozGetUserMedia({video: true, audio: false}, success, error);
}

//document.getElementById("qrimg").src="qrimg2.png";
//document.getElementById("webcamimg").src="webcam.png";
document.getElementById("qrimg").style.opacity=0.2;
document.getElementById("webcamimg").style.opacity=1.0;

stype=1;
setTimeout(captureToCanvas, 500);
}
function setimg()
{
document.getElementById("result").innerHTML="";
if(stype==2)
    return;
document.getElementById("outdiv").innerHTML = imghtml;
//document.getElementById("qrimg").src="qrimg.png";
//document.getElementById("webcamimg").src="webcam2.png";
document.getElementById("qrimg").style.opacity=1.0;
document.getElementById("webcamimg").style.opacity=0.2;
var qrfile = document.getElementById("qrfile");
qrfile.addEventListener("dragenter", dragenter, false);  
qrfile.addEventListener("dragover", dragover, false);  
qrfile.addEventListener("drop", drop, false);
stype=2;
}