switch from front camera to back camera JS

I apologize for my bad english. I have a QR scan camera but when I will open by cell phone in my website, the face camera turned on in my website. but I want to use the back camera

if someone can help me. I would be very grateful

function setwebcam()
{

 var constraints = { audio: true, video: true };




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(constraints, success, error);
else
if(n.webkitGetUserMedia)
{
    webkit=true;
    n.webkitGetUserMedia(constraints, success, error);
}
else
if(n.mozGetUserMedia)
{
    moz=true;
    n.mozGetUserMedia(constraints, success, error);
}