var constraints = {
	    audio: false,
	    video: {
	        facingMode: "environment",
	    	zoom: 0, 
	    	focusMode: {ideal: "continuous"}
	    }
	};

	var constraints = {
	    audio: false,
	    video: {
	        facingMode: "environment",
	    	zoom: 0, 
	    	focusMode: {ideal: "continuous"},
                deviceId: { exact: "89dkdnmdjhfssf" }
	    }
	};

// enumerate devices and select the first camera (mostly the back one)

navigator.mediaDevices.enumerateDevices().then(function(devices) {
    for (var i = 0; i !== devices.length; ++i) {
        if (devices[i].kind === 'videoinput') {
            //alert('Camera found: '+ devices[i].label || 'label not found' + devices[i].deviceId || 'id no found');
			alert('deviceid=>'+devices[i].label+' opc=>'+devices[i].deviceId);
            //videoConstraints.deviceId = { exact: devices[i].deviceId }
        }
    }
});