To enable rear camera, firstly use −
MediaStreamTrack.getSources(gotSources);
Now, select the source and pass it in as optional into getUserMedia method.
This method is useful for users to set permission to use up to one video input device −
var a = { audio: { optional: [{sourceId: audioSource}] }, video: { optional: [{sourceId: videoSource}] } }; navigator.getUserMedia(a, successCallback, errorCallback);