Can we use OpenCV from Javascript? Is there such implementation?
Is there any JS libraries that can be used for detecting face elements in the picture or video?
OpenCV has never been ported to JavaScript in its entirety, but individual parts and algorithms have:
For face and face element detection (and other parts / objects), you could use js-objectdetect or HAAR.js which are ports of the OpenCV Object Detection based on Haar Feature Cascades.
The very first face detection algorithm on the web found in ccv also deserves a mention. Its classifier seems to be less reliable than the one provided by OpenCV though.
The headtrackr library used for facetracking might also be of interest since it implements the camshift algorithm also found in OpenCV.