OpenCV has never been ported to JavaScript in its entirety, but individual parts and algorithms have:
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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.
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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.
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The headtrackr library used for facetracking might also be of interest since it implements the camshift algorithm also found in OpenCV.