How to create a mask in OpenCV.js?

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I can create a mask in OPENCV C++ using cv::Mat::zeros and Rect.But i cannot find these features on OPENCV.js.How can i create a mask on OPENCV.js?

cv::Mat mask = cv::Mat::zeros(8, 8, CV_8U); // all 0
mask(Rect(2,2,4,4)) = 1;