Building on Brian Nickel's answer, only the wanted single pixel of the source image is drawn onto a 1*1 pixel canvas, which is more efficient than drawing the entire image just to get a single pixel:

    function getPixel(img, x, y) {
    
    	let canvas = document.createElement('canvas');
    	canvas.width = 1;
    	canvas.height = 1;
    	canvas.getContext('2d').drawImage(img, x, y, 1, 1, 0, 0, 1, 1);;
    	let pixelData = canvas.getContext('2d').getImageData(0, 0, 1, 1).data;
    
    	return pixelData;	
    }