document.body.innerHTML = '<div id="m" style="width:100px;height:5px;background:black;"></div>';
px = 0;
py = 0;
angleDeg = (x1,y1,x2,y2) => Math.atan2(y2 - y1, x2 - x1) * 180 / Math.PI;
document.onmousemove = function(e) {
	document.querySelector('#m').style.transform = 'rotate(' + angleDeg(px, py, e.x, e.y) + 'deg)';
px = e.x; py = e.y;

}