JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="display">
<div id="c"></div>
<div id="b"></div>

CSS

#c {
  position: absolute;
  top: 190px; left: 190px;
  width: 20px; height: 20px;
  border-radius: 50%;
  background: red;
}
#b {
  position: absolute;
  top: 10px; left: 190px;
  width: 20px; height: 20px;
  border-radius: 50%;
  background: blue;
}

JavaScript

document.onmousemove = function(evt){
	let P1 = {x: 200, y: 200}
  let P2 = {x: 200, y: 0}
  let P3 = {x: evt.clientX, y: evt.clientY}
  let rad = Math.atan2(P3.y - P1.y, P3.x - P1.x) -
            Math.atan2(P2.y - P1.y, P2.x - P1.x)
            
	display.value = rad * (180 / Math.PI)
}