transform matrix mousemove spin

ctx.beginPath();
ctx.lineWidth = 1;

ctx.translate(this.cx, this.cy);               // translate to pivot

if (this.toAngle !== this.angle) {
  ctx.rotate(this.toAngle - this.angle);
}

ctx.strokeStyle = this.color;
ctx.arc(0, 0, this.radius, 0, Math.PI * 2);    // render at pivot

ctx.closePath();                               // must come before stroke() btw.
ctx.stroke();

ctx.beginPath();
ctx.fillStyle = 'black';
ctx.fillRect(-this.radius / 4, -this.radius / 4, 20, 20);  // render at pivot

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');

class Circle {
	constructor(options) {
  	this.cx = options.x;
    this.cy = options.y;
    this.radius = options.radius;
    this.color = options.color;
    this.angle = 0;
    this.toAngle = this.angle;
    
    this.binding();
  }
  
  binding() {
  	const self = this;
  	window.addEventListener('mousemove', (e) => {
    	self.update(e.clientX, e.clientY);
    });
  }
  
  update(nx, ny) {
  	this.toAngle = Math.atan2(ny - this.cy, nx - this.cx);
  }
  
  render() {
  	ctx.clearRect(0,0,canvas.width,canvas.height);
    
    ctx.save();
    
  	ctx.beginPath();

		ctx.lineWidth = 1;

	ctx.translate(this.cx, this.cy);
	
		if (this.toAngle !== this.angle) {
      ctx.rotate(this.toAngle - this.angle);
    }
    
	  ctx.strokeStyle = this.color;
	  ctx.arc(0, 0, this.radius, 0, Math.PI * 2);

	  ctx.stroke();
	  ctx.closePath();

    ctx.beginPath();
    
    ctx.fillStyle = 'black';
    ctx.fillRect(-this.radius / 4, -this.radius / 4, 20, 20);
    
    ctx.closePath();
    ctx.restore();
  }
}

var rotatingCircle = new Circle({
	x: 150,
  y: 100,
  radius: 40,
  color: 'black'
});

function animate() {
	rotatingCircle.render();
	requestAnimationFrame(animate);
}

animate();