Triangle

<!DOCTYPE html>
<html>

 <body>

   <canvas id='paper'></canvas>

   <script>

    var paper = document.getElementById('paper');
    paper.width = window.innerWidth;
    paper.height = window.innerHeight;
    paper.style.position = 'absolute';paper.style.left = 0;paper.style.top = 0;

     document.body.onmousemove = function(e)
     {

      paper.width = paper.width;
      var x = e.pageX;
      var y = e.pageY;

        var ctx = paper.getContext('2d');

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

       ctx.moveTo(50,window.innerHeight/2);
       ctx.lineTo(x,y);
       ctx.moveTo(x,y);
       ctx.lineTo(e.pageX,window.innerHeight/2);
       ctx.moveTo(e.pageX,window.innerHeight/2);
       ctx.lineTo(50,window.innerHeight/2);
       ctx.moveTo(50,window.innerHeight/2);

          var x = e.pageX - 50;
          var y = window.innerHeight/2 - e.pageY;
          var angle = Math.abs(  Math.atan(y/x)*180/Math.PI );

         ctx.font = '15px Calibri';
         ctx.fillStyle='blue';
         ctx.fillText(Math.round(angle)+'°',20,window.innerHeight/2);
         ctx.fillText((180-90-Math.round(angle))+'°',e.pageX+15,e.pageY);

       ctx.stroke();
     }

   </script>

 </body>

</html>