Mouse follow experiment fullscreen

follow the mouse fullscreen, because of jsfiddle frames this is not very obvious, but does work

HTML

<body>
    <canvas id="myCanvas" width="300" height="500"></canvas> 

</body>

JavaScript

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

var width = 300;
var height = 600;
var centerX = 100;
var centerY = 200;
update(0,0,centerX,centerY);

$(window).mousemove(function (event) {
    var pageCoords = "( " + event.pageX + ", " + event.pageY + " )";
    var clientCoords = "( " + event.clientX + ", " + event.clientY + " )";
    update(event.pageX, event.pageY, centerX, centerY);
    $('span').html(pageCoords + ' ' + clientCoords);
});

function update(mX,mY,cX,cY) {
    context.clearRect ( 0 , 0 , width , height );
    context.save();
    // center
    context.translate(cX,cY);
    context.fillStyle='black';
    context.fillRect(-3,-3,6,6);
    // set angle
    var radians = Math.atan2(mY - cY, mX - cX);
    var angle = 90 + (radians * (180 / Math.PI));
    // move
    drawSpike(angle);
    context.restore();
}

function drawSpike(angle) {
    context.save();
    context.rotate(deg2Rad(angle));
    context.beginPath();
    context.moveTo(-3, 0);
    context.lineTo(3, 0);
    context.lineTo(0, -50);
    context.fillStyle = 'red';
    context.closePath();
    context.fill();
    context.strokeStyle = 'black';
    context.stroke();
    context.restore();
}

function deg2Rad(deg) {
    return deg * Math.PI / 180;
}