JS Canvas Laser Pen Simulator SRC

<!DOCTYPE html>
<html>
<body onload="setInterval(doit,31);" bgcolor="black">

<canvas id="myCanvas" width="1000" height="1000" style="border:1px solid #black;">
Your browser does not support the HTML5 canvas tag.</canvas>

<script>
function doit(){
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");

var gradient = ctx.createLinearGradient(0, 0, Math.random()*170, 0);
gradient.addColorStop("0", "black");
gradient.addColorStop("0.5", "red");
gradient.addColorStop("1.0", "black");

ctx.fillStyle = gradient;
ctx.lineWidth = 10;
ctx.fillRect(0, 0, 500,250);
}
</script>
</body>
</html>