1GAM Javascript Particle System

I love particle systems, so for part of a One Game a Month entry I decided to create one using HTML5 canvas and Javascript. This system creates little explosions of circles of various shades of a base colour.

You can view a fullscreen demo here, and the source is on Github.

<canvas id="canvas" width="900" height="500"></canvas>

<script src="js/Utility.js"></script>
<script src="js/Particle.js"></script>
<script src="js/ParticleSystem.js"></script>

// include the Utility, Particle and ParticleSystem files first
var particles = (function() {
    var ctx;
    var canvas;
    var time;

    function init() {
        canvas = document.getElementById('canvas');
        ctx = canvas.getContext('2d');

        var colour = '#52ef1d';
        var particleSystem = new ParticleSystem(ctx);

        // listen for click events and trigger particle explosion
        canvas.addEventListener('mousedown', function(event) {
            var mousePos = Utility.getMouseClick(canvas, event);

            particleSystem.createExplosion(mousePos.x, mousePos.y, colour);
        }, false);

        // pick three random positions and create explosions there every 100ms
        setInterval(function() {
            for(var i = 0; i < 3; i++) {
            var random = randomPos(canvas);
                particleSystem.createExplosion(random.x, random.y, colour);
            }
        }, 100);

       // request animation frame
        (function animloop(){
            requestAnimFrame(animloop);
            var now = new Date().getTime();
            var dt = now - (time || now);
            time = now;
            particleSystem.update(dt);
        })();
    }

    // return a random co-ordinate in the canvas
    function randomPos(canvas) {
        return {
            x: Math.floor(Math.random() * canvas.width),
            y: Math.floor(Math.random() * canvas.height)
        };
    }

    return {
        init: init
    };
})();

// wait til everything's loaded then start
$(document).ready(function() {
    particles.init();
});