eyeofmidas.com/htdocs/js/circle.js

function Circle() {
	var x = 10, y = 10, radius = 5;
	var velocity = {x: 1, y: 1};
	this.init = function(canvas) {
		x = Math.floor(canvas.width * Math.random());
		y = Math.floor(canvas.height * Math.random());
		velocity.x = Math.floor(10 * Math.random() - 5);
		velocity.y = Math.floor(10 * Math.random() - 5);
	};
	this.update = function(canvas) {
		x += velocity.x;
		y += velocity.y;
		if(x + radius > canvas.width) {
			x = canvas.width - radius;
			velocity.x *= -1.1;
		}
		if(x < radius) {
			x = radius;
			velocity.x *= -1.1;
		}
		if(y + radius > canvas.height) {
			y = canvas.height - radius;
			velocity.y *= -1.1;
		}
		if(y < radius) {
			y = radius;
			velocity.y *= -1.1;
		}

		velocity.x = Math.max(-5, Math.min(5, velocity.x));
		velocity.y = Math.max(-5, Math.min(5, velocity.y));
	}, 
	this.draw = function(context) {
		context.strokeStyle = "#000000";
		context.beginPath();
		context.arc(x, y, radius, 0, 2 * Math.PI);
		context.stroke();
	}
}