examples/ch06/04-bouncing-1.html

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Bouncing 1</title>
    <link rel="stylesheet" href="../include/style.css">
  </head>
  <body>
    <header>
      Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
    </header>
    <canvas id="canvas" width="400" height="400"></canvas>

    <script src="../include/utils.js"></script>
    <script src="./classes/ball.js"></script>
    <script>
    window.onload = function () {
      var canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          ball = new Ball(),
          vx = Math.random() * 10 - 5,
          vy = Math.random() * 10 - 5;

      ball.x = canvas.width / 2;
      ball.y = canvas.height / 2;

      (function drawFrame () {
        window.requestAnimationFrame(drawFrame, canvas);
        context.clearRect(0, 0, canvas.width, canvas.height);

        var left = 0,
            right = canvas.width,
            top = 0,
            bottom = canvas.height;

        ball.x += vx;
        ball.y += vy;

        if (ball.x + ball.radius > right) {
          ball.x = right - ball.radius;
          vx *= -1;
        } else if (ball.x - ball.radius < left) {
          ball.x = left + ball.radius;
          vx *= -1;
        }
        if (ball.y + ball.radius > bottom) {
          ball.y = bottom - ball.radius;
          vy *= -1;
        } else if (ball.y - ball.radius < top) {
          ball.y = top + ball.radius;
          vy *= -1;
        }
        ball.draw(context);
      }());
    };
    </script>
  </body>
</html>