HTML5 Game - Gravity as acceleration

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Description

Gravity as acceleration

Demo

ResultView the demo in separate window 

<!doctype html>
<html>
  <body>
    <canvas id="canvas" width="400" height="400"></canvas>
    <aside>Press arrow keys.</aside>
    <script>
class Ball{    /*  w w w  . j ava 2  s. c  om*/
   constructor(radius, color) {  
     if (radius === undefined) { radius = 40; }  
     if (color === undefined) { color = "#ff0000"; }  
     this.x = 0;  
     this.y = 0;  
     this.radius = radius;  
     this.rotation = 0;  
     this.scaleX = 1;  
     this.scaleY = 1;  
     this.color = color;  
     this.lineWidth = 1;  
   }  
     
   draw(context) {  
     context.save();  
     context.translate(this.x, this.y);  
     context.rotate(this.rotation);  
     context.scale(this.scaleX, this.scaleY);  
     context.lineWidth = this.lineWidth;  
     context.fillStyle = this.color;  
     context.beginPath();  
     //x, y, radius, start_angle, end_angle, anti-clockwise  
     context.arc(0, 0, this.radius, 0, (Math.PI * 2), true);  
     context.closePath();  
     context.fill();  
     if (this.lineWidth > 0) {  
       context.stroke();  
     }  
     context.restore();  
   }
}
    window.onload = function () {
      let canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          ball = new Ball(),
          vx = 0,
          vy = 0,
          ax = 0,
          ay = 0,
          gravity = 0.02;

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

      window.addEventListener('keydown', function (event) {
        switch (event.keyCode) {
        case 37:      //left
          ax = -0.1;
          break;
        case 39:      //right
          ax = 0.1;
          break;
        case 38:      //up
          ay = -0.1;
          break;
        case 40:      //down
          ay = 0.1;
          break;
        }
      }, false);

      window.addEventListener('keyup', function () {
        ax = 0;
        ay = 0;
      }, false);

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

        vx += ax;
        vy += ay;
        vy += gravity;
        ball.x += vx;
        ball.y += vy;
        ball.draw(context);
      }());
    };
    </script>
  </body>
</html>

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