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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset=utf-8 />
<title>JS Bin</title>
  <script>
    var canvas, ctx;
    var width, height;
    var x, y;
    // Move 3 pixels left or right at each frame
    var incX = 3;
    var size = 10;
   
    // Called after the DOM is ready (page loaded)
    function init() {
      // init the different variables
      canvas = document.querySelector("#mycanvas");
      ctx = canvas.getContext('2d');
      
      width = canvas.width;
      height = canvas.height;
      
      // default values
      x=10; y = 10;
      ctx.fillStyle='red';
           
      // Start animation
      animationLoop();
    }
    
    function animationLoop() {
      // an animation is : 1) clear canvas and 2) draw shapes, 
      // 3) move shapes, 4) recall the loop with requestAnimationFrame
      
      // clear canvas
      ctx.clearRect(0, 0, width, height);
     
      ctx.fillRect(x, y, size, size);
      ctx.strokeRect(x, y, size, size);
     
      // move rectangle
      x += incX;
      
      // check collision on left or right
      if(((x+size) > width) || (x <= 0)) {
        // cancel move + inverse speed
        x -= incX;
        incX = -incX;
      }
      
      // animate again at 60 frames/s
      requestAnimationFrame(animationLoop);
    }  
  </script>
</head>
  
<body onload="init();">
  <canvas id="mycanvas" width="200" height="50" style="border: 2px solid black"></canvas>
</body>
</html>
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