text: var cvs = document.getElementById("canvas");
  var ctx = cvs.getContext("2d");
  
  // load images
  
  var bird = new Image();
  var bg = new Image();
  var fg = new Image();
  var pipeNorth = new Image();
  var pipeSouth = new Image();
  
  bird.src = "images/bird4.png";
  bg.src = "images/bg.jpg";
  
  pipeNorth.src = "images/pipeNorth.png";
  pipeSouth.src = "images/pipeSouth.png";
  
  
  // some variables
  
  var gap = 85;
  var constant;
  
  var bX = 600;
  var bY = 150;
  
  var gravity = 1.5;
  
  var score = 0;
  
  // audio files
  
  var fly = new Audio();
  var scor = new Audio();
  
  fly.src = "sounds/fly.mp3";
  scor.src = "sounds/score.mp3";
  
  // on key down
  
  document.addEventListener("keydown",moveUp);
  
  function moveUp(){
      bY -= 25;
      fly.play();
  }
  
  // pipe coordinates
  
  var pipe = [];
  
  pipe[0] = {
      x : cvs.width,
      y : 0
  };
  
  // draw images
  
  function draw(){
      
      ctx.drawImage(bg,0,0);
      
      
      for(var i = 0; i < pipe.length; i++){
          
          constant = pipeNorth.height+gap;
          ctx.drawImage(pipeNorth,pipe[i].x,pipe[i].y);
          ctx.drawImage(pipeSouth,pipe[i].x,pipe[i].y+constant);
               
          pipe[i].x--;
          
          if( pipe[i].x == cvs.width-400 ){
              pipe.push({
                  x : cvs.width,
                  y : Math.floor(Math.random()*pipeNorth.height)-pipeNorth.height
              }); 
          }
  
          // detect collision
          
          if( bX + bird.width >= pipe[i].x && bX <= pipe[i].x + pipeNorth.width && (bY <= pipe[i].y + pipeNorth.height || bY+bird.height >= pipe[i].y+constant) || bY + bird.height >=  cvs.height-118){
              location.reload(); // reload the page
          }
  
          if(pipe[i].x == 5){
              score++;
              scor.play();
          }
      }
      
  
     
      
      ctx.drawImage(bird,bX,bY);
      
      bY += gravity;
      
      ctx.fillStyle = "#000";
      ctx.font = "20px Verdana";
      ctx.fillText("Score : "+score,20,cvs.height-20);
      
      requestAnimationFrame(draw);
      
  }
  
  draw();