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HTML 

              
                      <html>
    <head>
        <title>
         Space Shoot with HTML5 Canvas
      </title>
    </head>
    <body background="">
      <div id="gameDiv">
        <canvas id='ballCanvas' style="user-select: none;"></canvas>
        <canvas id='spaceShip' style="user-select: none;"></canvas>
        <div id='scoreBoard' style="user-select: none;">Press Space Bar or click mouse to Shoot and Start the Game. Use Up and Down Keys to Move or use mouse to move.</div>
        <div id='gameOver' style="user-select: none;">Game Over</div>
      </div>
    </body>
</html>
!

CSS 

              
                #gameDiv {
  position: relative;
  width: 100%;
  height: 530px;
  background: black;
  margin: auto;
  cursor: none;
  user-select: none;
}

#gameOver {
  position: absolute;
  width: 100%;
  height: 530px;
  top: 0px;
  background: black;
  text-align: center;
  line-height: 530px;
  color: lightblue;
  font-family: 'Arial';
  font-size: 20px;
  user-select: none;
}

#ballCanvas {
  background: url("http://cloud-3.steamusercontent.com/ugc/720918725566294424/FCD5ECFE932139A7886585ECCBCA6586C6FF6E4A/");
  width: 100%;
  height: 520px;
  background-size: 100% 520px;
}

#scoreBoard {
  position: absolute;
  width: 100%;
  height: 50px;
  top: 520px;
  background: black;
  text-align: center;
  line-height: 50px;
  color: lightblue;
  font-family: 'Arial';
  font-size: 20px;
}
!

JS 

              
                document.onkeydown = function(event) {
    switch(event.keyCode) {
        case 13:
          init();
          $('#scoreBoard').html('Press Space Bar or click mouse to Shoot and Start the Game. Use Up and Down Keys to Move or use mouse to move.');
          break;
    }
}

function init() {
 $('#gameOver').hide();
var canvas = document.getElementById('ballCanvas'),
    context = canvas.getContext('2d'),
    ballBox=[], 
    bulletBox=[], 
    index= 0, 
    b_index= 0, 
    attacker_position = canvas.height / 2,
    hits_no=0,
    balls_no=0,
    bullets_no= 0,
    gameOver = false;

$('#gameOver').hide();

canvas.onkeydown = function(event) {
    switch(event.keyCode) {
        case 38:
            attacker_position = attacker_position - 10 ;
            break;
        case 40:
            attacker_position = attacker_position + 10;
            break;
        case 32:
            bulletBox[b_index] = Bullet(180, attacker_position + 30);
            b_index = b_index + 1 ;
            bullets_no = bullets_no +1;
            if(bullets_no === 1) {
              $('#scoreBoard').html('Game Started! Protect Spaceship!');
            }
            break;
    }
};


canvas.addEventListener('mousemove', function (e) {
  attacker_position = e.pageY ;
});

canvas.addEventListener('click', function (e) {
  bulletBox[b_index] = Bullet(180, attacker_position + 30);
  b_index = b_index + 1 ;
  bullets_no = bullets_no +1;
  if(bullets_no === 1) {
    $('#scoreBoard').html('Game Started! Protect Spaceship!');
  }
});

canvas.width = window.innerWidth;
canvas.height = window.innerHeight;

function drawImages() {
  var spaceShip = new Image();
  var asteroids = new Image();
  spaceShip.src = "http://vignette1.wikia.nocookie.net/thedimensionsaga/images/f/f0/Spiff's_spacecraft.png/revision/latest?cb=20130716184758";
  asteroids.src = 'http://www.asteroidmission.org/wp-content/themes/osiris/public_assets/images/bennu-rendered.png';
  spaceShip.onload = function() {
    asteroids.onload = function(){
      context.clearRect(0,0, window.innerWidth, window.innerHeight);
      context.drawImage(spaceShip, 100, attacker_position, 60, 60);
      for(var i=0;i<ballBox.length;i++){
        if (!_.isUndefined(ballBox[i]) && i < ballBox.length){
          context.drawImage(asteroids, ballBox[i].x, ballBox[i].y, 40, 40);
        }
      }
    };
  };
}

function animateSpace(){
  context.beginPath();       
  drawImages();
  for(var i=0;i<ballBox.length;i++) {
    if (!_.isUndefined(ballBox[i]) && ballBox[i].x !==0) {
      ballBox[i].x = ballBox[i].x - ballBox[i].howFast;
    }
    if(!_.isUndefined(ballBox[i]) && ballBox[i].x == 0){
      ballBox.splice(i,1);
      index--;
    }
  }
  
   if(bulletBox.length > 0) {
    drawBullet();
    for(var bulletHit=0; bulletHit<bulletBox.length; bulletHit++){
      for(var asteroidHit=0; asteroidHit<ballBox.length; asteroidHit++){
        if (!_.isUndefined(ballBox[asteroidHit]) && asteroidHit <= ballBox.length -1){
          if(((ballBox[asteroidHit].x) <= 140) && 
             (90 <= (ballBox[asteroidHit].x)) && 
             ((ballBox[asteroidHit].y) <= attacker_position+50) && 
             (attacker_position <=(ballBox[asteroidHit].y))
            ){
             gameOver = true;
           }
         }
         if (!_.isUndefined(bulletBox[bulletHit]) && !_.isUndefined(ballBox[asteroidHit]) && asteroidHit <= ballBox.length -1 && bulletHit <= bulletBox.length -1){
          if(((ballBox[asteroidHit].x) <= bulletBox[bulletHit].x) && (bulletBox[bulletHit].x <= (ballBox[asteroidHit].x + 50)) &&
             ((ballBox[asteroidHit].y) <= bulletBox[bulletHit].y) && (bulletBox[bulletHit].y <= (ballBox[asteroidHit].y + 50))
            ){
            //starBurst(bulletBox[bulletHit].x, bulletBox[bulletHit].y);
            ballBox.splice(asteroidHit,1);
            bulletBox.splice(bulletHit,1);
            hits_no++;
            if (hits_no == 1){
            $('#scoreBoard').html("HIT ! "+hits_no+" Asteroid Destroyed");
            } else {
            $('#scoreBoard').html("HIT! "+hits_no+" Asteroids Destroyed");
            }
            break;
          }
        }
      }
    }
  }
  if(bulletBox.length > 0) {
    for (var bul = 0; bul < bulletBox.length; bul++) {
      if (!_.isUndefined(bulletBox[bul])){
        bulletBox[bul].x = bulletBox[bul].x +bulletBox[bul].howFast;
      }
    }
  }
  if (!gameOver){
    requestAnimationFrame(animateSpace);
  } else {
    setTimeout(function(){ $('#gameOver').show() ;$('#scoreBoard').html('Ship Destroyed by Asteroids ! Total Hits: '+hits_no+' Press Enter to Restart Game');}, 1300);
  }
}

function Ball(xCord,yCord,coloR){
  return {
    'bs': 10,
    'x': xCord,
    'y': yCord,
    'coloR': coloR,
    'howFast': _.sample([0.1,0.5,1.5,3.5,5.5,7.5,13,17])
    // Varying velocities... ;)
  };
}


function Bullet(xCord, yCord){
  return {  
    'bs': 6,
    'x': xCord,
    'y': yCord,
    'howFast': 20
  };
}


/* Initailly used later replaced with images.

function drawAttacker(){
   context.beginPath();
   context.arc(100, attacker_position, 30, 0, 360);
   context.fillStyle = "lightgreen";
   context.fill();
   context.closePath();
} */


/* function drawBalls(){
    for(var i=0;i<ballBox.length;i++){
        context.beginPath();
        context.fillStyle = ballBox[i].coloR;
        context.arc(ballBox[i].x, ballBox[i].y, ballBox[i].bs, 0, 360);
        context.fill();
        context.closePath();
    }
} */


function drawBullet() {
    for(var i=0;i<bulletBox.length;i++){
       if(!_.isUndefined(bulletBox[i])){
        context.beginPath();
        context.fillStyle = "red";
        context.arc(bulletBox[i].x, bulletBox[i].y, bulletBox[i].bs, 0, 360);
        context.fill();
        context.closePath();
        }
    }
}

function randomAttack() {
    return Math.random() * (500 - 20) + 20;
}

function initAttack() {
    ballBox[index] = Ball(canvas.width, randomAttack(), "darkgrey");
    index++;
}


intVal = setInterval(function(){  
  initAttack(); 
},800);
animateSpace();



/*Random Asteroid Logic.. 
Doesn't seems to work with asteroids getting vanished midway

 function drawSpaceship(url) {
  var i;
  for(i=0;i<ballBox.length -1;i++){
    var spaceShip = new Image();
    var asteroids = new Image();
    spaceShip.src = url;
    asteroids.src = ballBox[i].url;
    spaceShip.onload = function() {
      asteroids.onload = function(){
        context.clearRect(0,0, 1000, 600);
        context.drawImage(spaceShip, 100, attacker_position, 60, 60);
        context.drawImage(asteroids, ballBox[i].x, ballBox[i].y, 40, 40);
      }
    }
  }
}


function Ball(xCord,yCord,coloR){
  return {
   'url': _.sample([ "http://i1104.photobucket.com/albums/h329/zorq1/Spinning-asteroid-8.gif",
                          "http://www.minerwars.com/ForumUploads/20101222063029_5025_Asteroid2.png",
                          "http://www.icon2s.com/wp-content/uploads/2013/09/asteroid.png",
                          "http://userpages.umbc.edu/~sohisa1/images/artwork/cosmoknights/backgrounds/asteroid.png",
                          "https://www.asteroidfight.com/wp-content/uploads/2015/06/asteroid_populated_small.png",
                          "http://orig15.deviantart.net/217f/f/2013/183/3/c/asteroid_belt_12a_by_mototsume-d6brdm4.png"]),
    'bs': 10,
    'x': xCord,
    'y': yCord,
    'coloR': coloR,
    'howFast': _.sample([0.1,0.5,1,2,3,4,5,6,7,8,9,10,11,12,13,14])
    // Varying velocities... ;)
  };
}

*/

function starBurst(xCor, yCor){
  for (var x = xCor; x<= xCor +100 ; x++){
    context.beginPath();
    context.clearRect(xCor, yCor, 50, 50);
    xCor = xCor + 2;
    context.fillStyle = "red";
    context.arc(xCor, yCor, 5, 0, 360);
    context.fill();
    context.closePath();
  }
}
}

init();
!
999px

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