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HTML 

              
                 <!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=">
  <title></title>
</head>
<body>

  <div class="container">  
    <h1>Space Invaders</h1>
     <h3>Score: <span id="result"></span></h3>
  <div class="grid">
    <div></div>
    <div></div>
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  </div>
  <ul>
    <li>move ship Left and Right arrow keys</li>
    <li>spacebar to shoot</li>
  </ul>
    
  </div>

 
</body>
</html>
!

CSS 

              
                *{
  margin:0;
  padding:0;
}
body{
  background:#2B2B2B;
}
h1{
  color:#fcfeff;
  font-size: 3.3em;
  line-height: 1.2;
  margin-bottom:10px;
}

h3{
  color:#fff;
}

ul{
  display:felx;
  justify-content:center;
  margin-top:10px;
  color:#fcfeff;
  font-size:1.8rem;
  text-decoration:none;
  list-style:none;
  text-align:center;
  font-family:sans-serif;
}
li{
  margin-top:10px;
}
.container{
  position:absolute;
  top:50%;
  left:50%;
  transform:translate(-50%, -50%);
}


.grid{
  display:flex;
  flex-wrap:wrap;
  margin:0 auto;
  border: solid 5px #5cb85c;
  width:300px;
  height:300px;
  background-image: url('https://img.freepik.com/free-vector/sky-night-background-many-stars_135176-451.jpg?size=626&ext=jpg');
  background-position: center;
  background-size: 300%;
  background-repeat: repeat-y;
  background-position-y: -3rem;
  animation: ani 30s ease-in-out infinite;
  animation-direction: alternate-reverse;
}

@keyframes ani{
  0%{
    background-position-y: -3rem;
  }
  100%{
    background-position-y: 20rem;
  }
}


.grid div{
  width:20px;
  height:20px;  
}

  .shooter{
    position: relative;
    display:block;
    width:20px;
    height:20px;
    background:transparent;
  }
  .shooter::after{
    content:"🚀";
    position:absolute;
    width:20px;
    height:20px;
    position:absolute;
    top:0;
    left:0;
    right:0;
    margin:0 auto;
    display:block;

  }
  .invader{
    position: relative;
    display:block;
    width:20px;
    height:20px;
    background:transparent;
  }
  .invader::after{
    content:"👾";
    position:absolute;
    width:20px;
    height:20px;
    position:absolute;
    top:0;
    left:0;
    right:0;
    margin:0 auto;
    display:block;
  }
.laser{
  background:transparent;
  position:relative;
}
.laser::after{
  content:"";
  position:absolute;
  width:2px;
  height:5px;
  position:absolute;
  top:0;
  left:0;
  right:0;
  margin:0 auto;
  display:block;
  background:orange; 
}
!

JS 

              
                document.addEventListener('DOMContentLoaded', () => {
  const squares = document.querySelectorAll('.grid div')
  const resultDisplay = document.querySelector('#result')
  let width = 15
  let currentShooterIndex = 202
  let currentInvaderIndex = 0
  let alienInvadersTakenDown = []
  let result = 0
  let direction = 1
  let invaderId
  
  
  // define the alien invaders
   const alienInvaders = [
     0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
     15,16,17,18,19,20,21,22,23,24,
     30,31,32,33,34,35,36,37,38,39
   ]
   
  //draw the alien invaders
  alienInvaders.forEach( invader => squares[currentInvaderIndex + invader].classList.add('invader'))

  //draw the shooter
  squares[currentShooterIndex].classList.add('shooter')
  
  //move the shooter along a line
  function moveShooter(e){
    squares[currentShooterIndex].classList.remove('shooter')
    switch(e.keyCode){
      case 37:
      if(currentShooterIndex % width !==0) currentShooterIndex -=1
      break
      
      case 39:
      if(currentShooterIndex % width < width -1) currentShooterIndex +=1
      break 
    }
    squares[currentShooterIndex].classList.add('shooter')  
  }
  document.addEventListener('keydown', moveShooter)
  
  //move the alien invaders
  function moveInvaders() {
    const leftEdge = alienInvaders[0] % width === 0
    const rightEdge = alienInvaders[alienInvaders.length -1] % width === width -1
    
    if((leftEdge && direction === -1)  || (rightEdge && direction === 1)){
      direction = width
    }else if(direction === width){
      if (leftEdge) direction = 1
      else direction = -1
    }
    for (let i = 0; i <= alienInvaders.length -1; i++ ){
      squares[alienInvaders[i]].classList.remove('invader')
    }
    for (let i = 0; i <= alienInvaders.length -1; i++ ){
      alienInvaders[i] += direction
    }
      for (let i = 0; i <= alienInvaders.length -1; i++ ){
        if(!alienInvadersTakenDown.includes(i)){
          squares[alienInvaders[i]].classList.add('invader')
        }
    }
  
  }
  
  //deside a game over
  if(squares[currentShooterIndex].classList.contains('invader', 'shooter')){
    resultDisplay.textContent = 'Game Over'
    squares[currentShooterIndex].classList.add('boom')
    clearInterval(invaderId)
  }
  
  for(let i = 0; i <= alienInvaders.length -1; i++){
    if(alienInvaders[i] > (squares.length - (width-1))){
      resultDisplay.textContent ='Game Over'
      clearInterval(invaderId)
    }
    
    //decide a win
    if(alienInvadersTakenDown.length === alienInvaders.length){
      resultDisplay.textContent = "You Win"
      clearInterval(invaderId)
    }
  }
  invaderId = setInterval(moveInvaders, 500)
  
  
  //shoot at aliens
  function shoot(e){
    let laserId
    let currentLaserIndex = currentShooterIndex
    
    //move the laser from the shooter to the alien Invader
    function moveLaser(){
      squares[currentLaserIndex].classList.remove('laser')
      currentLaserIndex -= width
      squares[currentLaserIndex].classList.add('laser')
      if(squares[currentLaserIndex].classList.contains('invader')){
        squares[currentLaserIndex].classList.remove('laser')
        squares[currentLaserIndex].classList.remove('invader')
        squares[currentLaserIndex].classList.add('boom')
        
        setTimeout(() => squares[currentLaserIndex].classList.remove('boom'), 250 )
        clearInterval(laserId)
        
        const alienTakenDown = alienInvaders.indexOf(currentLaserIndex)
        alienInvadersTakenDown.push(alienTakenDown)
        result++
        resultDisplay.textContent = result
      }
      
      if(currentLaserIndex < width){
        clearInterval(laserId)
        setTimeout(() => squares[currentLaserIndex].classList.remove('laser'), 100)
      }
    }
    
    
    switch(e.keyCode){
      case 32:
        laserId = setInterval(moveLaser, 100)
        break
    }
  }
  
  
    document.addEventListener('keyup', shoot)
  
  
  
  
  
  
  
  
  })
!
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