Javascript dodger browser game

dodger.js

const DODGER = document.getElementById('dodger')
const GAME = document.getElementById('game')
const GAME_HEIGHT = 400
const GAME_WIDTH = 400
const LEFT_ARROW = 37 // use e.which!
const RIGHT_ARROW = 39 // use e.which!
const START = document.getElementById('start')

const ROCKS = []
var SCORE = 0

var gameInterval = null

function checkCollision(rock) {
  const top = positionToInteger(rock.style.top)


  // rocks are 20px high
  // DODGER is 20px high
  // GAME_HEIGHT - 20 - 20 = 360px;
  if (top > 360) {
    const dodgerLeftEdge = positionToInteger(DODGER.style.left)
    const dodgerRightEdge = dodgerLeftEdge + 40;
    const rockLeftEdge = positionToInteger(rock.style.left)
    const rockRightEdge = rockLeftEdge + 20;

    return (
      (rockLeftEdge <= dodgerLeftEdge && rockRightEdge >= dodgerLeftEdge) ||
      (rockLeftEdge >= dodgerLeftEdge && rockRightEdge <= dodgerRightEdge) ||
      (rockLeftEdge <= dodgerRightEdge && rockRightEdge >= dodgerRightEdge)
    )
  }
  

}
function checkScore(rock) {
  const top = positionToInteger(rock.style.top)
  if (top > 360) {
   return true
  }
  return false

}

function createRock(x) {
  const rock = document.createElement('div')

  rock.className = 'rock'
  rock.style.left = `${x}px`

  var top = rock.style.top = 0

  GAME.appendChild(rock)

  function moveRock() {
    rock.style.top = `${top += 2}px`;

    if (checkCollision(rock)) {
      return endGame()
    }

    if (checkScore(rock)){
    	SCORE += 1
    }

    if (top < GAME_HEIGHT) {
      window.requestAnimationFrame(moveRock)
    } else {
      rock.remove()
    }
  }

  window.requestAnimationFrame(moveRock)

  ROCKS.push(rock)

  return rock
}

function endGame() {
  clearInterval(gameInterval)
  let l = ROCKS.length


  for(var i=0 ; i < l ; i++)
  	{
  		ROCKS[i].remove();
  	}
  

  document.removeEventListener('keydown', moveDodger)

  START.innerHTML = `SCORE: ${SCORE}\nPlay again?`
  START.style.display = 'inline'
  SCORE = 0

  return alert('YOU LOSE!')
}

function moveDodger(e) {
  const code = e.which

  if ([LEFT_ARROW, RIGHT_ARROW].indexOf(code) > -1) {
    e.preventDefault()
    e.stopPropagation()
  }

  if (code === LEFT_ARROW) {
    moveDodgerLeft()
  } else if (code === RIGHT_ARROW) {
    moveDodgerRight()
  }
}

function moveDodgerLeft() {
  window.requestAnimationFrame(function() {
    const left = positionToInteger(DODGER.style.left)

    if (left > 0) {
      DODGER.style.left = `${left - 10}px`;
    }
  })
}

function moveDodgerRight() {
  window.requestAnimationFrame(function() {
    const left = positionToInteger(DODGER.style.left)

    if (left < 360) {
      DODGER.style.left = `${left + 10}px`;
    }
  })
}

function positionToInteger(p) {
  return parseInt(p.split('px')[0]) || 0
}

function start() {
  document.addEventListener('keydown', moveDodger)

  START.style.display = `none`


  gameInterval = setInterval(function() {
    createRock(Math.floor(Math.random() *  (GAME_WIDTH - 20)))
  }, 1000)
}