我的想法是只有在食物被吃完后才改變背景圖像。然而，它只是在我列表中的圖像之間來回變化。我認為這是因為我在腳本中使用了 requestAnimationFrame。這是 script.jsimport { update as updateSnake, draw as drawSnake, getSnakeHead, snakeIntersection } from './snake.js'import { update as updateFood, draw as drawFood} from './food.js'import { outsideGrid } from './grid.js'let lastRenderTime = 0let gameOver = 0;let modal = document.getElementById('modal')let easy = document.getElementById('easy')let normal = document.getElementById('normal')let hard = document.getElementById('hard')let SNAKE_SPEEDeasy.onclick = function() {    modal.style.display = "none"    chooseDifficulty(easy)}normal.onclick = function() {    modal.style.display = "none"    chooseDifficulty(normal)}hard.onclick = function() {    modal.style.display = "none"    chooseDifficulty(hard)}const gameBoard = document.getElementById('game-board')function chooseDifficulty(difficulty) {    switch (difficulty) {        case easy:            SNAKE_SPEED = 5;            break;        case normal:            SNAKE_SPEED = 8;            break;        case hard:            SNAKE_SPEED = 12;            break;    }    }function main(currentTime) {    if (gameOver) {        if (confirm('You lost. Press OK to restart')) {            location.reload()        }        return    }    window.requestAnimationFrame(main)    const secondsSinceLastRender = (currentTime - lastRenderTime) / 1000    if (secondsSinceLastRender < 1 / SNAKE_SPEED) return    lastRenderTime = currentTime;    update()    draw()}window.requestAnimationFrame(main)function update() {    updateSnake()    updateFood()    checkDeath()}function draw() {    gameBoard.innerHTML = ''    drawSnake(gameBoard)    drawFood(gameBoard)}function checkDeath() {    gameOver = outsideGrid(getSnakeHead()) || snakeIntersection()}如何在吃完食物后只改變一次食物的圖像？