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HTML

              
                <div class="container">
  <header>
    <div class="contrast">100%</div>
    <div class="score">0</div>
  </header>
  <div class="grid"></div>
  <footer>Press an arrow key or space to start!
    <div>Ready for hard more? Press H</footer>
</div>

<a id="youtube" href="https://youtu.be/TAmYp4jKWoM" target="_blank">
  <span>See how this game was made</span>
</a>
<div id="youtube-card">
  How to create a snake game with JavaScript
</div>
              
            
!

CSS

              
                html,
body {
  height: 100%;
  margin: 0;
}

body {
  --size: 15px;
  --color: black;
  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
  color: var(--color);
  background-color: #ff8585;
  background: linear-gradient(
    162deg,
    rgba(255, 133, 133, 1) 0%,
    rgba(227, 84, 95, 1) 100%
  );
}

footer {
  font-size: 0.8em;
}

@media (min-height: 425px) {
  body {
    --size: 25px;
  }

  footer {
    height: 40px;
    font-size: 1em;
  }
}

.container {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  height: 100%;
}

header {
  display: flex;
  justify-content: space-between;
  width: calc(var(--size) * 17);
  font-size: 2em;
  font-weight: 900;
}

.grid {
  display: grid;
  grid-template-columns: repeat(15, auto);
  grid-template-rows: repeat(15, auto);
  border: var(--size) solid var(--color);
}
.tile {
  position: relative;
  width: var(--size);
  height: var(--size);
}
.content {
  position: absolute;
  width: 100%;
  height: 100%;
}

footer {
  margin-top: 20px;
  max-width: calc(var(--size) * 17);
  text-align: center;
}

footer a:visited {
  color: inherit;
}

#youtube,
#youtube-card {
  display: none;
}

@media (min-height: 425px) {
  /** Youtube logo by https://codepen.io/alvaromontoro */
  #youtube {
    z-index: 2;
    display: block;
    width: 100px;
    height: 70px;
    position: absolute;
    bottom: 20px;
    right: 20px;
    background: white;
    border-radius: 50% / 11%;
    transform: scale(0.8);
    transition: transform 0.5s;
  }

  #youtube:hover,
  #youtube:focus {
    transform: scale(0.9);
    background: red;
  }

  #youtube::before {
    content: "";
    display: block;
    position: absolute;
    top: 7.5%;
    left: -6%;
    width: 112%;
    height: 85%;
    background: white;
    border-radius: 9% / 50%;
  }

  #youtube:hover::before,
  #youtube:focus::before {
    background: red;
  }

  #youtube::after {
    content: "";
    display: block;
    position: absolute;
    top: 20px;
    left: 40px;
    width: 45px;
    height: 30px;
    border: 15px solid transparent;
    box-sizing: border-box;
    border-left: 30px solid #ff8585;
  }

  #youtube:hover::after,
  #youtube:focus::after {
    border-left: 30px solid white;
  }

  #youtube span {
    font-size: 0;
    position: absolute;
    width: 0;
    height: 0;
    overflow: hidden;
  }

  #youtube:hover + #youtube-card {
    display: block;
    position: absolute;
    bottom: 12px;
    right: 10px;
    padding: 25px 130px 25px 25px;
    width: 300px;
    background-color: white;
  }
}

              
            
!

JS

              
                /*



If you want to learn how this game was made, check out this video, that walks through the main ideas: 

YouTube: https://youtu.be/TAmYp4jKWoM
Skillshare: https://skl.sh/3nudJ1o

Follow me on twitter for more: https://twitter.com/HunorBorbely



*/

window.addEventListener("DOMContentLoaded", function (event) {
  window.focus(); // Capture keys right away (by default focus is on editor)

  // Game data
  let snakePositions; // An array of snake positions, starting head first
  let applePosition; // The position of the apple

  let startTimestamp; // The starting timestamp of the animation
  let lastTimestamp; // The previous timestamp of the animation
  let stepsTaken; // How many steps did the snake take
  let score;
  let contrast;

  let inputs; // A list of directions the snake still has to take in order

  let gameStarted = false;
  let hardMode = false;

  // Configuration
  const width = 15; // Grid width
  const height = 15; // Grid height

  const speed = 200; // Milliseconds it takes for the snake to take a step in the grid
  let fadeSpeed = 5000; // milliseconds it takes the grid to disappear (initially)
  let fadeExponential = 1.024; // after each score it will gradually take more time for the grid to fade
  const contrastIncrease = 0.5; // contrast you gain after each score
  const color = "black"; // Primary color

  // Setup: Build up the grid
  // The grid consists of (width x height) tiles
  // The tiles take the the shape of a grid using CSS grid
  // The tile can represent a part of the snake or an apple
  // Each tile has a content div that takes an absolute position
  // The content can fill the tile or slide in or out from any direction to take the shape of a transitioning snake head or tail
  const grid = document.querySelector(".grid");
  for (let i = 0; i < width * height; i++) {
    const content = document.createElement("div");
    content.setAttribute("class", "content");
    content.setAttribute("id", i); // Just for debugging, not used

    const tile = document.createElement("div");
    tile.setAttribute("class", "tile");
    tile.appendChild(content);

    grid.appendChild(tile);
  }

  const tiles = document.querySelectorAll(".grid .tile .content");

  const containerElement = document.querySelector(".container");
  const noteElement = document.querySelector("footer");
  const contrastElement = document.querySelector(".contrast");
  const scoreElement = document.querySelector(".score");

  // Initialize layout
  resetGame();

  // Resets game variables and layouts but does not start the game (game starts on keypress)
  function resetGame() {
    // Reset positions
    snakePositions = [168, 169, 170, 171];
    applePosition = 100; // Initially the apple is always at the same position to make sure it's reachable

    // Reset game progress
    startTimestamp = undefined;
    lastTimestamp = undefined;
    stepsTaken = -1; // It's -1 because then the snake will start with a step
    score = 0;
    contrast = 1;

    // Reset inputs
    inputs = [];

    // Reset header
    contrastElement.innerText = `${Math.floor(contrast * 100)}%`;
    scoreElement.innerText = hardMode ? `H ${score}` : score;

    // Reset tiles
    for (const tile of tiles) setTile(tile);

    // Render apple
    setTile(tiles[applePosition], {
      "background-color": color,
      "border-radius": "50%"
    });

    // Render snake
    // Ignore the last part (the snake just moved out from it)
    for (const i of snakePositions.slice(1)) {
      const snakePart = tiles[i];
      snakePart.style.backgroundColor = color;

      // Set up transition directions for head and tail
      if (i == snakePositions[snakePositions.length - 1])
        snakePart.style.left = 0;
      if (i == snakePositions[0]) snakePart.style.right = 0;
    }
  }

  // Handle user inputs (e.g. start the game)
  window.addEventListener("keydown", function (event) {
    // If not an arrow key or space or H was pressed then return
    if (
      ![
        "ArrowLeft",
        "ArrowUp",
        "ArrowRight",
        "ArrowDown",
        " ",
        "H",
        "h",
        "E",
        "e"
      ].includes(event.key)
    )
      return;

    // If an arrow key was pressed then first prevent default
    event.preventDefault();

    // If space was pressed restart the game
    if (event.key == " ") {
      resetGame();
      startGame();
      return;
    }

    // Set Hard mode
    if (event.key == "H" || event.key == "h") {
      hardMode = true;
      fadeSpeed = 4000;
      fadeExponential = 1.025;
      noteElement.innerHTML = `Hard mode. Press space to start!`;
      noteElement.style.opacity = 1;
      resetGame();
      return;
    }

    // Set Easy mode
    if (event.key == "E" || event.key == "e") {
      hardMode = false;
      fadeSpeed = 5000;
      fadeExponential = 1.024;
      noteElement.innerHTML = `Easy mode. Press space to start!`;
      noteElement.style.opacity = 1;
      resetGame();
      return;
    }

    // If an arrow key was pressed add the direction to the next moves
    // Do not allow to add the same direction twice consecutively
    // The snake can't do a full turn either
    // Also start the game if it hasn't started yet
    if (
      event.key == "ArrowLeft" &&
      inputs[inputs.length - 1] != "left" &&
      headDirection() != "right"
    ) {
      inputs.push("left");
      if (!gameStarted) startGame();
      return;
    }
    if (
      event.key == "ArrowUp" &&
      inputs[inputs.length - 1] != "up" &&
      headDirection() != "down"
    ) {
      inputs.push("up");
      if (!gameStarted) startGame();
      return;
    }
    if (
      event.key == "ArrowRight" &&
      inputs[inputs.length - 1] != "right" &&
      headDirection() != "left"
    ) {
      inputs.push("right");
      if (!gameStarted) startGame();
      return;
    }
    if (
      event.key == "ArrowDown" &&
      inputs[inputs.length - 1] != "down" &&
      headDirection() != "up"
    ) {
      inputs.push("down");
      if (!gameStarted) startGame();
      return;
    }
  });

  // Start the game
  function startGame() {
    gameStarted = true;
    noteElement.style.opacity = 0;
    window.requestAnimationFrame(main);
  }

  // The main game loop
  // This function gets invoked approximately 60 times per second to render the game
  // It keeps track of the total elapsed time and time elapsed since last call
  // Based on that animates the snake either by transitioning it in between tiles or stepping it to the next tile
  function main(timestamp) {
    try {
      if (startTimestamp === undefined) startTimestamp = timestamp;
      const totalElapsedTime = timestamp - startTimestamp;
      const timeElapsedSinceLastCall = timestamp - lastTimestamp;

      const stepsShouldHaveTaken = Math.floor(totalElapsedTime / speed);
      const percentageOfStep = (totalElapsedTime % speed) / speed;

      // If the snake took a step from a tile to another one
      if (stepsTaken != stepsShouldHaveTaken) {
        stepAndTransition(percentageOfStep);

        // If it’s time to take a step
        const headPosition = snakePositions[snakePositions.length - 1];
        if (headPosition == applePosition) {
          // Increase score
          score++;
          scoreElement.innerText = hardMode ? `H ${score}` : score;

          // Generate another apple
          addNewApple();

          // Increase the contrast after each score
          // Don't let the contrast go above 1
          contrast = Math.min(1, contrast + contrastIncrease);

          // Debugging
          console.log(`Contrast increased by ${contrastIncrease * 100}%`);
          console.log(
            "New fade speed (from 100% to 0% in milliseconds)",
            Math.pow(fadeExponential, score) * fadeSpeed
          );
        }

        stepsTaken++;
      } else {
        transition(percentageOfStep);
      }

      if (lastTimestamp) {
        // Decrease the contrast based on the time passed an the current score
        // With a higher score the contrast decreases slower
        const contrastDecrease =
          timeElapsedSinceLastCall /
          (Math.pow(fadeExponential, score) * fadeSpeed);
        // Don't let the contrast drop below zero
        contrast = Math.max(0, contrast - contrastDecrease);
      }

      contrastElement.innerText = `${Math.floor(contrast * 100)}%`;
      containerElement.style.opacity = contrast;

      window.requestAnimationFrame(main);
    } catch (error) {
      // Write a note about restarting game and setting difficulty
      const pressSpaceToStart = "Press space to reset the game.";
      const changeMode = hardMode
        ? "Back to easy mode? Press the letter E."
        : "Ready for hard more? Press the letter H.";
      const followMe =
        'Follow me <a href="https://twitter.com/HunorBorbely" , target="_blank">@HunorBorbely</a>';
      noteElement.innerHTML = `${error.message}. ${pressSpaceToStart} <div>${changeMode}</div> ${followMe}`;
      noteElement.style.opacity = 1;
      containerElement.style.opacity = 1;
    }

    lastTimestamp = timestamp;
  }

  // Moves the snake and sets up tiles for the transition function so the transition function will be more effective (the transition function gets called more frequently)
  function stepAndTransition(percentageOfStep) {
    // Calculate the next position and add it to the snake
    const newHeadPosition = getNextPosition();
    console.log(`Snake stepping into tile ${newHeadPosition}`);
    snakePositions.push(newHeadPosition);

    // Start with tail instead of head
    // Because the head might step into the previous position of the tail

    // Clear tile, yet keep it in the array if the snake grows.
    // Whenever the snake steps into a new tile, it will leave the last one.
    // Yet the last tile stays in the array if the snake just grows.
    // As a sideeffect in case the snake just eats an apple,
    // the tail transitioning will happen on a this "hidden" tile
    // (so the tail appears as stationary).
    const previousTail = tiles[snakePositions[0]];
    setTile(previousTail);

    if (newHeadPosition != applePosition) {
      // Drop the previous tail
      snakePositions.shift();

      // Set up and start transition for new tail
      // Make sure it heads to the right direction and set initial size
      const tail = tiles[snakePositions[0]];
      const tailDi = tailDirection();
      // The tail value is inverse because it slides out not in
      const tailValue = `${100 - percentageOfStep * 100}%`;

      if (tailDi == "right")
        setTile(tail, {
          left: 0,
          width: tailValue,
          "background-color": color
        });

      if (tailDi == "left")
        setTile(tail, {
          right: 0,
          width: tailValue,
          "background-color": color
        });

      if (tailDi == "down")
        setTile(tail, {
          top: 0,
          height: tailValue,
          "background-color": color
        });

      if (tailDi == "up")
        setTile(tail, {
          bottom: 0,
          height: tailValue,
          "background-color": color
        });
    }

    // Set previous head to full size
    const previousHead = tiles[snakePositions[snakePositions.length - 2]];
    setTile(previousHead, { "background-color": color });

    // Set up and start transitioning for new head
    // Make sure it heads to the right direction and set initial size
    const head = tiles[newHeadPosition];
    const headDi = headDirection();
    const headValue = `${percentageOfStep * 100}%`;

    if (headDi == "right")
      setTile(head, {
        left: 0, // Slide in from left
        width: headValue,
        "background-color": color,
        "border-radius": 0
      });

    if (headDi == "left")
      setTile(head, {
        right: 0, // Slide in from right
        width: headValue,
        "background-color": color,
        "border-radius": 0
      });

    if (headDi == "down")
      setTile(head, {
        top: 0, // Slide in from top
        height: headValue,
        "background-color": color,
        "border-radius": 0
      });

    if (headDi == "up")
      setTile(head, {
        bottom: 0, // Slide in from bottom
        height: headValue,
        "background-color": color,
        "border-radius": 0
      });
  }

  // Transition head and tail between two steps
  // Called with every animation frame, except when stepping to a new tile
  function transition(percentageOfStep) {
    // Transition head
    const head = tiles[snakePositions[snakePositions.length - 1]];
    const headDi = headDirection();
    const headValue = `${percentageOfStep * 100}%`;
    if (headDi == "right" || headDi == "left") head.style.width = headValue;
    if (headDi == "down" || headDi == "up") head.style.height = headValue;

    // Transition tail
    const tail = tiles[snakePositions[0]];
    const tailDi = tailDirection();
    const tailValue = `${100 - percentageOfStep * 100}%`;
    if (tailDi == "right" || tailDi == "left") tail.style.width = tailValue;
    if (tailDi == "down" || tailDi == "up") tail.style.height = tailValue;
  }

  // Calculate to which tile will the snake step into
  // Throw error if the snake bites its tail or hits the wall
  function getNextPosition() {
    const headPosition = snakePositions[snakePositions.length - 1];
    const snakeDirection = inputs.shift() || headDirection();
    switch (snakeDirection) {
      case "right": {
        const nextPosition = headPosition + 1;
        if (nextPosition % width == 0) throw Error("The snake hit the wall");
        // Ignore the last snake part, it'll move out as the head moves in
        if (snakePositions.slice(1).includes(nextPosition))
          throw Error("The snake bit itself");
        return nextPosition;
      }
      case "left": {
        const nextPosition = headPosition - 1;
        if (nextPosition % width == width - 1 || nextPosition < 0)
          throw Error("The snake hit the wall");
        // Ignore the last snake part, it'll move out as the head moves in
        if (snakePositions.slice(1).includes(nextPosition))
          throw Error("The snake bit itself");
        return nextPosition;
      }
      case "down": {
        const nextPosition = headPosition + width;
        if (nextPosition > width * height - 1)
          throw Error("The snake hit the wall");
        // Ignore the last snake part, it'll move out as the head moves in
        if (snakePositions.slice(1).includes(nextPosition))
          throw Error("The snake bit itself");
        return nextPosition;
      }
      case "up": {
        const nextPosition = headPosition - width;
        if (nextPosition < 0) throw Error("The snake hit the wall");
        // Ignore the last snake part, it'll move out as the head moves in
        if (snakePositions.slice(1).includes(nextPosition))
          throw Error("The snake bit itself");
        return nextPosition;
      }
    }
  }

  // Calculate in which direction the snake's head is moving
  function headDirection() {
    const head = snakePositions[snakePositions.length - 1];
    const neck = snakePositions[snakePositions.length - 2];
    return getDirection(head, neck);
  }

  // Calculate in which direction of the snake's tail
  function tailDirection() {
    const tail1 = snakePositions[0];
    const tail2 = snakePositions[1];
    return getDirection(tail1, tail2);
  }

  function getDirection(first, second) {
    if (first - 1 == second) return "right";
    if (first + 1 == second) return "left";
    if (first - width == second) return "down";
    if (first + width == second) return "up";
    throw Error("the two tile are not connected");
  }

  // Generates a new apple on the field
  function addNewApple() {
    // Find a position for the new apple that is not yet taken by the snake
    let newPosition;
    do {
      newPosition = Math.floor(Math.random() * width * height);
    } while (snakePositions.includes(newPosition));

    // Set new apple
    setTile(tiles[newPosition], {
      "background-color": color,
      "border-radius": "50%"
    });

    // Note that the apple is here
    applePosition = newPosition;
  }

  // Resets size and position related CSS properties
  function setTile(element, overrides = {}) {
    const defaults = {
      width: "100%",
      height: "100%",
      top: "auto",
      right: "auto",
      bottom: "auto",
      left: "auto",
      "background-color": "transparent"
    };
    const cssProperties = { ...defaults, ...overrides };
    element.style.cssText = Object.entries(cssProperties)
      .map(([key, value]) => `${key}: ${value};`)
      .join(" ");
  }
});

              
            
!
999px

Console