Fading Snake

fading-snake.markdown

Fading Snake

No libraries, no preprocessors, no imports not even SVG or canvas. Just plain HTML, CSS, and JavaScript. Check out the source code, for detailed comments.

If you want to know how this game was made, watch the quick introduction for it on YouTube that explains the main game logic and the transitioning effect.

Follow me on Twitter for more

A Pen by vineeth on CodePen.

License.

index.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>

script.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(" ");
  }
});

style.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;
  }
}