弾幕シューティングゲーム

以下は、JavaScriptを使用して実装した弾幕シューティングゲームの例です。

HTML:

```
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>弾幕シューティングゲーム</title>
    <style>
        #canvas {
            border: 1px solid black;
        }
    </style>
</head>
<body>
    <canvas id="canvas" width="600" height="400"></canvas>
</body>
<script src="game.js"></script>
</html>
```

JavaScript:

```
// Game loop timer
var gameLoop;

// Set canvas
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

// Set player variables
var playerWidth = 20;
var playerHeight = 20;
var playerX = canvas.width / 2 - playerWidth / 2;
var playerY = canvas.height - playerHeight - 10;
var playerSpeed = 10;

// Set bullet variables
var bulletWidth = 5;
var bulletHeight = 10;
var bulletSpeed = 7;
var bullets = [];

// Set enemy variables
var enemyWidth = 20;
var enemyHeight = 20;
var enemySpeed = 2;
var enemies = [];

// Set game variables
var score = 0;
var gameStarted = false;

// Start game
function startGame() {
    gameStarted = true;
    gameLoop = setInterval(update, 30);
}

// Stop game
function stopGame() {
    gameStarted = false;
    clearInterval(gameLoop);
}

// Update game loop
function update() {
    // Clear canvas
    ctx.clearRect(0, 0, canvas.width, canvas.height);

    // Draw player
    ctx.fillStyle = "blue";
    ctx.fillRect(playerX, playerY, playerWidth, playerHeight);

    // Move bullets
    for (var i = 0; i < bullets.length; i++) {
        bullets[i].y -= bulletSpeed;
        ctx.fillStyle = "red";
        ctx.fillRect(bullets[i].x, bullets[i].y, bulletWidth, bulletHeight);
    }

    // Move enemies
    for (var i = 0; i < enemies.length; i++) {
        enemies[i].y += enemySpeed;
        ctx.fillStyle = "green";
        ctx.fillRect(enemies[i].x, enemies[i].y, enemyWidth, enemyHeight);

        // Check collision with player
        if (enemies[i].y + enemyHeight >= playerY &&
            enemies[i].x >= playerX &&
            enemies[i].x <= playerX + playerWidth) {
            gameOver();
            return;
        }
    }

    // Check collision with bullets
    for (var i = 0; i < bullets.length; i++) {
        for (var j = 0; j < enemies.length; j++) {
            if (bullets[i].y <= enemies[j].y + enemyHeight &&
                bullets[i].x >= enemies[j].x &&
                bullets[i].x <= enemies[j].x + enemyWidth) {
                bullets.splice(i, 1);
                enemies.splice(j, 1);
                score++;
                break;
            }
        }
    }

    // Draw score
    ctx.fillStyle = "black";
    ctx.font = "20px Arial";
    ctx.fillText("Score: " + score, 10, 25);

    // Add new enemy
    if (Math.random() < 0.05) {
        enemies.push({
            x: Math.floor(Math.random() * (canvas.width - enemyWidth)),
            y: -enemyHeight
        });
    }
}

// Handle key inputs
document.addEventListener("keydown", function(e) {
    switch(e.keyCode) {
        case 37: // Left arrow
            playerX -= playerSpeed;
            break;
        case 39: // Right arrow
            playerX += playerSpeed;
            break;
        case 32: // Space bar
            bullets.push({
                x: playerX + playerWidth / 2 - bulletWidth / 2,
                y: playerY - bulletHeight
            });
            break;
    }
});

// Game over function
function gameOver() {
    stopGame();
    alert("Game over! Score: " + score);
}

// Start game when user clicks canvas
canvas.addEventListener("click", function(e) {
    if (!gameStarted) {
        startGame();
    }
});
```

このゲームは、キーボードの左右矢印キーでプレイヤーを移動し、スペースキーで弾を発射します。敵を倒し、スコアを上げることが目的です。敵に触れてしまうとゲームオーバーになります。弾幕シューティングゲームなので、敵の攻撃をかわしながら戦いましょう！