game.js

var sprites = {
  ship: { sx: 0, sy: 0, w: 37, h: 42, frames: 1 },
  missile: { sx: 0, sy: 30, w: 2, h: 10, frames: 1 },
  enemy_purple: { sx: 37, sy: 0, w: 42, h: 43, frames: 1 },
  enemy_bee: { sx: 79, sy: 0, w: 37, h: 43, frames: 1 },
  enemy_ship: { sx: 116, sy: 0, w: 42, h: 43, frames: 1 },
  enemy_circle: { sx: 158, sy: 0, w: 32, h: 33, frames: 1 },
  explosion: { sx: 0, sy: 64, w: 64, h: 64, frames: 12 },
  enemy_missile: { sx: 9, sy: 42, w: 3, h: 20, frame: 1 },
};

var enemies = {
  straight: { x: 0, y: -50, sprite: "enemy_ship", health: 10, E: 100 },
  ltr: {
    x: 0,
    y: -100,
    sprite: "enemy_purple",
    health: 10,
    B: 75,
    C: 1,
    E: 100,
    missiles: 2,
  },
  circle: {
    x: 250,
    y: -50,
    sprite: "enemy_circle",
    health: 10,
    A: 0,
    B: -100,
    C: 1,
    E: 20,
    F: 100,
    G: 1,
    H: Math.PI / 2,
  },
  wiggle: {
    x: 100,
    y: -50,
    sprite: "enemy_bee",
    health: 20,
    B: 50,
    C: 4,
    E: 100,
    firePercentage: 0.001,
    missiles: 2,
  },
  step: {
    x: 0,
    y: -50,
    sprite: "enemy_circle",
    health: 10,
    B: 150,
    C: 1.2,
    E: 75,
  },
};

var OBJECT_PLAYER = 1,
  OBJECT_PLAYER_PROJECTILE = 2,
  OBJECT_ENEMY = 4,
  OBJECT_ENEMY_PROJECTILE = 8,
  OBJECT_POWERUP = 16;

var startGame = function () {
  var ua = navigator.userAgent.toLowerCase();

  // Only 1 row of stars
  if (ua.match(/android/)) {
    Game.setBoard(0, new Starfield(50, 0.6, 100, true));
  } else {
    Game.setBoard(0, new Starfield(20, 0.4, 100, true));
    Game.setBoard(1, new Starfield(50, 0.6, 100));
    Game.setBoard(2, new Starfield(100, 1.0, 50));
  }
  Game.setBoard(
    3,
    new TitleScreen("Alien Invasion", "Press fire to start playing", playGame)
  );
};

var level1 = [
  // Start,   End, Gap,  Type,   Override
  [0, 4000, 500, "step"],
  [6000, 13000, 800, "ltr"],
  [10000, 16000, 400, "circle"],
  [17800, 20000, 500, "straight", { x: 50 }],
  [18200, 20000, 500, "straight", { x: 90 }],
  [18200, 20000, 500, "straight", { x: 10 }],
  [22000, 25000, 400, "wiggle", { x: 150 }],
  [22000, 25000, 400, "wiggle", { x: 100 }],
];

var playGame = function () {
  var board = new GameBoard();
  board.add(new PlayerShip());
  board.add(new Level(level1, winGame));
  Game.setBoard(3, board);
  Game.setBoard(5, new GamePoints(0));
};

var winGame = function () {
  Game.setBoard(
    3,
    new TitleScreen("You win!", "Press fire to play again", playGame)
  );
};

var loseGame = function () {
  Game.setBoard(
    3,
    new TitleScreen("You lose!", "Press fire to play again", playGame)
  );
};

var Starfield = function (speed, opacity, numStars, clear) {
  // Set up the offscreen canvas
  var stars = document.createElement("canvas");
  stars.width = Game.width;
  stars.height = Game.height;
  var starCtx = stars.getContext("2d");

  var offset = 0;

  // If the clear option is set,
  // make the background black instead of transparent
  if (clear) {
    starCtx.fillStyle = "#000";
    starCtx.fillRect(0, 0, stars.width, stars.height);
  }

  // Now draw a bunch of random 2 pixel
  // rectangles onto the offscreen canvas
  starCtx.fillStyle = "#FFF";
  starCtx.globalAlpha = opacity;
  for (var i = 0; i < numStars; i++) {
    starCtx.fillRect(
      Math.floor(Math.random() * stars.width),
      Math.floor(Math.random() * stars.height),
      2,
      2
    );
  }

  // This method is called every frame
  // to draw the starfield onto the canvas
  this.draw = function (ctx) {
    var intOffset = Math.floor(offset);
    var remaining = stars.height - intOffset;

    // Draw the top half of the starfield
    if (intOffset > 0) {
      ctx.drawImage(
        stars,
        0,
        remaining,
        stars.width,
        intOffset,
        0,
        0,
        stars.width,
        intOffset
      );
    }

    // Draw the bottom half of the starfield
    if (remaining > 0) {
      ctx.drawImage(
        stars,
        0,
        0,
        stars.width,
        remaining,
        0,
        intOffset,
        stars.width,
        remaining
      );
    }
  };

  // This method is called to update
  // the starfield
  this.step = function (dt) {
    offset += dt * speed;
    offset = offset % stars.height;
  };
};

var PlayerShip = function () {
  this.setup("ship", { vx: 0, reloadTime: 0.25, maxVel: 200 });

  this.reload = this.reloadTime;
  this.x = Game.width / 2 - this.w / 2;
  this.y = Game.height - Game.playerOffset - this.h;

  this.step = function (dt) {
    if (Game.keys["left"]) {
      this.vx = -this.maxVel;
    } else if (Game.keys["right"]) {
      this.vx = this.maxVel;
    } else {
      this.vx = 0;
    }

    this.x += this.vx * dt;

    if (this.x < 0) {
      this.x = 0;
    } else if (this.x > Game.width - this.w) {
      this.x = Game.width - this.w;
    }

    this.reload -= dt;
    if (Game.keys["fire"] && this.reload < 0) {
      Game.keys["fire"] = false;
      this.reload = this.reloadTime;

      this.board.add(new PlayerMissile(this.x, this.y + this.h / 2));
      this.board.add(new PlayerMissile(this.x + this.w, this.y + this.h / 2));
    }
  };
};

PlayerShip.prototype = new Sprite();
PlayerShip.prototype.type = OBJECT_PLAYER;

PlayerShip.prototype.hit = function (damage) {
  if (this.board.remove(this)) {
    loseGame();
  }
};

var PlayerMissile = function (x, y) {
  this.setup("missile", { vy: -700, damage: 10 });
  this.x = x - this.w / 2;
  this.y = y - this.h;
};

PlayerMissile.prototype = new Sprite();
PlayerMissile.prototype.type = OBJECT_PLAYER_PROJECTILE;

PlayerMissile.prototype.step = function (dt) {
  this.y += this.vy * dt;
  var collision = this.board.collide(this, OBJECT_ENEMY);
  if (collision) {
    collision.hit(this.damage);
    this.board.remove(this);
  } else if (this.y < -this.h) {
    this.board.remove(this);
  }
};

var Enemy = function (blueprint, override) {
  this.merge(this.baseParameters);
  this.setup(blueprint.sprite, blueprint);
  this.merge(override);
};

Enemy.prototype = new Sprite();
Enemy.prototype.type = OBJECT_ENEMY;

Enemy.prototype.baseParameters = {
  A: 0,
  B: 0,
  C: 0,
  D: 0,
  E: 0,
  F: 0,
  G: 0,
  H: 0,
  t: 0,
  reloadTime: 0.75,
  reload: 0,
};

Enemy.prototype.step = function (dt) {
  this.t += dt;

  this.vx = this.A + this.B * Math.sin(this.C * this.t + this.D);
  this.vy = this.E + this.F * Math.sin(this.G * this.t + this.H);

  this.x += this.vx * dt;
  this.y += this.vy * dt;

  var collision = this.board.collide(this, OBJECT_PLAYER);
  if (collision) {
    collision.hit(this.damage);
    this.board.remove(this);
  }

  if (Math.random() < 0.01 && this.reload <= 0) {
    this.reload = this.reloadTime;
    if (this.missiles == 2) {
      this.board.add(new EnemyMissile(this.x + this.w - 2, this.y + this.h));
      this.board.add(new EnemyMissile(this.x + 2, this.y + this.h));
    } else {
      this.board.add(new EnemyMissile(this.x + this.w / 2, this.y + this.h));
    }
  }
  this.reload -= dt;

  if (this.y > Game.height || this.x < -this.w || this.x > Game.width) {
    this.board.remove(this);
  }
};

Enemy.prototype.hit = function (damage) {
  this.health -= damage;
  if (this.health <= 0) {
    if (this.board.remove(this)) {
      Game.points += this.points || 100;
      this.board.add(new Explosion(this.x + this.w / 2, this.y + this.h / 2));
    }
  }
};

var EnemyMissile = function (x, y) {
  this.setup("enemy_missile", { vy: 200, damage: 10 });
  this.x = x - this.w / 2;
  this.y = y;
};

EnemyMissile.prototype = new Sprite();
EnemyMissile.prototype.type = OBJECT_ENEMY_PROJECTILE;

EnemyMissile.prototype.step = function (dt) {
  this.y += this.vy * dt;
  var collision = this.board.collide(this, OBJECT_PLAYER);
  if (collision) {
    collision.hit(this.damage);
    this.board.remove(this);
  } else if (this.y > Game.height) {
    this.board.remove(this);
  }
};

var Explosion = function (centerX, centerY) {
  this.setup("explosion", { frame: 0 });
  this.x = centerX - this.w / 2;
  this.y = centerY - this.h / 2;
};

Explosion.prototype = new Sprite();

Explosion.prototype.step = function (dt) {
  this.frame++;
  if (this.frame >= 12) {
    this.board.remove(this);
  }
};

window.addEventListener("load", function () {
  Game.initialize("game", sprites, startGame);
});