P2: Steps 11-15

Be sure that you completed Coding Steps 6-10 on the previous page.

IMPORTANT: Take the time to read each step, and try to understand how the code works.

Step 11: Add Player Shields (Health Bar)

Right now, our game is very unforgiving — if the spaceship collides with just one asteroid, the game is over.

Let's give the spaceship some "shields" that will be damaged every time the ship collides with an asteroid. Once all the shields are gone, then the ship will explode.

Many games have some kind of health bar for the player's character — when the health reaches zero, the player dies in the game. The shields that we're going to add to our game are just a health bar for the spaceship.

Phaser has built-in properties and methods to keep track of the health of a sprite. When a sprite's health reaches zero, Phaser will automatically kill() the sprite.

• health is the property that represents a sprite's health value. You can set it to any value (but many game designers use 100 as the starting value for a sprite's health).

• maxHealth is a property that sets a maximum value for the sprite's health. You can set this to any value (but many game designers also use 100 for the maximum value). If your game has ways for the sprite's health to increase (e.g., by collecting health packs, resting, etc.), this will prevent the health from exceeding this limit.

• damage() is a method to decrease a sprite's health value. Inside the parentheses, you include a value representing the amount of damage.

• heal() is a method to increase a sprite's health value. Inside the parentheses, you include a value representing the amount of healing.

Add Player Health and Damage

Add this Phaser code in your create() function (after the command that set the angle of the player):

player.health = 100;
player.maxHealth = 100;

This will set the starting value of the player's health to 100 (and also set 100 as the maximum value).

Instead of killing the player sprite when it collides with an asteroid, let's damage the player's health by 25. After 4 collisions, the player's health will be zero, and Phaser will automatically kill() the player sprite.

In your collideAsteriod() function, replace the command that kills the player with this command instead:

player.damage(25);

Refresh your HTML preview to verify that it takes 4 collisions with asteroids before the spaceship explodes.

Add Health Bar for Shields

Now we just need to add a health bar, so the player has some visual feedback to track the amount of "shields" remaining for their spaceship.

Typically, a health bar is a rectangle that decreases (or increases) in width (or height) to represent the amount of health remaining. An easy way to do this in Phaser is to scale an image of a rectangle in one direction (either its width or its height).

We're going to use images of a red rectangle and a green rectangle to create our health bar:

Notice that these two bars are the exact same size. We're going to position them on top of each other as layers, so the red bar is behind the green bar.

The width of the green bar will represent the amount of health ("shields") remaining. When the shields are at maximum, the green bar will completely cover and hide the red bar.

When the player's health is damaged, we will scale down the width of the green bar, which will also reveal some of the red bar behind it. This will make it easier for the player to quickly see how much or how little health is remaining.

Let's first add a text label for the health bar (so it will be clear to the player what this bar represents):

• Add a global variable for healthText

• In your create() function (after the code that added scoreText), add healthText as a text object at position 210, 20. Set the text to display: 'Shields'. Use the same font styling that you used for scoreText (Arial, 20px, bold, #ffffff)

• You do not need to set the text's anchor or add a text shadow.

Refresh your HTML preview to verify that the text label (Shields) appears at the top of the game to the right of the score.

We're going to add the red and green bars to the game as images instead of as sprites.

• Add a global variable for healthBar, which will represent the image of the green bar (we don't need to create a variable for the red bar)

• Preload assets/images/health-red.png as the image for the red bar. Assign it a unique asset key name, such as red-bar. (Hint: If you need help, look back at Step 4 to see how to preload an image.)

• Preload assets/images/health-green.png as the image for the green bar. Assign it a unique asset key name, such as green-bar.

Add this Phaser command in your create() function (after the code that added healthText):

game.add.image(300, 20, 'red-bar');

This command does just what you think — it adds an image at position 300, 20 using the red-bar asset. Because we're not going to scale or change the red bar in any way, we don't have to assign it to a variable — we can just add it to the game.

Refresh your HTML preview to verify that the red bar appears at the top of the game to the right of the "Shields" text.

Now add this Phaser command (immediately after the command that added the red bar):

healthBar = game.add.image(300, 20, 'green-bar');

This command adds the green bar at the exact same position and assigns it to the healthBar variable (so that we can scale the green bar when the health value changes).

Because you added the red bar first and then added the green bar, the green bar will be layered on top of the red bar.

Refresh your HTML preview to verify that the green bar appears at the top of the game (and hides the red bar, which is actually behind it).

Scale the Health Bar

Now we just need to scale the width of the green bar, so it will shrink to represent the decrease in the player's health.

Phaser has a scale property for game objects (such as: text, images, sprites, etc.) that allows you to scale the object's width and height to make the object larger or smaller.

• To shrink the width or height, use a scale value that is between 0 and 1.

• To enlarge the width or height, use a scale value that is greater than 1.

• To keep the width or height the same size, use a scale value of exactly 1.

For our health bar, we only want to change the width of the green bar. We'll keep the height of the green bar the same size.

We can calculate the scale needed for the width of the healthBar by using a simple fraction — the scale should be the player's current health value divided by its maximum possible health value.

• For example, if the player's health is currently 25 and its maximum health value is 100, the scale for the width of the health bar should be 0.25 (25/100), which would look like this:

Add this Phaser command inside your collideAsteroid() function (immediately after the command that damages the player):

healthBar.scale.setTo(player.health / player.maxHealth, 1);

This command sets the scale values for the width and height (in order) of the healthbar image. The scale for the width will be calculated by dividing the player's health by its maximum health. The scale for the height will be kept at 1, so the height of the image doesn't change.

Refresh your HTML preview to verify that the width of the green health bar shrinks in proportion to the damage from each collision.

In our case, each collision should shrink the width of the health bar by 25% of its original width until it reaches zero (100 → 75 → 50 → 25 → 0). When the shields reach zero, the spaceship will explode.

Step 12: Add Player Lives

Some games only give the player one life during the game — when the player's character is killed, the game is over. Other games give the player unlimited lives, allowing the player to restart the game again and again (often by sending the player back to a previous checkpoint in the game). Some games give the player a limited number of lives.

The original Asteroids game (like many video games from the 1970s and '80s) gave the player 3 lives at the start of the game. The player could also earn extra lives by reaching certain scores in the game.

We're going to add both of these features to our game. In this step, we'll give the player 3 lives (spaceships) at the start of the game. When the spaceship explodes, the player loses one life — but a new spaceship will appear as long as the player has at least one life left. When the player is out of lives, the game will be over.

Our game will also give the player a chance to earn an extra life as a bonus after every 10,000 score points (10,000 → 20,000 → 30,000 → 40,000, etc.). However, our game will also limit the total number of lives (spaceships) to a maximum of 5 lives.

Add Lives Bar

Our game could just list the number of lives as text on the screen. Instead, we'll show spaceship icons to represent the number of lives. Visually, this will make it easier for the player to see at a glance how many lives remain. It will sort of be like a health bar for the number of lives left.

Here's the image for the number of lives remaining. It shows 5 spaceship icons because that's the maximum number of lives we decided to allow the player to have:

If we were to scale this image, we could make it look larger or smaller — but it would still show 5 spaceships.

So instead of scaling this image, we are going to hide part of it when the number of lives is less than 5.

Phaser has a method called crop() that allows you to only show part of an image (and hide the rest). You can also change the "crop" later to show less (or more) of the original image. The "crop" can be changed or reversed whenever you need because it doesn't actually alter the original image.

Let's first add a text label for the spaceship icons (so it will be absolutely clear to the player what these icons represent):

• Add a global variable for livesText

• In your create() function (after the code that added healthBar), add livesText as a text object at position 590, 20. Set the text to display: 'Ships'. Use the same font styling that you used for scoreText and healthText (Arial, 20px, bold, #ffffff)

• You do not need to set the text's anchor or add a text shadow.

Refresh your HTML preview to verify that the text label (Ships) appears at the top of the game to the right of the Shields health bar.

Now you're going to add the spaceship icons to the game as an image:

• Add a global variable for livesBar

• Preload assets/images/ship-lives.png as the image for the spaceship icons. Assign it a unique asset key name, such as lives.

• Add the image to the game at position 655, 20 and assign it to the livesBar variable.

Refresh your HTML preview to verify that the 5 spaceship icons appear at the top of the game to the right of the "Ships" text.

Crop the Lives Bar

Next we're going to start the player with 3 lives and crop the livesBar to only show 3 spaceships.

In order to crop an image, you create a rectangle of a specified width and height to represent the crop that you want to apply to an image. Then you can crop() the image using this rectangle. Any part of the original image that is outside of the crop rectangle will be hidden.

The spaceship icons image is similar to a spritesheet containing frames. The image has 5 identical spaceship icons, which are each 25 pixels in width and 25 pixels in height (including the transparent area around each icon). The entire image is 125 pixels in width and 25 pixels in height.

So we can set the width of the crop rectangle based on how many lives the player has left. For example, if the player has 3 lives left, then the crop rectangle should be set to a width of 75 pixels (3×25).

We want the height of the crop rectangle to be the same as the height of the spaceship icons image (which is 25 pixels in height).

Add these variables to your code:

• Add a global variable named shipLives and assign it an initial value of 3

• Add a global variable called livesCrop to represent the crop rectangle

Add this Phaser code to your create() function (after the command that added livesBar):

    livesCrop = new Phaser.Rectangle(0, 0, shipLives * 25, 25);
    livesBar.crop(livesCrop);

Let's examine this code:

• The first line of code creates a rectangle that you'll use for the crop. 0, 0 represent the position of the top-left corner of the crop rectangle (relative to the top-left corner of the image that it will crop). The next two numbers represent the width and height (in order) of the rectangle: shipLives * 25 is the width, and 25 is the height.

• The second line applies the crop for the livesBar image using the livesCrop rectangle.

Refresh your HTML preview to verify that the only 3 spaceship icons appear at the top of the game.

Subtract Life When Player Killed

Now we need to actually change the number of shipLives every time the player's spaceship is destroyed. Then we will want to re-crop the livesBar using a new width (based on how many lives remain).

As a reminder, in your create() function, you added an event that will run a function whenever the player is killed. This is where your game resets the explosion sprite and plays its animation. Now you're going to add some more code inside this event function to change the number of lives remaining and adjust the crop for the livesBar.

Add Phaser commands in your player.events.onKilled function to do the following (in order):

• Decrease the value of shipLives by 1

• Change the width of livesCrop by calculating its new value: livesBar.width = shipLives * 25;

• Apply the crop for livesBar using the livesCrop rectangle

Refresh your HTML preview to verify that only 2 spaceship icons will be shown after the spaceship is destroyed.

Spawn New Spaceship If Lives Remain

Now we need to respawn the player's spaceship if the player still has lives yet. We can use an if-else statement to determine if the number of lives left is greater than zero (otherwise the game is over).

Add this Phaser code in your player.events.onKilled function (after the code that updated the crop for livesBar):

        // respawn player if lives are left
        if (shipLives > 0) {
            player.x = game.world.centerX;
            player.y = game.world.centerY;
            player.angle = -90;
            player.body.velocity.set(0);
            player.body.acceleration.set(0);
            player.revive(player.maxHealth);
            player.alpha = 0; // start as transparent
            game.add.tween(player).to({alpha:1}, 2000, Phaser.Easing.Cubic.Out, true);

        }
        else {
            // game over

        }

If the player still has at least one life left, this code will:

• position the player sprite back at the center of the game with the spaceship pointing up (-90° angle)

• sets the player sprite's velocity and acceleration to zero

• brings the player sprite back to life with maximum health using the revive() method (alternatively, we could have used the reset() method)

• fades the player sprite into view using a to tween on the sprite's alpha property (2000 means the tween will take 2 seconds to complete)

Otherwise, the player has zero lives left, which means the game is over. In Step 14, we'll add some code to run when the game's over.

Refresh your HTML preview to verify that a new spaceship will appear after the spaceship is destroyed. After 3 spaceships have been destroyed, the game should be over.

Add Teleport Sound Effect

Let's add some audio feedback to make it clear to the player that a new spaceship has appeared.

Add a sound effect that will play one time whenever a new spaceship appears:

• Use teleportSound as the name of the global variable for the sound.

• Use assets/sounds/teleport.mp3 for the sound file. Assign a unique asset key name to the sound, such as teleport.

• Add teleportSound to the game with a volume of 0.5.

• Add a Phaser command to play teleportSound inside your player.events.onKilled function (after the code for the tween that fades in the new spaceship).

Refresh your HTML preview to verify that the teleport sound plays whenever a new spaceship appears.

Step 13: Add Extra Life Reward

Let's reward the player with an extra life every 10,000 score points (10,000 → 20,000 → 30,000 → 40,000, etc.). However, we will set a limit of 5 lives that the player can have at any point.

If the player already has 5 lives, we'll reward the player by setting the shields (health) back to maximum.

Let's play a sound effect as feedback to the player when either reward is received. We'll also do a visual effect when a new life is awarded.

If the player already has 5 lives and maximum health, then no reward will be given, but the player will get another chance to earn a reward after another 10,000 points.

Add code to your game to do the following:

• Add a global variable named maxLives and assign it a value of 5

• Add a global variable named newLife and assign it an initial value of 10000

• Add a global variable named lifeSound for the sound effect

• Preload assets/sounds/extra-life.wav and assign it a unique asset key name, such as life

• Add lifeSound to the game with a volume of 0.5

Now let's plan out the code that we'll need to check to see if the player has earned an extra life reward.

If the player's score is greater than or equal to newLife, then the player has earned a reward:

• Next we need to check if the number of shipLives is less than maxLives. If that's true, we'll give the player an extra life.

• Otherwise the player already has the maximum number of lives, so we'll check if the player's health is less than its maxHealth. If that's true, we'll set the health back to its maximum.

• In either case, we need to increase newLife by another 10000 points, so the player can earn another reward later.

Let's add the code for this check as a separate function in your game code.

Add a new custom function named checkNewLife() (after your shootAsteroid() function).

Add this code inside the curly braces { } of your checkNewLife() function:

    if (score >= newLife) {
        if (shipLives < maxLives) {
            // award extra life

        }
        // maxLives already reached
        else if (player.health < player.maxHealth) {
            // replenish health instead

        }
        // increase score needed for next new life
        newLife = newLife + 10000;
    }

As you can see, this code will perform the check that we planned out. However, there is some missing code that you'll need to add inside the if-else statements.

Add Phaser commands to award an extra life:

• Increase the value of shipLives by 1

• Change the width of livesCrop by calculating its new value based on shipLives

• Set the crop for livesBar using the livesCrop rectangle

• Update the crop for livesBar

• Play lifeSound

• Make the game screen flash a bright green color by using this command: game.camera.flash(0x00ff00, 500);

Add Phaser commands to replenish the player's health (if the maximum number of lives is already reached):

• Change the value of player.health to be equal to player.maxHealth

• Scale the healthBar

• Play lifeSound

Most importantly, we need to call the checkNewLife() function, in order for it to actually run. We could call this function either inside the shootAsteroid() function (after the code that increases the score) OR inside the update() function (after the collide() commands is a good spot) — choose just one of those places to call the function with this command:

    checkNewLife();

Refresh your HTML preview to verify that an extra life is awarded when the score reaches 10000 points. Another extra life should be awarded at 20000 points. If possible, verify that the shields will be replenished to maximum if you already have 5 ships when you reach the next new life score.

Step 14: Add Game Start and Game Over

At this point, you have a working version of Asteroids with lots of features. Now we're going to add a simple "start screen" and "game over screen" to finish it.

Add Game Title

At the start of the game, let's show an image of our game's title (Asteroids 2084).

Add code to do the following:

• Add a global variable named gameTitle for the title image.

• Preload assets/images/asteroids-2084-title.png for the title image. Assign a unique asset key name to the image, such as title.

• At the end of your create() function (after the code that added and cropped the livesBar), add the title image 100 pixels above the game's center, and assign the image to the gameTitle variable.

• Set the title image's anchor to be its center.

• Set the scale for the title image to be 0.75 for both its width and height.

Refresh your HTML preview to verify that the game title appears on the screen. Currently, it will remain on-screen during the game, but we'll fix that in a bit.

Add Start Instruction

Let's add a way for the player to start the game when he or she is ready. We'll wait to show the spaceship until the player has pressed the fire key (spacebar). Once the fire key is pressed, we'll remove the game title (and a start instruction) from the screen, show the spaceship, and let the gameplay begin.

Add code to do the following:

• Add a global variable named startText for the start instruction text.

• In your create() function (after the code that added the gameTitle), add startText as a text object positioned 200 pixels below the game's center. Set the text to display: 'Press Fire to Start Mission'. For the font style, use Arial, 30px, bold, #00ff00 (which will make the text slightly larger and bright green).

• Set the text's anchor to be its center.

Refresh your HTML preview to verify that the start instruction text appears on the screen. (Just like the game title, it currently remains on-screen during the game, but we'll change that soon.)

Hide Spaceship Until Game Started by Player

Now let's hide the spaceship until the player actually presses the fire key to start the mission.

Add these Phaser commands (inside your create() function after the code that added the player animation):

    // hide player until game starts
    player.exists = false;

This code hides the player sprite by removing it from the game. Later, your code will add the sprite back into the game by setting the exists property back to true.

Refresh your HTML preview to verify that the spaceship no longer appears on the screen.

You probably discovered that pressing the fire key (spacebar) doesn't do anything yet. We're about to change that.

You might have also discovered that pressing the up arrow key will cause the engineSound to get louder (as if the spaceship were accelerating, except it doesn't exist yet).

Modify your existing if statement for the arrowKey.up, so it will check whether the arrowKey.up is being pressed AND the player exists.

• Hint: Look at your if statement for the fireKey as an example of how to do this

Refresh your HTML preview to verify that the engine sound will not get louder if you press the up arrow key when the spaceship is not visible.

Add Signal and Custom Function to Start Game

Now we want to add a signal that will "start" the game when the player presses the fire key. However, we only want this signal to run one time. Once the game is "started", we want the fire key to fire the spaceship's laser.

Add this Phaser command in your create() function (after the code that added the fireKey as a keyboard input):

fireKey.onDown.addOnce(startGame, this);

This command will add a one-time signal to the fireKey: when this key is pressed down, it will call a custom function named startGame (which you will need to create).

Add a new custom function named startGame() (after your checkNewLife() function).

Add this code inside the curly braces { } of your startGame() function:

    // fade out start text
    game.add.tween(startText).to({alpha: 0}, 250, Phaser.Easing.Cubic.Out, true);

    // fade out and zoom out game title (after slight delay)
    game.add.tween(gameTitle).to({alpha: 0}, 3000, Phaser.Easing.Cubic.Out, true, 250);
    game.add.tween(gameTitle.scale).to({x: 3, y: 3}, 3000, Phaser.Easing.Cubic.Out, true, 250);

    // fade in player

As you can see, this code adds several to tweens that change the startText and gameTitle. All these tweens use the Phaser.Easing.Cubic.Out pattern, and true means the tweens will start automatically (once the startGame() function is called).

• The startText will fade out (to an alpha value of zero, which is transparent) over 0.25 seconds (250 milliseconds).

• The gameTitle will fade out over 3 seconds but only starts after a 0.25 second delay (the 250 after true represents a delay value).

• At the same time, the gameTitle will also zoom out over 3 seconds (3000) by changing the scale of the image to 3 in both directions. This will also only start after a 0.25 second delay.

Now you need to add some missing code in this function to fade in the player sprite:

• Play the teleportSound (as audio feedback to the player)

• Make the player sprite exist (by changing its exists property to true)

• Add a to tween to make the player sprite fade into view by changing its alpha value to 1 over 2 seconds. Use the Phaser.Easing.Cubic.Out pattern, and set the tween to start automatically (with no delay).

Refresh your HTML preview to verify that pressing the fire key will "start" the game (fading out the start instruction, fading and zooming out the game title, fading in the spaceship and playing the teleport sound).

Show Game Over Text When Player is Out of Lives

Now let's add a simple "game over" screen once the player is out of lives. We'll display some text and allow the player to restart a new game (by pressing the fire key).

Add code to do the following:

• Add a global variable named gameOverText

• In your create() function (after the code that added the startText), add gameOverText as a text object positioned 100 pixels above the game's center. Set the text to display: 'Game Over'. For the font style, use Arial, 48px, bold, #ff0000 (which will make the text red).

• Set the text's anchor to be its center

• Make the text invisible (i.e., transparent) by setting its visible property to false

Now we need to display the gameOverText and the startText when the player has run out of lives.

Add Phaser code in your player.events.onKilled function (inside the else curly braces for when the player has no lives left) to do the following:

• Make gameOverText visible

• Set the scale for gameOverText to be 3 for both its width and height

• Add a to tween to make gameOverText fade into view by changing its alpha value to 1 over 1 second. Use the Phaser.Easing.Cubic.Out pattern, and set the tween to start automatically (with no delay).

• Add a to tween to make gameOverText.scale zoom in by changing its value to 1 (for both directions) over 1 second. Use the Phaser.Easing.Cubic.Out pattern, and set the tween to start automatically (with no delay).

• Add a to tween to make startText fade into view by changing its alpha value to 1 over 0.5 seconds. Use the Phaser.Easing.Cubic.Out pattern, and set the tween to start automatically after a 2 second delay.

Refresh your HTML preview to verify that the game over text and start instruction text appear on the screen when the player has run out of lives (game over text should fade and zoom in first, then start instruction should fade in).

Add Signal and Custom Function to Restart Game

Finally, when the game is over, we want to add a signal that will "restart" the game when the player presses the fire key again. However, we only want this signal to run one time.

Add a Phaser command in your player.events.onKilled function (when the game is over) to create a one-time signal that will call a custom function named restartGame when the firekey is pressed.

Add a new custom function named restartGame() (after your startGame function).

The restartGame() function needs to reset some of the game variables (such as: score, shipLives, etc.) back to their original values, and then restart the game.

Add code inside the curly braces { } of your restartGame() function to do the following:

• Change the value of score back to 0

• Change the value of shipLives back to 3

• Change the value of newLife back to 10000

• Change the value of the asteroids' maxSpeed back to 100

• Restart the game state with this command:

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  game.state.restart();

Refresh your HTML preview to verify that a new game can be restarted after the current game is over.

Congratulations, you've completed your second practice game! Hopefully, the new Phaser commands made sense — you'll get the chance to use them again in the next practice game.

If you have the time and interest, you can add an enemy spaceship in Step 15. Be forewarned that it will require a lot of additional code, and only a small amount will be provided to you. Luckily, most of the necessary code is similar to things you've already done (so you'll be able to copy and modify portions of your existing game code).

Step 15: Add Enemy Ship (Extra Credit)

The original Asteroids game had enemy spaceships that would periodically appear and shoot at the player, adding an extra challenge to the game — but rewarding the player with a large score point bonus if the player destroys the enemy.

In this step, you can add an enemy ship to your game. You'll add a timer event to make the enemy ship appear every 30 seconds and move across the screen in a random direction and speed. You'll also add a simple AI (artificial intelligence) to make the enemy aim and shoot at the player.

Adding this enemy ship will involve lots of sub-steps with lots of additional code (but most of it is similar to things you already have in your code). However, adding the enemy ship will be worth it because it really increases the excitement and challenge of the gameplay experience.

Add Global Variables for Enemy

Add global variables for the following:

• enemy (enemy spaceship sprite)

• enemyLaser (enemy weapon)

• enemyExplosion (animated explosion sprite for enemy)

• enemyFireSound (sound played when enemy weapon is fired)

• enemyAlarmSound (sound played when enemy is alive)

Preload Assets for Enemy

In your preload() function, load the following assets:

• Load assets/images/enemy-ship.png as a spritesheet, and assign it a unique asset key name, such as enemy-ship. Each animation frame in this spritesheet is 64 pixels in width and 64 pixels in height.

• Load assets/images/enemy-laser.png as an image, and assign it a unique asset key name, such as enemy-bullet

• Load assets/sounds/enemy-fire.wav as audio, and assign it a unique asset key name, such as enemy-fire

• Load assets/images/alarm.mp3 as audio, and assign it a unique asset key name, such as enemy-alarm

The enemyExplosion will just use the same spritesheet as the player explosion does (so your code does not need to load it a second time).

Add Enemy

In your create() function (after the code that added the player sprite), add code for the enemy sprite:

• Add the enemy sprite at position 0, 0 (which is a temporary position) using the enemy-ship asset

• Set the sprite's anchor to its center

• Enable Arcade Physics for the sprite

• Set the sprite's body collision area to be a circle with a radius of 30 pixels that is offset 2 pixels from the left edge of the frame and 2 pixels from the top edge

• Have the game automatically kill the sprite when it leaves the game world boundaries by adding this code:

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    enemy.outOfBoundsKill = true;
    enemy.checkWorldBounds = true;

• Add an animation to the sprite to play all 3 frames (in order 0-2) at 10 frames per second in a continuous loop

• Play the animation (so it will already be playing anytime the enemy is on-screen)

• Hide the sprite (until it is spawned) by setting its exists property to false

Add Enemy Laser

In your create() function (before the code that adds the enemy sprite), add code for the enemyLaser weapon:

• Add the enemyLaser weapon to the game with 5 bullets using the enemy-bullet asset

• Set the bullets to be killed automatically if they leave the camera boundaries

• Set the bullet speed to 400 pixels per second

• Set the fire rate to 750 (once every 0.75 seconds)

• Set the bullet body collision area to be a rectangle 16 pixels in width and 16 pixels in height that is offset 4 pixels from the left edge and 4 pixels from the top edge

After the code that adds the enemy sprite, add a command to:

• Set the enemyLaser weapon to track the enemy sprite with no offset but don't track the sprite's rotation (set to false)

Add Enemy Explosion

In your create() function (after the code that adds the explosion sprite used for the player), add code for the enemyExplosion sprite:

• Add the enemyExplosion sprite at position 0, 0 (which is a temporary position) using the explosion asset (same one used for player's explosion)

• Set the sprite's anchor to its center

• Add an animation to the sprite to play all 16 frames (in order 0-15) at 30 frames per second (with no loop)

• Hide the sprite (until it is needed) by setting its visible property to false

Add Enemy Fire Sound

In your create() function, add code for enemyFireSound:

• Add enemyFireSound to the game with a volume of 0.5

• After the code that adds the enemyLaser, add an enemyLaser.onFire function containing a command to play the sound.

Add Function to Spawn Enemy

You're going to create a custom function to spawn (generate) an enemy spaceship. Later, you'll add a timer to call (run) this custom function every 30 seconds.

Every time a new enemy is spawned, we want it to appear at a random location along one of the edges of the screen. The enemy should then move across the screen at a random speed and in a random direction.

After your restartGame() function, add a new custom function called spawnEnemy().

Add this Phaser code inside the curly braces { } of the function:

    if (player.exists == false) return; // don't spawn enemy if game over or not started

    // generate random location (1-4) for enemy spawn site
    // 1 = top, 2 = right, 3 = bottom, 4 = left
    var spawnSite = Math.floor(Math.random() * 4) + 1;

    // generate random starting position and velocity to move enemy across screen
    if (spawnSite == 1) {
        enemy.x = Math.random() * 400 + 200; // 200 to 600
        enemy.y = 0; // top edge
        enemy.body.velocity.x = Math.random() * 100 - 50; // -50 to 50
        enemy.body.velocity.y = Math.random() * 50 + 100; // 100 to 150
    } else if (spawnSite == 2) {
        enemy.x = game.world.width; // right edge
        enemy.y = Math.random() * 200 + 200; // 200 to 400
        enemy.body.velocity.x = Math.random() * 50 - 150; // -150 to -100
        enemy.body.velocity.y = Math.random() * 100 - 50; // -50 to 50
    } else if (spawnSite == 3) {
        enemy.x = Math.random() * 400 + 200; // 200 to 600
        enemy.y = game.world.height; // bottom edge
        enemy.body.velocity.x = Math.random() * 100 - 50; // -50 to 50
        enemy.body.velocity.y = Math.random() * 50 - 150; // -150 to -100
    } else {
        enemy.x = 0; // left edge
        enemy.y = Math.random() * 200 + 200; // 200 to 400
        enemy.body.velocity.x = Math.random() * 50 + 100; // 100 to 150
        enemy.body.velocity.y = Math.random() * 100 - 50; // -50 to 50
    }

    enemy.revive();

The first line of this code checks to see whether the player sprite exists. If the player hasn't started the game yet or the game is over, player.exists will be false. If that's the case, the function will simply return (i.e., end) without performing the other code in this function.

The rest of the code in this function generates a random number between 1-4 to decide whether the enemy will be spawned from the top, right, bottom, or left of the game screen. Then it generates a random starting position to place the enemy somewhere along the edge of the screen. It also generates a random velocity to move the enemy across the screen towards the opposite side (in a straight line). The ranges for the random values have been chosen to keep the enemy on-screen for several seconds.

The last command brings the enemy sprite back to life.

Add Enemy Alarm Sound

• In your create() function, add enemyAlarmSound to the game with a volume of 0.1, and set the sound to loop continuously.

• In your spawnEnemy() function, play the sound when the enemy is brought back to life.

• In your create() function (after the code that adds the enemy), add an enemy.events.onKilled function containing a command to stop the sound.

Add Enemy AI to Rotate and Fire

We want to add a simple AI (artificial intelligence) to control the enemy. The enemy will automatically move across the screen based on the velocity it was given when it was spawned. However, as it moves, we want the enemy to rotate to face towards the player, so the enemy can fire its laser at the player.

Add this Phaser code in your update() function (after the code that spawns new asteroids):

    if (enemy.exists) {
        enemy.rotation = game.physics.arcade.angleBetween(enemy, player); // rotate enemy towards player
        var fireError = Math.floor(Math.random() * 20) - 10; // random number between -10 and 10
        enemyLaser.fireAngle = enemy.angle + fireError; // add error so enemy not perfect shot
        enemyLaser.fire();
    }

When the enemy exists on-screen, this code will rotate the enemy sprite to face towards the player (by calculating the angle between these two sprites). Then it adds a small amount of error to the enemyLaser.fireAngle. (Otherwise, the enemy is always a perfect shot, making it too difficult for the player to avoid being hit.) Then the enemyLaser fires towards the direction of the player. The enemyLaser.fireRate (which you set in the create() function) will automatically determine how frequently the enemy actually fires its weapon.

Add Timer to Spawn Enemy

In your create() function (after the code that adds the enemy), add this Phaser command:

game.time.events.loop(Phaser.Timer.SECOND * 30, spawnEnemy, this);

This command adds a timer event that will run in a loop: Every 30 seconds, it will call (run) the custom function named spawnEnemy.

Refresh your HTML preview to verify that the enemy will appear once every 30 seconds (from a random edge location - top, right, bottom, or left). As the enemy flies across the screen, it should rotate to face towards the player and fire its weapon. The alarm sound and enemy fire sound should play.

However, at this point, the enemy and its laser bullets don't affect anything else in the game because you haven't added any collisions involving the enemy or its bullets.

Add Collision for Player and Enemy

1. In your update() function, add a collide() command for player and enemy (list in this order) that will call a custom function named collideEnemy.

2. At the end of your code, add a new custom function called collideEnemy().

3. Inside the parentheses ( ) after the function's name, list these parameters (in order): player, enemy

4. Inside the curly braces { } of the function, add Phaser commands to do the following:

   • Kill the enemy

   • Reset enemyExplosion to the x and y position of the enemy

   • Play the enemyExplosion animation

   • Play boomSound

   • Shake the game camera

   • Damage the player by 25 health points

   • Scale the player's healthBar

Refresh your HTML preview to verify that the player's spaceship is damaged and the enemy spaceship explodes when they collide.

Add Collision for Player's Bullets and Enemy

1. In your update() function, add a collide() command for laser.bullets and enemy (list in this order) that will call a custom function named shootEnemy.

2. At the end of your code, add a new custom function called shootEnemy().

3. Inside the parentheses () after the function's name, list these parameters (in order): bullet, enemy

4. Inside the curly braces { } of the function, add Phaser commands to do the following:

   • Kill the bullet

   • Kill the enemy

   • Reset enemyExplosion to the x and y position of the enemy

   • Play the enemyExplosion animation

   • Play boomSound

   • Add 2500 to the player's score

   • Update scoreText to display the new score value

   • Call the checkNewLife() function (NOTE: This is not needed if a call to checkNewLife() is already listed in your update() function)

Refresh your HTML preview to verify that the player's laser can destroy the enemy spaceship, awarding the player 2500 score points.

Add Overlap for Player and Enemy's Bullets

1. In your update() function, add an overlap() command for player and enemyLaser.bullets (list in this order) that will call a custom function named shootPlayer. As a reminder, overlap() is similar to collide() except overlap() doesn't transfer momentum between the two objects. (If we used a collide() function here, the enemy's bullets would slow down the player's spaceship or even push it backwards, which seems unnatural.)

2. At the end of your code, add a new custom function called shootPlayer().

3. Inside the parentheses () after the function's name, list these parameters (in order): player, bullet

4. Inside the curly braces { } of the function, add Phaser commands to do the following:

   • Kill the bullet

   • Play boomSound

   • Shake the game camera

   • Damage the player by 25 health points

   • Scale the player's healthBar

Refresh your HTML preview to verify that the enemy's laser can damage the player's spaceship.

Add Collision for Enemy's Bullets and Asteroids

1. In your update() function, add a collide() command for enemyLaser.bullets and asteroidGroup that will call a custom function named removeAsteroid.

2. At the end of your code, add a new custom function called removeAsteroid().

3. Inside the parentheses ( ) after the function's name, list these parameters (in order): bullet, asteroid

4. Inside the curly braces { } of the function, add Phaser commands to do the following:

   • Kill the bullet

   • Set the x and y positions of asteroidParticles to the x and y positions of the asteroid

   • Kill the asteroid

   • Play the explode() particle effect for asteroidParticles

   • Play boomSound

Refresh your HTML preview to verify that the enemy's laser will destroy an asteroid if hit.

You might have noticed that we did not add a collision between the enemy and the asteroidGroup. This was intentional, so that the player has to either destroy or avoid the enemy (rather than letting an asteroid randomly hit and destroy the enemy). However, if you want, you could add this last type of collision to make the game slightly more realistic. It's your choice.

Congratulations, you've completed everything in Step 15! You should be feeling much more confident about your ability to use Phaser to code a video game.