Web-Dev-For-Beginners/translations/en/6-space-game/5-keeping-score

Build a Space Game Part 5: Scoring and Lives

Pre-Lecture Quiz

Pre-lecture quiz

In this lesson, you'll learn how to add scoring to a game and manage lives.

Draw text on the screen

To display a game score on the screen, you need to know how to render text. The solution is using the fillText() method on the canvas object. You can also customize aspects like the font, text color, and alignment (left, right, center). Below is an example of code that draws text on the screen.

ctx.font = "30px Arial";
ctx.fillStyle = "red";
ctx.textAlign = "right";
ctx.fillText("show this on the screen", 0, 0);

✅ Learn more about how to add text to a canvas, and feel free to make your text look more stylish!

Life, as a game concept

In games, the concept of "life" is simply a number. In a space game, it's common to assign a set number of lives that decrease one by one when your ship takes damage. It's even better if you can visually represent this with icons like small ships or hearts instead of just a number.

What to build

Let's add the following features to your game:

• Game score: Award points for every enemy ship destroyed. We suggest 100 points per ship. The score should be displayed in the bottom left corner.
• Life: Your ship starts with three lives. You lose a life whenever an enemy ship collides with you. Lives should be displayed in the bottom right corner using the following graphic: .

Recommended steps

Locate the files provided in the your-work subfolder. It should contain the following:

-| assets
  -| enemyShip.png
  -| player.png
  -| laserRed.png
-| index.html
-| app.js
-| package.json

Start your project in the your_work folder by typing:

cd your-work
npm start

This will start an HTTP server at http://localhost:5000. Open a browser and navigate to that address. At this point, you should see the hero and all the enemies. You can move the hero using the left and right arrow keys and shoot down enemies.

Add code

1. Copy the required assets from the solution/assets/ folder into the your-work folder. This includes the life.png asset. Add the lifeImg to the window.onload function:

   lifeImg = await loadTexture("assets/life.png");
   

2. Add the lifeImg to the list of assets:

   let heroImg,
   ...
   lifeImg,
   ...
   eventEmitter = new EventEmitter();
   

3. Add variables. Create variables to represent the total score (starting at 0) and remaining lives (starting at 3). Display these values on the screen.

4. Extend the updateGameObjects() function. Modify the updateGameObjects() function to handle collisions with enemies:

   enemies.forEach(enemy => {
       const heroRect = hero.rectFromGameObject();
       if (intersectRect(heroRect, enemy.rectFromGameObject())) {
         eventEmitter.emit(Messages.COLLISION_ENEMY_HERO, { enemy });
       }
     })
   

5. Add life and points.

   1. Initialize variables. In the Hero class, under this.cooldown = 0, initialize the life and points variables:

      this.life = 3;
      this.points = 0;
      

   2. Draw variables on the screen. Render these values on the screen:

      function drawLife() {
        // TODO, 35, 27
        const START_POS = canvas.width - 180;
        for(let i=0; i < hero.life; i++ ) {
          ctx.drawImage(
            lifeImg, 
            START_POS + (45 * (i+1) ), 
            canvas.height - 37);
        }
      }
      
      function drawPoints() {
        ctx.font = "30px Arial";
        ctx.fillStyle = "red";
        ctx.textAlign = "left";
        drawText("Points: " + hero.points, 10, canvas.height-20);
      }
      
      function drawText(message, x, y) {
        ctx.fillText(message, x, y);
      }
      
      

   3. Add methods to the game loop. Ensure these functions are added to the window.onload function under updateGameObjects():

      drawPoints();
      drawLife();
      

6. Implement game rules. Add the following rules to your game:

   1. Deduct a life for every collision between the hero and an enemy.

      Extend the Hero class to handle life deduction:

      decrementLife() {
        this.life--;
        if (this.life === 0) {
          this.dead = true;
        }
      }
      

   2. Increase the score by 100 points for every laser that hits an enemy.

      Extend the Hero class to handle score increments:

        incrementPoints() {
          this.points += 100;
        }
      

      Add these functions to your collision event emitters:

      eventEmitter.on(Messages.COLLISION_ENEMY_LASER, (_, { first, second }) => {
         first.dead = true;
         second.dead = true;
         hero.incrementPoints();
      })
      
      eventEmitter.on(Messages.COLLISION_ENEMY_HERO, (_, { enemy }) => {
         enemy.dead = true;
         hero.decrementLife();
      });
      

✅ Explore other games built using JavaScript/Canvas. What common features do they share?

By the end of this task, you should see small "life" icons in the bottom right corner, points displayed in the bottom left corner, and observe your life count decrease when colliding with enemies and your score increase when shooting enemies. Great job! Your game is almost finished.

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🚀 Challenge

Your code is nearly complete. What do you think your next steps should be?

Post-Lecture Quiz

Post-lecture quiz

Review & Self Study

Research different ways to increment and decrement game scores and lives. There are some interesting game engines like PlayFab. How could using one of these enhance your game?

Assignment

Build a Scoring Game

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Disclaimer:
This document has been translated using the AI translation service Co-op Translator. While we strive for accuracy, please note that automated translations may contain errors or inaccuracies. The original document in its native language should be regarded as the authoritative source. For critical information, professional human translation is recommended. We are not responsible for any misunderstandings or misinterpretations resulting from the use of this translation.