Web-Dev-For-Beginners/6-space-game/1-introduction

Build a Space Game Part 1: Introduction

Just like NASA's mission control coordinates multiple systems during a space launch, we're going to build a space game that demonstrates how different parts of a program can work together seamlessly. While creating something you can actually play, you'll learn essential programming concepts that apply to any software project.

We'll explore two fundamental approaches to organizing code: inheritance and composition. These aren't just academic concepts – they're the same patterns that power everything from video games to banking systems. We'll also implement a communication system called pub/sub that works like the communication networks used in spacecraft, allowing different components to share information without creating dependencies.

By the end of this series, you'll understand how to build applications that can scale and evolve – whether you're developing games, web applications, or any other software system.

Pre-Lecture Quiz

Pre-lecture quiz

Inheritance and Composition in Game Development

As projects grow in complexity, code organization becomes critical. What begins as a simple script can become difficult to maintain without proper structure – much like how the Apollo missions required careful coordination between thousands of components.

We'll explore two fundamental approaches for organizing code: inheritance and composition. Each has distinct advantages, and understanding both helps you choose the right approach for different situations. We'll demonstrate these concepts through our space game, where heroes, enemies, power-ups, and other objects must interact efficiently.

✅ One of the most famous programming books ever written has to do with design patterns.

In any game, you have game objects – the interactive elements that populate your game world. Heroes, enemies, power-ups, and visual effects are all game objects. Each exists at specific screen coordinates using x and y values, similar to plotting points on a coordinate plane.

Despite their visual differences, these objects often share fundamental behaviors:

• They exist somewhere – Every object has x and y coordinates so the game knows where to draw it
• Many can move around – Heroes run, enemies chase, bullets fly across the screen
• They have a lifespan – Some stick around forever, others (like explosions) appear briefly and vanish
• They react to stuff – When things collide, power-ups get collected, health bars update

✅ Think about a game like Pac-Man. Can you identify the four object types listed above in this game?

Expressing Behavior Through Code

Now that you understand the common behaviors game objects share, let's explore how to implement these behaviors in JavaScript. You can express object behavior through methods attached to either classes or individual objects, and there are several approaches to choose from.

The Class-Based Approach

Classes and inheritance provide a structured approach to organizing game objects. Like the taxonomic classification system developed by Carl Linnaeus, you start with a base class containing common properties, then create specialized classes that inherit these fundamentals while adding specific capabilities.

✅ Inheritance is an important concept to understand. Learn more on MDN's article about inheritance.

Here's how you can implement game objects using classes and inheritance:

// Step 1: Create the base GameObject class
class GameObject {
  constructor(x, y, type) {
    this.x = x;
    this.y = y;
    this.type = type;
  }
}

Let's break this down step by step:

• We're creating a basic template that every game object can use
• The constructor saves where the object is (x, y) and what kind of thing it is
• This becomes the foundation that all your game objects will build on

// Step 2: Add movement capability through inheritance
class Movable extends GameObject {
  constructor(x, y, type) {
    super(x, y, type); // Call parent constructor
  }

  // Add the ability to move to a new position
  moveTo(x, y) {
    this.x = x;
    this.y = y;
  }
}

In the above, we've:

• Extended the GameObject class to add movement functionality
• Called the parent constructor using super() to initialize inherited properties
• Added a moveTo() method that updates the object's position

// Step 3: Create specific game object types
class Hero extends Movable {
  constructor(x, y) {
    super(x, y, 'Hero'); // Set type automatically
  }
}

class Tree extends GameObject {
  constructor(x, y) {
    super(x, y, 'Tree'); // Trees don't need movement
  }
}

// Step 4: Use your game objects
const hero = new Hero(0, 0);
hero.moveTo(5, 5); // Hero can move!

const tree = new Tree(10, 15);
// tree.moveTo() would cause an error - trees can't move

Understanding these concepts:

• Creates specialized object types that inherit appropriate behaviors
• Demonstrates how inheritance allows selective feature inclusion
• Shows that heroes can move while trees remain stationary
• Illustrates how the class hierarchy prevents inappropriate actions

✅ Take a few minutes to re-envision a Pac-Man hero (Inky, Pinky or Blinky, for example) and how it would be written in JavaScript.

The Composition Approach

Composition follows a modular design philosophy, similar to how engineers design spacecraft with interchangeable components. Instead of inheriting from a parent class, you combine specific behaviors to create objects with exactly the functionality they need. This approach offers flexibility without rigid hierarchical constraints.

// Step 1: Create base behavior objects
const gameObject = {
  x: 0,
  y: 0,
  type: ''
};

const movable = {
  moveTo(x, y) {
    this.x = x;
    this.y = y;
  }
};

Here's what this code does:

• Defines a base gameObject with position and type properties
• Creates a separate movable behavior object with movement functionality
• Separates concerns by keeping position data and movement logic independent

// Step 2: Compose objects by combining behaviors
const movableObject = { ...gameObject, ...movable };

// Step 3: Create factory functions for different object types
function createHero(x, y) {
  return {
    ...movableObject,
    x,
    y,
    type: 'Hero'
  };
}

function createStatic(x, y, type) {
  return {
    ...gameObject,
    x,
    y,
    type
  };
}

In the above, we've:

• Combined base object properties with movement behavior using spread syntax
• Created factory functions that return customized objects
• Enabled flexible object creation without rigid class hierarchies
• Allowed objects to have exactly the behaviors they need

// Step 4: Create and use your composed objects
const hero = createHero(10, 10);
hero.moveTo(5, 5); // Works perfectly!

const tree = createStatic(0, 0, 'Tree');
// tree.moveTo() is undefined - no movement behavior was composed

Key points to remember:

• Composes objects by mixing behaviors rather than inheriting them
• Provides more flexibility than rigid inheritance hierarchies
• Allows objects to have exactly the features they need
• Uses modern JavaScript spread syntax for clean object combination

**Which Pattern Should You Choose?**

> 💡 **Pro Tip**: Both patterns have their place in modern JavaScript development. Classes work well for clearly defined hierarchies, while composition shines when you need maximum flexibility.
> 
**Here's when to use each approach:**
- **Choose** inheritance when you have clear "is-a" relationships (a Hero *is-a* Movable object)
- **Select** composition when you need "has-a" relationships (a Hero *has* movement abilities)
- **Consider** your team's preferences and project requirements
- **Remember** that you can mix both approaches in the same application

## Communication Patterns: The Pub/Sub System

As applications grow complex, managing communication between components becomes challenging. The publish-subscribe pattern (pub/sub) solves this problem using principles similar to radio broadcasting – one transmitter can reach multiple receivers without knowing who's listening.

Consider what happens when a hero takes damage: the health bar updates, sound effects play, visual feedback appears. Rather than coupling the hero object directly to these systems, pub/sub allows the hero to broadcast a "damage taken" message. Any system that needs to respond can subscribe to this message type and react accordingly.

✅ **Pub/Sub** stands for 'publish-subscribe'

### Understanding the Pub/Sub Architecture

The pub/sub pattern keeps different parts of your application loosely coupled, meaning they can work together without being directly dependent on each other. This separation makes your code more maintainable, testable, and flexible to changes.

**The key players in pub/sub:**
- **Messages** – Simple text labels like `'PLAYER_SCORED'` that describe what happened (plus any extra info)
- **Publishers** – The objects that shout out "Something happened!" to anyone who's listening
- **Subscribers** – The objects that say "I care about that event" and react when it happens
- **Event System** – The middleman that makes sure messages get to the right listeners

### Building an Event System

Let's create a simple but powerful event system that demonstrates these concepts:

```javascript
// Step 1: Create the EventEmitter class
class EventEmitter {
  constructor() {
    this.listeners = {}; // Store all event listeners
  }
  
  // Register a listener for a specific message type
  on(message, listener) {
    if (!this.listeners[message]) {
      this.listeners[message] = [];
    }
    this.listeners[message].push(listener);
  }
  
  // Send a message to all registered listeners
  emit(message, payload = null) {
    if (this.listeners[message]) {
      this.listeners[message].forEach(listener => {
        listener(message, payload);
      });
    }
  }
}

Breaking down what happens here:

• Creates a central event management system using a simple class
• Stores listeners in an object organized by message type
• Registers new listeners using the on() method
• Broadcasts messages to all interested listeners using emit()
• Supports optional data payloads for passing relevant information

Putting It All Together: A Practical Example

Alright, let's see this in action! We'll build a simple movement system that shows how clean and flexible pub/sub can be:

// Step 1: Define your message types
const Messages = {
  HERO_MOVE_LEFT: 'HERO_MOVE_LEFT',
  HERO_MOVE_RIGHT: 'HERO_MOVE_RIGHT',
  ENEMY_SPOTTED: 'ENEMY_SPOTTED'
};

// Step 2: Create your event system and game objects
const eventEmitter = new EventEmitter();
const hero = createHero(0, 0);

Here's what this code does:

• Defines a constants object to prevent typos in message names
• Creates an event emitter instance to handle all communication
• Initializes a hero object at the starting position

// Step 3: Set up event listeners (subscribers)
eventEmitter.on(Messages.HERO_MOVE_LEFT, () => {
  hero.moveTo(hero.x - 5, hero.y);
  console.log(`Hero moved to position: ${hero.x}, ${hero.y}`);
});

eventEmitter.on(Messages.HERO_MOVE_RIGHT, () => {
  hero.moveTo(hero.x + 5, hero.y);
  console.log(`Hero moved to position: ${hero.x}, ${hero.y}`);
});

In the above, we've:

• Registered event listeners that respond to movement messages
• Updated the hero's position based on the movement direction
• Added console logging to track the hero's position changes
• Separated the movement logic from the input handling

// Step 4: Connect keyboard input to events (publishers)
window.addEventListener('keydown', (event) => {
  switch(event.key) {
    case 'ArrowLeft':
      eventEmitter.emit(Messages.HERO_MOVE_LEFT);
      break;
    case 'ArrowRight':
      eventEmitter.emit(Messages.HERO_MOVE_RIGHT);
      break;
  }
});

Understanding these concepts:

• Connects keyboard input to game events without tight coupling
• Enables the input system to communicate with game objects indirectly
• Allows multiple systems to respond to the same keyboard events
• Makes it easy to change key bindings or add new input methods

💡 Pro Tip: The beauty of this pattern is flexibility! You can easily add sound effects, screen shake, or particle effects by simply adding more event listeners – no need to modify the existing keyboard or movement code.

Here's why you'll love this approach:

• Adding new features becomes super easy – just listen for the events you care about
• Multiple things can react to the same event without stepping on each other
• Testing gets way simpler because each piece works independently
• When something breaks, you know exactly where to look

Why Pub/Sub Scales Effectively

The pub/sub pattern maintains simplicity as applications grow in complexity. Whether managing dozens of enemies, dynamic UI updates, or sound systems, the pattern handles increased scale without architectural changes. New features integrate into the existing event system without affecting established functionality.

⚠️ Common Mistake: Don't create too many specific message types early on. Start with broad categories and refine them as your game's needs become clearer.

Best practices to follow:

• Groups related messages into logical categories
• Uses descriptive names that clearly indicate what happened
• Keeps message payloads simple and focused
• Documents your message types for team collaboration

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GitHub Copilot Agent Challenge 🚀

Use the Agent mode to complete the following challenge:

Description: Create a simple game object system using both inheritance and the pub/sub pattern. You'll implement a basic game where different objects can communicate through events without directly knowing about each other.

Prompt: Create a JavaScript game system with the following requirements: 1) Create a base GameObject class with x, y coordinates and a type property. 2) Create a Hero class that extends GameObject and can move. 3) Create an Enemy class that extends GameObject and can chase the hero. 4) Implement an EventEmitter class for the pub/sub pattern. 5) Set up event listeners so when the hero moves, nearby enemies receive a 'HERO_MOVED' event and update their position to move toward the hero. Include console.log statements to show the communication between objects.

Learn more about agent mode here.

🚀 Challenge

Consider how the pub-sub pattern can enhance game architecture. Identify which components should emit events and how the system should respond. Design a game concept and map out the communication patterns between its components.

Post-Lecture Quiz

Post-lecture quiz

Review & Self Study

Learn more about Pub/Sub by reading about it.

Assignment

Mock up a game