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Build a Space Game Part 3: Adding Motion

Movement is the heart of any engaging game experience. Without motion, your space game would be nothing more than a static image – interesting to look at, but hardly exciting to play! In this lesson, you'll breathe life into your game by learning how to create dynamic, interactive movement that responds to player input and creates autonomous behavior for game objects.

You'll discover the fundamental principles behind all game movement: coordinate manipulation, screen clearing, and redrawing. These concepts form the foundation of game animation and will serve you well whether you're building simple 2D games or complex interactive applications. Think of this as learning the core "engine" that powers visual interactivity.

By the end of this lesson, you'll have a fully interactive space game where players can control their hero spaceship and watch enemy ships move across the screen. You'll understand how keyboard events work, how to create smooth animations, and how to organize your code using modern JavaScript patterns. Let's transform your static game into an exciting interactive experience!

Pre-Lecture Quiz

Pre-lecture quiz

Understanding Game Movement

Games aren't much fun until you have aliens running around on screen! In this game, we will make use of two types of movements:

Keyboard/Mouse movement: when the user interacts with the keyboard or mouse to move an object on the screen.
Game induced movement: when the game moves an object with a certain time interval.

So how do we move things on a screen? It's all about cartesian coordinates: we change the location (x,y) of the object and then redraw the screen.

Typically you need the following steps to accomplish movement on a screen:

Set a new location for an object; this is needed to perceive the object as having moved.
Clear the screen, the screen needs to be cleared in between draws. We can clear it by drawing a rectangle that we fill with a background color.
Redraw object at new location. By doing this we finally accomplish moving the object from one location to the other.

Here's what it can look like in code:

// Set the hero's location
hero.x += 5;
// Clear the rectangle that hosts the hero
ctx.clearRect(0, 0, canvas.width, canvas.height);
// Redraw the game background and hero
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = "black";
ctx.drawImage(heroImg, hero.x, hero.y);

Here's what this code does:

Updates the hero's x-coordinate by 5 pixels to move it horizontally
Clears the entire canvas area to remove the previous frame
Fills the canvas with a black background color
Redraws the hero image at its new position

✅ Can you think of a reason why redrawing your hero many frames per second might accrue performance costs? Read about alternatives to this pattern.

Handle keyboard events

Interactive games respond to player input, and keyboard events are one of the most common ways players control their game characters. Understanding how to capture and respond to keyboard input is essential for creating engaging gameplay experiences.

You handle events by attaching specific events to code. Keyboard events are triggered on the whole window whereas mouse events like a click can be connected to clicking a specific element. We will use keyboard events throughout this project.

To handle an event you need to use the window's addEventListener() method and provide it with two input parameters. The first parameter is the name of the event, for example keyup. The second parameter is the function that should be invoked as a result of the event taking place.

Here's an example:

window.addEventListener('keyup', (evt) => {
  // evt.key = string representation of the key
  if (evt.key === 'ArrowUp') {
    // do something
  }
});

Breaking down what happens here:

Listens for keyboard events on the entire window
Captures the event object which contains information about which key was pressed
Checks if the pressed key matches a specific key (in this case, the up arrow)
Executes code when the condition is met

For key events there are two properties on the event you can use to see what key was pressed:

key - this is a string representation of the pressed key, for example 'ArrowUp'
keyCode - this is a number representation, for example 37, corresponds to ArrowLeft

✅ Key event manipulation is useful outside of game development. What other uses can you think of for this technique?

Special keys: a caveat

There are some special keys that affect the window. That means that if you are listening to a keyup event and you use these special keys to move your hero it will also perform horizontal scrolling. For that reason you might want to shut-off this built-in browser behavior as you build out your game. You need code like this:

const onKeyDown = function (e) {
  console.log(e.keyCode);
  switch (e.keyCode) {
    case 37:
    case 39:
    case 38:
    case 40: // Arrow keys
    case 32:
      e.preventDefault();
      break; // Space
    default:
      break; // do not block other keys
  }
};

window.addEventListener('keydown', onKeyDown);

Understanding this prevention code:

Checks for specific key codes that might cause unwanted browser behavior
Prevents the default browser action for arrow keys and spacebar
Allows other keys to function normally
Uses e.preventDefault() to stop the browser's built-in behavior

Game induced movement

Not all movement in games comes from player input. Many game objects need to move on their own – think of enemies patrolling, bullets flying, or background elements scrolling. This type of automatic movement creates the dynamic environment that makes games feel alive.

We can make things move by themselves by using timers such as the setTimeout() or setInterval() function that update the location of the object on each tick, or time interval. Here's what that can look like:

const id = setInterval(() => {
  // Move the enemy on the y axis
  enemy.y += 10;
}, 100);

Here's what this movement code does:

Creates a timer that runs every 100 milliseconds
Updates the enemy's y-coordinate by 10 pixels each time
Stores the interval ID so we can stop it later if needed
Moves the enemy downward on the screen automatically

The game loop

The game loop is the heartbeat of your game – it's what makes everything come together and creates the illusion of continuous motion. Think of it as the conductor of an orchestra, coordinating all the different elements to create a smooth, synchronized experience.

The game loop is a concept that is essentially a function that is invoked at regular intervals. It's called the game loop as everything that should be visible to the user is drawn into the loop. The game loop makes use of all the game objects that are part of the game, drawing all of them unless for some reason shouldn't be part of the game any more. For example if an object is an enemy that was hit by a laser and blows up, it's no longer part of the current game loop (you'll learn more on this in subsequent lessons).

Here's what a game loop can typically look like, expressed in code:

const gameLoopId = setInterval(() => {
  function gameLoop() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.fillStyle = "black";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    drawHero();
    drawEnemies();
    drawStaticObjects();
  }
  gameLoop();
}, 200);

Understanding the game loop structure:

Clears the entire canvas to remove the previous frame
Fills the background with a solid color
Draws all game objects in their current positions
Repeats this process every 200 milliseconds to create smooth animation
Manages the frame rate by controlling the interval timing

Continuing the Space Game

Now it's time to put theory into practice! You'll take the existing code and extend it with the movement concepts you've just learned. This is where your static space game transforms into an interactive experience.

You will take the existing code and extend it. Either start with the code that you completed during part I or use the code in Part II- starter.

What you'll implement:

Moving the hero: you will add code to ensure you can move the hero using the arrow keys
Move enemies: you will also need to add code to ensure the enemies move from top to bottom at a given rate

Recommended steps

Locate the files that have been created for you in the your-work sub folder. It should contain the following:

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

You start your project in the your-work folder by typing:

cd your-work
npm start

What this command does:

Navigates to your project directory
Starts a HTTP Server on address http://localhost:5000
Serves your game files so you can test them in a browser

The above will start a HTTP Server on address http://localhost:5000. Open up a browser and input that address, right now it should render the hero and all the enemies; nothing is moving - yet!

Add code

Add dedicated objects for hero and enemy and game object, they should have x and y properties. (Remember the portion on Inheritance or composition).

HINT game object should be the one with x and y and the ability to draw itself to a canvas.

Tip: Start by adding a new GameObject class with its constructor delineated as below, and then draw it to the canvas:

class GameObject {
  constructor(x, y) {
    this.x = x;
    this.y = y;
    this.dead = false;
    this.type = "";
    this.width = 0;
    this.height = 0;
    this.img = undefined;
  }

  draw(ctx) {
    ctx.drawImage(this.img, this.x, this.y, this.width, this.height);
  }
}

Understanding this base class:

Defines common properties that all game objects share (position, size, image)
Includes a dead flag to track whether the object should be removed
Provides a draw() method that renders the object on the canvas
Sets default values for all properties that child classes can override

Now, extend this GameObject to create the Hero and Enemy:

class Hero extends GameObject {
  constructor(x, y) {
    super(x, y);
    this.width = 98;
    this.height = 75;
    this.type = "Hero";
    this.speed = 5;
  }
}

class Enemy extends GameObject {
  constructor(x, y) {
    super(x, y);
    this.width = 98;
    this.height = 50;
    this.type = "Enemy";
    const id = setInterval(() => {
      if (this.y < canvas.height - this.height) {
        this.y += 5;
      } else {
        console.log('Stopped at', this.y);
        clearInterval(id);
      }
    }, 300);
  }
}

Key concepts in these classes:

Inherits from GameObject using the extends keyword
Calls the parent constructor with super(x, y)
Sets specific dimensions and properties for each object type
Implements automatic movement for enemies using setInterval()

Add key-event handlers to handle key navigation (move hero up/down left/right)

REMEMBER it's a cartesian system, top-left is 0,0. Also remember to add code to stop default behavior

Tip: Create your onKeyDown function and attach it to the window:
```
const onKeyDown = function (e) {
  console.log(e.keyCode);
  // Add the code from the lesson above to stop default behavior
  switch (e.keyCode) {
    case 37:
    case 39:
    case 38:
    case 40: // Arrow keys
    case 32:
      e.preventDefault();
      break; // Space
    default:
      break; // do not block other keys
  }
};

window.addEventListener("keydown", onKeyDown);
```
What this event handler does:
- Listens for keydown events on the entire window
- Logs the key code to help you debug which keys are being pressed
- Prevents default browser behavior for arrow keys and spacebar
- Allows other keys to function normally
Check your browser console at this point, and watch the keystrokes being logged.

Implement the Pub sub pattern, this will keep your code clean as you follow the remaining parts.

The Publish-Subscribe pattern helps organize your code by separating event detection from event handling. This makes your code more modular and easier to maintain.

To do this last part, you can:

Add an event listener on the window:

window.addEventListener("keyup", (evt) => {
  if (evt.key === "ArrowUp") {
    eventEmitter.emit(Messages.KEY_EVENT_UP);
  } else if (evt.key === "ArrowDown") {
    eventEmitter.emit(Messages.KEY_EVENT_DOWN);
  } else if (evt.key === "ArrowLeft") {
    eventEmitter.emit(Messages.KEY_EVENT_LEFT);
  } else if (evt.key === "ArrowRight") {
    eventEmitter.emit(Messages.KEY_EVENT_RIGHT);
  }
});

What this event system does:

Detects keyboard input and converts it to custom game events
Separates input detection from game logic
Makes it easy to change controls later without affecting game code
Allows multiple systems to respond to the same input

Create an EventEmitter class to publish and subscribe to messages:

class EventEmitter {
  constructor() {
    this.listeners = {};
  }

  on(message, listener) {
    if (!this.listeners[message]) {
      this.listeners[message] = [];
    }
    this.listeners[message].push(listener);
  }

Add constants and set up the EventEmitter:

const Messages = {
  KEY_EVENT_UP: "KEY_EVENT_UP",
  KEY_EVENT_DOWN: "KEY_EVENT_DOWN",
  KEY_EVENT_LEFT: "KEY_EVENT_LEFT",
  KEY_EVENT_RIGHT: "KEY_EVENT_RIGHT",
};

let heroImg, 
    enemyImg, 
    laserImg,
    canvas, ctx, 
    gameObjects = [], 
    hero, 
    eventEmitter = new EventEmitter();

Understanding the setup:

Defines message constants to avoid typos and make refactoring easier
Declares variables for images, canvas context, and game state
Creates a global event emitter for the pub-sub system
Initializes an array to hold all game objects

Initialize the game

function initGame() {
  gameObjects = [];
  createEnemies();
  createHero();

  eventEmitter.on(Messages.KEY_EVENT_UP, () => {
    hero.y -= 5;
  });

  eventEmitter.on(Messages.KEY_EVENT_DOWN, () => {
    hero.y += 5;
  });

  eventEmitter.on(Messages.KEY_EVENT_LEFT, () => {
    hero.x -= 5;
  });

Setup the game loop

Refactor the window.onload function to initialize the game and set up a game loop on a good interval. You'll also add a laser beam:

window.onload = async () => {
  canvas = document.getElementById("canvas");
  ctx = canvas.getContext("2d");
  heroImg = await loadTexture("assets/player.png");
  enemyImg = await loadTexture("assets/enemyShip.png");
  laserImg = await loadTexture("assets/laserRed.png");

  initGame();
  const gameLoopId = setInterval(() => {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.fillStyle = "black";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    drawGameObjects(ctx);
  }, 100);
};

Understanding the game setup:

Waits for the page to load completely before starting
Gets the canvas element and its 2D rendering context
Loads all image assets asynchronously using await
Starts the game loop running at 100ms intervals (10 FPS)
Clears and redraws the entire screen each frame

Add code to move enemies at a certain interval

Refactor the createEnemies() function to create the enemies and push them into the new gameObjects class:

function createEnemies() {
  const MONSTER_TOTAL = 5;
  const MONSTER_WIDTH = MONSTER_TOTAL * 98;
  const START_X = (canvas.width - MONSTER_WIDTH) / 2;
  const STOP_X = START_X + MONSTER_WIDTH;

  for (let x = START_X; x < STOP_X; x += 98) {
    for (let y = 0; y < 50 * 5; y += 50) {
      const enemy = new Enemy(x, y);
      enemy.img = enemyImg;
      gameObjects.push(enemy);
    }
  }
}

What the enemy creation does:

Calculates positions to center enemies on the screen
Creates a grid of enemies using nested loops
Assigns the enemy image to each enemy object
Adds each enemy to the global game objects array

and add a createHero() function to do a similar process for the hero.

function createHero() {
  hero = new Hero(
    canvas.width / 2 - 45,
    canvas.height - canvas.height / 4
  );
  hero.img = heroImg;
  gameObjects.push(hero);
}

What the hero creation does:

Positions the hero at the bottom center of the screen
Assigns the hero image to the hero object
Adds the hero to the game objects array for rendering

and finally, add a drawGameObjects() function to start the drawing:

function drawGameObjects(ctx) {
  gameObjects.forEach(go => go.draw(ctx));
}

Understanding the drawing function:

Iterates through all game objects in the array
Calls the draw() method on each object
Passes the canvas context so objects can render themselves

Your enemies should start advancing on your hero spaceship! } }


and add a `createHero()` function to do a similar process for the hero.

```javascript
function createHero() {
  hero = new Hero(
    canvas.width / 2 - 45,
    canvas.height - canvas.height / 4
  );
  hero.img = heroImg;
  gameObjects.push(hero);
}

and finally, add a drawGameObjects() function to start the drawing:

function drawGameObjects(ctx) {
  gameObjects.forEach(go => go.draw(ctx));
}

Your enemies should start advancing on your hero spaceship!

GitHub Copilot Agent Challenge 🚀

Use the Agent mode to complete the following challenge:

Description: Enhance the space game by implementing boundary detection and smooth movement controls. This challenge will help you practice game object manipulation, event handling, and canvas rendering techniques.

Prompt: Create a boundary detection system that prevents the hero spaceship from moving outside the canvas boundaries. Additionally, implement a smooth movement system where holding down arrow keys creates continuous movement instead of single-step movement. Make sure the hero stops immediately when keys are released and add visual feedback when the hero hits a boundary (like a brief color change or glow effect).

🚀 Challenge

As you can see, your code can turn into 'spaghetti code' when you start adding functions and variables and classes. How can you better organize your code so that it is more readable?

Your challenge: Sketch out a system to organize your code, even if it still resides in one file. Consider these approaches:

Grouping related functions together with clear comment headers
Separating concerns - keep game logic separate from rendering
Using consistent naming conventions for variables and functions
Creating modules or namespaces to organize different aspects of your game
Adding documentation that explains the purpose of each major section

Reflection questions:

Which parts of your code are hardest to understand when you come back to them?
How could you organize your code to make it easier for someone else to contribute?
What would happen if you wanted to add new features like power-ups or different enemy types?

Post-Lecture Quiz

Post-lecture quiz

Review & Self Study

While we're writing our game without using frameworks, there are many JavaScript-based canvas frameworks for game development. Take some time to do some reading about these and consider how they might solve some of the organizational challenges you identified.

Explore these concepts:

Game engine architecture and design patterns
Entity-Component-System (ECS) patterns in game development
Performance optimization techniques for canvas-based games
Modern JavaScript features that can improve game code organization

Assignment

Comment your code

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