In this challenge, you are expected to use the bouncing balls demo from the previous article as a starting point, and add some new and interesting features to it.
To start this challenge, make a local copy of index-finished.html, style.css, and main-finished.js from our last article in a new directory in your local computer.
Alternatively, you could use an online editor such as CodePen or JSFiddle. If the online editor you are using doesn't have a separate JavaScript panel, you can put it inline in a <script> element inside the HTML page.
Note: If you get stuck, you can reach out to us in one of our communication channels.
A couple of pointers before you get started.
Our bouncy ball demo is fun, but now we want to make it a little bit more interactive by adding a user-controlled evil circle, which will eat the balls if it catches them. We also want to test your object-building skills by creating a generic Shape() object that our balls and evil circle can inherit from. Finally, we want to add a score counter to track the number of balls left to capture.
The following screenshot gives you an idea of what the finished program should look like:
To give you more of an idea, have a look at the finished example (no peeking at the source code!)
The following sections describe what you need to do.
First of all, create a new Shape class. This has only a constructor. The Shape constructor should define the x, y, velX, and velY properties in the same way as the Ball() constructor did originally, but not the color and size properties.
The Ball class should be made to derive from Shape using extends. The constructor for Ball should:
x, y, velX, velY, size, and colorShape constructor using super(), passing in the x, y, velX, and velY argumentscolor and size properties from the parameters it is given.Note: Make sure to create the Shape class above the existing Ball class, otherwise you'll get some error like: "Uncaught ReferenceError: Cannot access 'Shape' before initialization"
The Ball constructor should define a new property called exists, which is used to track whether the balls exist in the program (have not been eaten by the evil circle). This should be a boolean (true/false), initialized to true in the constructor.
The collisionDetect() method of the Ball class needs a small update. A ball needs to be considered for collision detection only if the exists property is true. So, replace the existing collisionDetect() code with the following code:
class Ball {
// …
collisionDetect() {
for (const ball of balls) {
if (!(this === ball) && ball.exists) {
const dx = this.x - ball.x;
const dy = this.y - ball.y;
const distance = Math.sqrt(dx * dx + dy * dy);
if (distance < this.size + ball.size) {
ball.color = this.color = randomRGB();
}
}
}
}
// …
}As discussed above, the only addition is to check if the ball exists — by using ball.exists in the if conditional.
The ball draw() and update() method definitions should be able to stay exactly the same as they were before.
At this point, try reloading the code — it should work just the same as it did before, with our redesigned objects.
Now it's time to meet the bad guy — the EvilCircle()! Our game is only going to involve one evil circle, but we are still going to define it using a constructor that inherits from Shape(), to give you some practice. You might want to add another circle to the app later on that can be controlled by another player, or have several computer-controlled evil circles. You're probably not going to take over the world with a single evil circle, but it will do for this challenge.
Create a definition for an EvilCircle class. It should inherit from Shape using extends.
The constructor for EvilCircle should:
x, y argumentsx, y arguments up to the Shape superclass along with values for velX and velY hardcoded to 20. You should do this with code like super(x, y, 20, 20);color to white and size to 10.Finally, the constructor should set up the code enabling the user to move the evil circle around the screen:
window.addEventListener("keydown", (e) => {
switch (e.key) {
case "a":
this.x -= this.velX;
break;
case "d":
this.x += this.velX;
break;
case "w":
this.y -= this.velY;
break;
case "s":
this.y += this.velY;
break;
}
});This adds a keydown event listener to the window object so that when a key is pressed, the event object's key property is consulted to see which key is pressed. If it is one of the four specified keys, then the evil circle will move left/right/up/down.
The EvilCircle class should have three methods, as described below.
This method has the same purpose as the draw() method for Ball: it draws the object instance on the canvas. The draw() method for EvilCircle will work in a very similar way, so you can start by copying the draw() method for Ball. You should then make the following changes:
fillStyle and fill() to strokeStyle and stroke() respectively.lineWidth somewhere after the beginPath() call (3 will do).This method will do the same thing as the first part of the update() method for Ball — look to see whether the evil circle is going to go off the edge of the screen, and stop it from doing so. Again, you can mostly just copy the update() method for Ball, but there are a few changes you should make:
if () statements, if the tests return true we don't want to update velX/velY; we want to instead change the value of x/y so the evil circle is bounced back onto the screen slightly. Adding or subtracting (as appropriate) the evil circle's size property would make sense.This method will act in a very similar way to the collisionDetect() method for Ball method, so you can use a copy of that as the basis of this new method. But there are a couple of differences:
if statement, you no longer need to check whether the current ball in the iteration is the same as the ball that is doing the checking — because it is no longer a ball, it is the evil circle! Instead, you need to do a test to see if the ball being checked exists (with which property could you do this with?). If it doesn't exist, it has already been eaten by the evil circle, so there is no need to check it again.if statement, you no longer want to make the objects change color when a collision is detected — instead, you want to set any balls that collide with the evil circle to not exist any more (again, how do you think you'd do that?).Now we've defined the evil circle, we need to actually make it appear in our scene. To do this, you need to make some changes to the loop() function.
draw(), update(), and collisionDetect() functions for each one, make it so that these functions are only called if the current ball exists.draw(), checkBounds(), and collisionDetect() methods on every iteration of the loop.To implement the score counter, follow the following steps:
In your HTML file, add a <p> element just below the h1 element containing the text "Ball count: ".
In your CSS file, add the following rule at the bottom:
p {
position: absolute;
margin: 0;
top: 35px;
right: 5px;
color: #aaaaaa;
}In your JavaScript, make the following updates: