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Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

We're going to create a simple game where a box moves across the screen at an increased speed after each click.

When you are done it should look like this

Our goal for this game is to learn how to bring together HTML, CSS, and JavaScript. We use HTML to define our structure, CSS to define the style of that structure, and JavaScript in order to implement behavior. One of the primary ways we can implement behavior in JavaScript is by making modifications to the HTML and CSS in response to events which we will demonstrate by making this simple game.

Let's get started

For this project you will need to clone the template at and a web server to run it. You can do this via a web server installed on your system, something like mongoose, or Cloud 9 IDE pointed at the public directory. There is also a basic node web server packaged into the application that you can start with

> npm install
> npm start

And then opening your Chrome browser to http://localhost:8080.

Browsers cache files so if your browser has already fetched and it is requested again, it can just get it out of the local cache, rather than having to go online and download it one more time. Since we are going to be changing files, we do not want this behavior! To disable it for development, open up your browser's developer tools (F12 or Ctrl+Shift+<C ), click the gear icon in the upper right, then check Disable cache (while DevTools is open).

Disable cache while DevTools is open

Review the environment

You will be working in the following files

  • index.html - Contains the html for *content and structure of the page. Since the content is largely just the box itself, we will not be doing much here except understanding the structure.
  • style.css - Contains the css for how things look on the page. We will play around with this a bit.
  • bouncing-box.js - Contains the javascript code for how the page behaves. This is largely the file we will be working with.

A note about jQuery

We are going to be using jQuery for this exercise. You can see that we've included it in our web page with the following HTML

<script src="//"></script>

jQuery is a powerful library which makes building web pages easier. It is also tremendously popular. If you are doing web development in 2015, you will likely run into jQuery. That is why we are introducing just a tiny bit of it here.

You can recognize jQuery by its use of a very curious function $() Here is some of the jQuery code we use in this page:

box = $('.box');
boardWidth = $('.board').width();

TODO 1: Create and Style Box

The HTML for our box has already been created for us:

<body class="board">
  <div class="box">?</div>

Find the CSS that styles the box and change the following properties:

  • left
  • top
  • width
  • height

Notice how you can change the appearance of the box using CSS! Now return those to their original values

width: 70px;
height: 70px;
top: 100px;
left: 0px;

TODO 2: Learn how to move the box

You can also change the appearance of the box using JavaScript.

Declare and initialize position and points variables to zero

// TODO 2
var position;
var points;
var speed;

position = 0;
points = 0;
speed = 10;

Now add the following code under TODO 2

box.css('left', position);

Now change the value of the position variable and watch the box move

JavaScript lets us change HTML as well. Add the following JavaScript code under TODO 2


Change the points variable and watch how it changes the box.

Before we move on, lets reset those variables to their starting values

position = 0;  
points = 0;  
speed = 10;

TODO 3: Animating the box

You can create animation on a web page by changing the appearance of an object over time. A traditional animation is made up of individual "frames" of still images. If you flip between these images rapidly and each image is just slightly different than the previous image, the viewer sees the scene as motion. We do the same thing in programming.

The setInterval function allows us to setup a timer, where we call a function every so often. How often, the time between function calls, is called the interval. That interval is expressed in milliseconds, or thousandths of a second.

The following code:

setInterval(update, 50);

Calls our update function ever 50 milliseconds, which is about 20 times per second.

To animate the box, add the following code to the update function

update = function() {
    position = position + speed;
    box.css('left', position);

This changes our position on every call to update and then also moves the box to that position.

TODO 4: Hey box, come back!

Each time we call the update function the position variable gets larger and larger until eventually our box has gone off the screen. The position of our box should never be greater than the width of the board. Add the following code inside the update function

if(position > boardWidth) {
    position = 0;

Your update function should look like this:

update = function() {
    position = position + speed;
    if(position > boardWidth) {
     position = 0;
    box.css('left', position);

TODO 5: Handling events

An event is just a particular thing that has happened. Some examples of events are:

  • A timer going off
  • The user clicking on something
  • the web page has finished loading

JavaScript allows us to change the web page in response to events. The following code calls the handleBoxClick function every time the box is clicked.

box.on('click', handleBoxClick);

Every time the user clicks the box, we want to reset the box to its starting position and make the game harder by increasing the speed of the box. Add the following code to the handleBoxClick function

handleBoxClick = function() {
  speed = speed + 3;
  position = 0;

TODO 6: Keeping Score

We want to keep track of how many times the user has clicked on the box.

Add the following code to the handleBoxClick function

 points = points + 1;

and then add the following code to the update function


What's going on here?

TODO 7: Make It Bounce

So we have the box loop accross the screen, but don't we want it to bounce off of the walls? Well, at least we want to make it look that way.

Before we can make it bounce we have to figure out how to make the box move from right to left. Right now our motion comes from the following line in the update function:

position = position + speed;

Since speed is positive, position keeps getting bigger, so to make the box move the other way we need to subtract the speed instead of adding it. To do this we can add a variable direction that will tell us whether to add or subract the speed. Let's declare and initialize it up top:

var direction;
direction = 1;

Now we modify the update function to be as follows:

position = position + (speed * direction);

When direction is set to 1 then this increases the position by speed, sending the box to the right. But when direction is set to -1, the speed is subracted from the position, sending the box to the left.

Lastly, we need to change the bounds-check in the update function:

if(position > boardWidth) {
    position = 0;

so that instead of looping to position 0 we change the direction to -1. Do this and confirm that the box bounces off the right wall. However, you will need to add another bounds-check (if...) to make the box bounce off the left wall. Do this yourself!

##Hint: At what position value do you want the box to "bounce" off the left wall?

Good Job

You've written your first game! Here are some ways you can try and make your game even more awesome.

Can you move the box up and down?

Can you change the size or color of the box with each click?

Use the background-image CSS property to change your box or the background


Tutorial that investigates tying HTML, CSS, and Javascript together to form a simple game



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