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Webpage metadata</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Headings_and_paragraphs">Headings and paragraphs in HTML</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Emphasis_and_importance">Emphasis and importance</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Lists">Lists</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_documents">Structuring documents</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Advanced_text_features">Advanced text features</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Creating_links">Creating links</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Marking_up_a_letter">Challenge: Marking up a letter</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_a_page_of_content">Challenge: Structuring a page of content</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_images">HTML images</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_video_and_audio">HTML video and audio</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Mozilla_splash_page">Challenge: Mozilla splash page</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_table_basics">HTML table basics</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Table_accessibility">HTML table accessibility</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Planet_data_table">Challenge: Structuring a planet data table</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_forms">Forms and buttons in HTML</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Debugging_HTML">Debugging HTML</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Styling_basics">CSS styling basics</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/What_is_CSS">What is CSS?</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Getting_started">Getting started with CSS</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Styling_a_bio_page">Challenge: Styling a biography page</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Basic_selectors">Basic CSS selectors</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Attribute_selectors">Attribute selectors</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Pseudo_classes_and_elements">Pseudo-classes and pseudo-elements</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Combinators">Combinators</a></li><li><a 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href="/en-US/docs/Learn_web_development/Core/CSS_layout/Fundamental_Layout_Comprehension">Challenge: Fundamental layout comprehension</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Scripting">Dynamic scripting with JavaScript</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript">What is JavaScript?</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/A_first_splash">A first splash into JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/What_went_wrong">What went wrong? Troubleshooting JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Variables">Storing the information you need — Variables</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Math">Basic math in JavaScript — numbers and operators</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Strings">Handling text — strings in JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Useful_string_methods">Useful string methods</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Arrays">Arrays</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Silly_story_generator">Challenge: Silly story generator</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Conditionals">Making decisions in your code — conditionals</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Loops">Looping code</a></li><li><a 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JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/JSON">Working with JSON</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Debugging_JavaScript">Debugging JavaScript and handling errors</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries">JavaScript frameworks and libraries</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Introduction">Introduction to client-side frameworks</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Main_features">Framework main features</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_getting_started">Getting started with React</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_todo_list_beginning">Beginning our React todo list</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_components">Componentizing our React app</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_events_state">React interactivity: Events and state</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_filtering_conditional_rendering">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_accessibility">Accessibility in React</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_resources">React resources</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Accessibility">Accessibility</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/What_is_accessibility">What is accessibility?</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Tooling">Accessibility tooling and assistive technology</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/HTML">HTML: A good basis for accessibility</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/CSS_and_JavaScript">CSS and JavaScript accessibility best practices</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/WAI-ARIA_basics">WAI-ARIA basics</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Multimedia">Accessible multimedia</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Mobile">Mobile accessibility</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Accessibility_troubleshooting">Challenge: Accessibility troubleshooting</a></li></ol></details></li><li><a href="/en-US/docs/Learn_web_development/Core/Design_for_developers">Design for developers</a></li><li><a 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href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/django_assessment_blog">Assessment: DIY Django mini blog</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs">Express web framework (Node.js)</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Introduction">Express/Node introduction</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/development_environment">Setting up a Node development environment</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Tutorial_local_library_website">Express Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/skeleton_website">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a 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to automated testing</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/Your_own_automation_environment">Setting up your own test automation environment</a></li></ol></details></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Transform_animate">Transform and animate CSS</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Security_privacy">Security and privacy</a></li><li class="section">Further resources</li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Howto">How to solve common problems</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Howto/Solve_HTML_problems">Solve common HTML problems</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Solve_CSS_problems">Solve common CSS problems</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Solve_JavaScript_problems">Solve common JavaScript problems</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Web_mechanics">Web mechanics</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Tools_and_setup">Tools and setup</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Design_and_accessibility">Design and accessibility</a></li></ol></details></li><li><a href="/en-US/docs/Learn_web_development/About">About</a></li><li><a href="/en-US/docs/Learn_web_development/Educators">Resources for educators</a></li><li><a href="/en-US/docs/Learn_web_development/Changelog">Changelog</a></li></ol></div></div><section class="place side"></section></nav></aside><div class="toc-container"><aside class="toc"><nav><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#starting_point">Starting point</a></li><li class="document-toc-item "><a class="document-toc-link" href="#hints_and_tips">Hints and tips</a></li><li class="document-toc-item "><a class="document-toc-link" href="#project_brief">Project brief</a></li><li class="document-toc-item "><a class="document-toc-link" href="#steps_to_complete">Steps to complete</a></li></ul></section></div></nav></aside><section class="place side"></section></div></div><main id="content" class="main-content "><article class="main-page-content" lang="en-US"><header><h1>Challenge: Adding features to our bouncing balls demo</h1></header><div class="section-content"><ul class="prev-next"><li><a class="button secondary" href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice"><span class="button-wrap"> Previous </span></a></li><li><a class="button secondary" href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects"><span class="button-wrap"> Overview: Advanced JavaScript objects</span></a></li></ul> <p>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.</p></div><section aria-labelledby="starting_point"><h2 id="starting_point"><a href="#starting_point">Starting point</a></h2><div class="section-content"><p>To start this challenge, make a local copy of <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/index-finished.html" class="external" target="_blank">index-finished.html</a>, <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/style.css" class="external" target="_blank">style.css</a>, and <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/main-finished.js" class="external" target="_blank">main-finished.js</a> from our last article in a new directory in your local computer.</p> <p>Alternatively, you could use an online editor such as <a href="https://codepen.io/" class="external" target="_blank">CodePen</a>, <a href="https://jsfiddle.net/" class="external" target="_blank">JSFiddle</a>, or <a href="https://glitch.com/" class="external" target="_blank">Glitch</a>. You could paste the HTML, CSS and JavaScript into one of these online editors. If the online editor you are using doesn't have a separate JavaScript panel, feel free to put it inline in a <code><script></code> element inside the HTML page.</p> <div class="notecard note"> <p><strong>Note:</strong> If you get stuck, you can reach out to us in one of our <a href="/en-US/docs/MDN/Community/Communication_channels">communication channels</a>.</p> </div></div></section><section aria-labelledby="hints_and_tips"><h2 id="hints_and_tips"><a href="#hints_and_tips">Hints and tips</a></h2><div class="section-content"><p>A couple of pointers before you get started.</p> <ul> <li>This challenge is quite difficult. Read all of the instructions before you start coding, and take each step slowly and carefully.</li> <li>It might be a good idea to save a separate copy of the demo after you get each stage working, so you can refer back to it if you find yourself in trouble later on.</li> </ul></div></section><section aria-labelledby="project_brief"><h2 id="project_brief"><a href="#project_brief">Project brief</a></h2><div class="section-content"><p>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 <code>Shape()</code> 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.</p> <p>The following screenshot gives you an idea of what the finished program should look like:</p> <p><img src="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features/bouncing-evil-circle.png" alt="Screenshot of the bouncing balls demo page. A white-outlined circle is visible in addition to the colored balls, and the text "Ball count: 23" is visible under the heading." width="800" height="533" loading="lazy"></p> <p>To give you more of an idea, have a look at the <a href="https://mdn.github.io/learning-area/javascript/oojs/assessment/" class="external" target="_blank">finished example</a> (no peeking at the source code!)</p></div></section><section aria-labelledby="steps_to_complete"><h2 id="steps_to_complete"><a href="#steps_to_complete">Steps to complete</a></h2><div class="section-content"><p>The following sections describe what you need to do.</p></div></section><section aria-labelledby="create_a_shape_class"><h3 id="create_a_shape_class"><a href="#create_a_shape_class">Create a Shape class</a></h3><div class="section-content"><p>First of all, create a new <code>Shape</code> class. This has only a constructor. The <code>Shape</code> constructor should define the <code>x</code>, <code>y</code>, <code>velX</code>, and <code>velY</code> properties in the same way as the <code>Ball()</code> constructor did originally, but not the <code>color</code> and <code>size</code> properties.</p> <p>The <code>Ball</code> class should be made to derive from <code>Shape</code> using <code>extends</code>. The constructor for <code>Ball</code> should:</p> <ul> <li>take the same arguments as before: <code>x</code>, <code>y</code>, <code>velX</code>, <code>velY</code>, <code>size</code>, and <code>color</code></li> <li>call the <code>Shape</code> constructor using <code>super()</code>, passing in the <code>x</code>, <code>y</code>, <code>velX</code>, and <code>velY</code> arguments</li> <li>initialize its own <code>color</code> and <code>size</code> properties from the parameters it is given.</li> </ul> <div class="notecard note"> <p><strong>Note:</strong> Make sure to create the <code>Shape</code> class above the existing <code>Ball</code> class, otherwise you'll get some error like: "Uncaught ReferenceError: Cannot access 'Shape' before initialization"</p> </div> <p>The <code>Ball</code> constructor should define a new property called <code>exists</code>, 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 (<code>true</code>/<code>false</code>), initialized to <code>true</code> in the constructor.</p> <p>The <code>collisionDetect()</code> method of the <code>Ball</code> class needs a small update. A ball needs to be considered for collision detection only if the <code>exists</code> property is <code>true</code>. So, replace the existing <code>collisionDetect()</code> code with the following code:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>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(); } } } } </code></pre></div> <p>As discussed above, the only addition is to check if the ball exists — by using <code>ball.exists</code> in the <code>if</code> conditional.</p> <p>The ball <code>draw()</code> and <code>update()</code> method definitions should be able to stay exactly the same as they were before.</p> <p>At this point, try reloading the code — it should work just the same as it did before, with our redesigned objects.</p></div></section><section aria-labelledby="defining_evilcircle"><h3 id="defining_evilcircle"><a href="#defining_evilcircle">Defining EvilCircle</a></h3><div class="section-content"><p>Now it's time to meet the bad guy — the <code>EvilCircle()</code>! Our game is only going to involve one evil circle, but we are still going to define it using a constructor that inherits from <code>Shape()</code>, 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.</p> <p>Create a definition for an <code>EvilCircle</code> class. It should inherit from <code>Shape</code> using <code>extends</code>.</p> <h4 id="evilcircle_constructor">EvilCircle constructor</h4> <p>The constructor for <code>EvilCircle</code> should:</p> <ul> <li>be passed just <code>x</code>, <code>y</code> arguments</li> <li>pass the <code>x</code>, <code>y</code> arguments up to the <code>Shape</code> superclass along with values for <code>velX</code> and <code>velY</code> hardcoded to 20. You should do this with code like <code>super(x, y, 20, 20);</code></li> <li>set <code>color</code> to <code>white</code> and <code>size</code> to <code>10</code>.</li> </ul> <p>Finally, the constructor should set up the code enabling the user to move the evil circle around the screen:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>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; } }); </code></pre></div> <p>This adds a <code>keydown</code> event listener to the <code>window</code> object so that when a key is pressed, the event object's <a href="/en-US/docs/Web/API/KeyboardEvent/key"><code>key</code></a> 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.</p></div></section><section aria-labelledby="defining_methods_for_evilcircle"><h3 id="defining_methods_for_evilcircle"><a href="#defining_methods_for_evilcircle">Defining methods for EvilCircle</a></h3><div class="section-content"><p>The <code>EvilCircle</code> class should have three methods, as described below.</p> <h4 id="draw">draw()</h4> <p>This method has the same purpose as the <code>draw()</code> method for <code>Ball</code>: it draws the object instance on the canvas. The <code>draw()</code> method for <code>EvilCircle</code> will work in a very similar way, so you can start by copying the <code>draw()</code> method for <code>Ball</code>. You should then make the following changes:</p> <ul> <li>We want the evil circle to not be filled in, but rather just have an outer line (stroke). You can achieve this by updating <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/fillStyle"><code>fillStyle</code></a> and <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/fill"><code>fill()</code></a> to <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/strokeStyle"><code>strokeStyle</code></a> and <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/stroke"><code>stroke()</code></a> respectively.</li> <li>We also want to make the stroke a bit thicker, so you can see the evil circle a bit more easily. This can be achieved by setting a value for <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/lineWidth"><code>lineWidth</code></a> somewhere after the <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/beginPath"><code>beginPath()</code></a> call (3 will do).</li> </ul> <h4 id="checkbounds">checkBounds()</h4> <p>This method will do the same thing as the first part of the <code>update()</code> method for <code>Ball</code> — 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 <code>update()</code> method for <code>Ball</code>, but there are a few changes you should make:</p> <ul> <li>Get rid of the last two lines — we don't want to automatically update the evil circle's position on every frame, because we will be moving it in some other way, as you'll see below.</li> <li>Inside the <code>if ()</code> statements, if the tests return true we don't want to update <code>velX</code>/<code>velY</code>; we want to instead change the value of <code>x</code>/<code>y</code> so the evil circle is bounced back onto the screen slightly. Adding or subtracting (as appropriate) the evil circle's <code>size</code> property would make sense.</li> </ul> <h4 id="collisiondetect">collisionDetect()</h4> <p>This method will act in a very similar way to the <code>collisionDetect()</code> method for <code>Ball</code> method, so you can use a copy of that as the basis of this new method. But there are a couple of differences:</p> <ul> <li>In the outer <code>if</code> 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.</li> <li>In the inner <code>if</code> 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?).</li> </ul></div></section><section aria-labelledby="bringing_the_evil_circle_into_the_program"><h3 id="bringing_the_evil_circle_into_the_program"><a href="#bringing_the_evil_circle_into_the_program">Bringing the evil circle into the program</a></h3><div class="section-content"><p>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 <code>loop()</code> function.</p> <ul> <li>First of all, create a new evil circle object instance (specifying the necessary parameters). You only need to do this once, not on every iteration of the loop.</li> <li>At the point where you loop through every ball and call the <code>draw()</code>, <code>update()</code>, and <code>collisionDetect()</code> functions for each one, make it so that these functions are only called if the current ball exists.</li> <li>Call the evil circle instance's <code>draw()</code>, <code>checkBounds()</code>, and <code>collisionDetect()</code> methods on every iteration of the loop.</li> </ul></div></section><section aria-labelledby="implementing_the_score_counter"><h3 id="implementing_the_score_counter"><a href="#implementing_the_score_counter">Implementing the score counter</a></h3><div class="section-content"><p>To implement the score counter, follow the following steps:</p> <ol> <li> <p>In your HTML file, add a <a href="/en-US/docs/Web/HTML/Element/p"><code><p></code></a> element just below the <a href="/en-US/docs/Web/HTML/Element/Heading_Elements">h1</a> element containing the text "Ball count: ".</p> </li> <li> <p>In your CSS file, add the following rule at the bottom:</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>p { position: absolute; margin: 0; top: 35px; right: 5px; color: #aaa; } </code></pre></div> </li> <li> <p>In your JavaScript, make the following updates:</p> <ul> <li>Create a variable that stores a reference to the paragraph.</li> <li>Keep a count of the number of balls on screen in some way.</li> <li>Increment the count and display the updated number of balls each time a ball is added to the scene.</li> <li>Decrement the count and display the updated number of balls each time the evil circle eats a ball (causes it not to exist).</li> </ul> </li> </ol> <ul class="prev-next"><li><a class="button secondary" href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice"><span class="button-wrap"> Previous 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content are ©1998–2025 by individual mozilla.org contributors. Content available under <a href="/en-US/docs/MDN/Writing_guidelines/Attrib_copyright_license">a Creative Commons license</a>.</p></div></div></footer></div><script type="application/json" id="hydration">{"url":"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features","doc":{"body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<ul class=\"prev-next\"><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice\"><span class=\"button-wrap\"> Previous </span></a></li><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects\"><span class=\"button-wrap\"> Overview: Advanced JavaScript objects</span></a></li></ul>\n<p>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.</p>"}},{"type":"prose","value":{"id":"starting_point","title":"Starting point","isH3":false,"content":"<p>To start this challenge, make a local copy of <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/index-finished.html\" class=\"external\" target=\"_blank\">index-finished.html</a>, <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/style.css\" class=\"external\" target=\"_blank\">style.css</a>, and <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/main-finished.js\" class=\"external\" target=\"_blank\">main-finished.js</a> from our last article in a new directory in your local computer.</p>\n<p>Alternatively, you could use an online editor such as <a href=\"https://codepen.io/\" class=\"external\" target=\"_blank\">CodePen</a>, <a href=\"https://jsfiddle.net/\" class=\"external\" target=\"_blank\">JSFiddle</a>, or <a href=\"https://glitch.com/\" class=\"external\" target=\"_blank\">Glitch</a>. You could paste the HTML, CSS and JavaScript into one of these online editors. If the online editor you are using doesn't have a separate JavaScript panel, feel free to put it inline in a <code><script></code> element inside the HTML page.</p>\n<div class=\"notecard note\">\n<p><strong>Note:</strong>\nIf you get stuck, you can reach out to us in one of our <a href=\"/en-US/docs/MDN/Community/Communication_channels\">communication channels</a>.</p>\n</div>"}},{"type":"prose","value":{"id":"hints_and_tips","title":"Hints and tips","isH3":false,"content":"<p>A couple of pointers before you get started.</p>\n<ul>\n<li>This challenge is quite difficult. Read all of the instructions before you start coding, and take each step slowly and carefully.</li>\n<li>It might be a good idea to save a separate copy of the demo after you get each stage working, so you can refer back to it if you find yourself in trouble later on.</li>\n</ul>"}},{"type":"prose","value":{"id":"project_brief","title":"Project brief","isH3":false,"content":"<p>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 <code>Shape()</code> 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.</p>\n<p>The following screenshot gives you an idea of what the finished program should look like:</p>\n<p><img src=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features/bouncing-evil-circle.png\" alt=\"Screenshot of the bouncing balls demo page. A white-outlined circle is visible in addition to the colored balls, and the text "Ball count: 23" is visible under the heading.\" width=\"800\" height=\"533\" loading=\"lazy\"></p>\n<p>To give you more of an idea, have a look at the <a href=\"https://mdn.github.io/learning-area/javascript/oojs/assessment/\" class=\"external\" target=\"_blank\">finished example</a> (no peeking at the source code!)</p>"}},{"type":"prose","value":{"id":"steps_to_complete","title":"Steps to complete","isH3":false,"content":"<p>The following sections describe what you need to do.</p>"}},{"type":"prose","value":{"id":"create_a_shape_class","title":"Create a Shape class","isH3":true,"content":"<p>First of all, create a new <code>Shape</code> class. This has only a constructor. The <code>Shape</code> constructor should define the <code>x</code>, <code>y</code>, <code>velX</code>, and <code>velY</code> properties in the same way as the <code>Ball()</code> constructor did originally, but not the <code>color</code> and <code>size</code> properties.</p>\n<p>The <code>Ball</code> class should be made to derive from <code>Shape</code> using <code>extends</code>. The constructor for <code>Ball</code> should:</p>\n<ul>\n<li>take the same arguments as before: <code>x</code>, <code>y</code>, <code>velX</code>, <code>velY</code>, <code>size</code>, and <code>color</code></li>\n<li>call the <code>Shape</code> constructor using <code>super()</code>, passing in the <code>x</code>, <code>y</code>, <code>velX</code>, and <code>velY</code> arguments</li>\n<li>initialize its own <code>color</code> and <code>size</code> properties from the parameters it is given.</li>\n</ul>\n<div class=\"notecard note\">\n<p><strong>Note:</strong>\nMake sure to create the <code>Shape</code> class above the existing <code>Ball</code> class, otherwise you'll get some error like: \"Uncaught ReferenceError: Cannot access 'Shape' before initialization\"</p>\n</div>\n<p>The <code>Ball</code> constructor should define a new property called <code>exists</code>, 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 (<code>true</code>/<code>false</code>), initialized to <code>true</code> in the constructor.</p>\n<p>The <code>collisionDetect()</code> method of the <code>Ball</code> class needs a small update. A ball needs to be considered for collision detection only if the <code>exists</code> property is <code>true</code>. So, replace the existing <code>collisionDetect()</code> code with the following code:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>collisionDetect() {\n for (const ball of balls) {\n if (!(this === ball) && ball.exists) {\n const dx = this.x - ball.x;\n const dy = this.y - ball.y;\n const distance = Math.sqrt(dx * dx + dy * dy);\n\n if (distance < this.size + ball.size) {\n ball.color = this.color = randomRGB();\n }\n }\n }\n}\n</code></pre></div>\n<p>As discussed above, the only addition is to check if the ball exists — by using <code>ball.exists</code> in the <code>if</code> conditional.</p>\n<p>The ball <code>draw()</code> and <code>update()</code> method definitions should be able to stay exactly the same as they were before.</p>\n<p>At this point, try reloading the code — it should work just the same as it did before, with our redesigned objects.</p>"}},{"type":"prose","value":{"id":"defining_evilcircle","title":"Defining EvilCircle","isH3":true,"content":"<p>Now it's time to meet the bad guy — the <code>EvilCircle()</code>! Our game is only going to involve one evil circle, but we are still going to define it using a constructor that inherits from <code>Shape()</code>, 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.</p>\n<p>Create a definition for an <code>EvilCircle</code> class. It should inherit from <code>Shape</code> using <code>extends</code>.</p>\n<h4 id=\"evilcircle_constructor\">EvilCircle constructor</h4>\n<p>The constructor for <code>EvilCircle</code> should:</p>\n<ul>\n<li>be passed just <code>x</code>, <code>y</code> arguments</li>\n<li>pass the <code>x</code>, <code>y</code> arguments up to the <code>Shape</code> superclass along with values for <code>velX</code> and <code>velY</code> hardcoded to 20. You should do this with code like <code>super(x, y, 20, 20);</code></li>\n<li>set <code>color</code> to <code>white</code> and <code>size</code> to <code>10</code>.</li>\n</ul>\n<p>Finally, the constructor should set up the code enabling the user to move the evil circle around the screen:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>window.addEventListener(\"keydown\", (e) => {\n switch (e.key) {\n case \"a\":\n this.x -= this.velX;\n break;\n case \"d\":\n this.x += this.velX;\n break;\n case \"w\":\n this.y -= this.velY;\n break;\n case \"s\":\n this.y += this.velY;\n break;\n }\n});\n</code></pre></div>\n<p>This adds a <code>keydown</code> event listener to the <code>window</code> object so that when a key is pressed, the event object's <a href=\"/en-US/docs/Web/API/KeyboardEvent/key\"><code>key</code></a> 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.</p>"}},{"type":"prose","value":{"id":"defining_methods_for_evilcircle","title":"Defining methods for EvilCircle","isH3":true,"content":"<p>The <code>EvilCircle</code> class should have three methods, as described below.</p>\n<h4 id=\"draw\">draw()</h4>\n<p>This method has the same purpose as the <code>draw()</code> method for <code>Ball</code>: it draws the object instance on the canvas. The <code>draw()</code> method for <code>EvilCircle</code> will work in a very similar way, so you can start by copying the <code>draw()</code> method for <code>Ball</code>. You should then make the following changes:</p>\n<ul>\n<li>We want the evil circle to not be filled in, but rather just have an outer line (stroke). You can achieve this by updating <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/fillStyle\"><code>fillStyle</code></a> and <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/fill\"><code>fill()</code></a> to <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/strokeStyle\"><code>strokeStyle</code></a> and <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/stroke\"><code>stroke()</code></a> respectively.</li>\n<li>We also want to make the stroke a bit thicker, so you can see the evil circle a bit more easily. This can be achieved by setting a value for <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/lineWidth\"><code>lineWidth</code></a> somewhere after the <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/beginPath\"><code>beginPath()</code></a> call (3 will do).</li>\n</ul>\n<h4 id=\"checkbounds\">checkBounds()</h4>\n<p>This method will do the same thing as the first part of the <code>update()</code> method for <code>Ball</code> — 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 <code>update()</code> method for <code>Ball</code>, but there are a few changes you should make:</p>\n<ul>\n<li>Get rid of the last two lines — we don't want to automatically update the evil circle's position on every frame, because we will be moving it in some other way, as you'll see below.</li>\n<li>Inside the <code>if ()</code> statements, if the tests return true we don't want to update <code>velX</code>/<code>velY</code>; we want to instead change the value of <code>x</code>/<code>y</code> so the evil circle is bounced back onto the screen slightly. Adding or subtracting (as appropriate) the evil circle's <code>size</code> property would make sense.</li>\n</ul>\n<h4 id=\"collisiondetect\">collisionDetect()</h4>\n<p>This method will act in a very similar way to the <code>collisionDetect()</code> method for <code>Ball</code> method, so you can use a copy of that as the basis of this new method. But there are a couple of differences:</p>\n<ul>\n<li>In the outer <code>if</code> 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.</li>\n<li>In the inner <code>if</code> 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?).</li>\n</ul>"}},{"type":"prose","value":{"id":"bringing_the_evil_circle_into_the_program","title":"Bringing the evil circle into the program","isH3":true,"content":"<p>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 <code>loop()</code> function.</p>\n<ul>\n<li>First of all, create a new evil circle object instance (specifying the necessary parameters). You only need to do this once, not on every iteration of the loop.</li>\n<li>At the point where you loop through every ball and call the <code>draw()</code>, <code>update()</code>, and <code>collisionDetect()</code> functions for each one, make it so that these functions are only called if the current ball exists.</li>\n<li>Call the evil circle instance's <code>draw()</code>, <code>checkBounds()</code>, and <code>collisionDetect()</code> methods on every iteration of the loop.</li>\n</ul>"}},{"type":"prose","value":{"id":"implementing_the_score_counter","title":"Implementing the score counter","isH3":true,"content":"<p>To implement the score counter, follow the following steps:</p>\n<ol>\n<li>\n<p>In your HTML file, add a <a href=\"/en-US/docs/Web/HTML/Element/p\"><code><p></code></a> element just below the <a href=\"/en-US/docs/Web/HTML/Element/Heading_Elements\">h1</a> element containing the text \"Ball count: \".</p>\n</li>\n<li>\n<p>In your CSS file, add the following rule at the bottom:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>p {\n position: absolute;\n margin: 0;\n top: 35px;\n right: 5px;\n color: #aaa;\n}\n</code></pre></div>\n</li>\n<li>\n<p>In your JavaScript, make the following updates:</p>\n<ul>\n<li>Create a variable that stores a reference to the paragraph.</li>\n<li>Keep a count of the number of balls on screen in some way.</li>\n<li>Increment the count and display the updated number of balls each time a ball is added to the scene.</li>\n<li>Decrement the count and display the updated number of balls each time the evil circle eats a ball (causes it not to exist).</li>\n</ul>\n</li>\n</ol>\n<ul class=\"prev-next\"><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice\"><span class=\"button-wrap\"> Previous </span></a></li><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects\"><span class=\"button-wrap\"> Overview: Advanced JavaScript objects</span></a></li></ul>"}}],"isActive":true,"isMarkdown":true,"isTranslated":false,"locale":"en-US","mdn_url":"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features","modified":"2024-12-19T15:37:45.000Z","native":"English (US)","noIndexing":false,"other_translations":[{"locale":"de","title":"Herausforderung: Hinzufügen von Funktionen zu unserem hüpfenden Ball-Demo","native":"Deutsch"},{"locale":"es","title":"Añadiendo características a nuestra demo de bouncing balls","native":"Español"},{"locale":"fr","title":"Ajouter des fonctionnalités à notre exercice des balles rebondissantes","native":"Français"},{"locale":"ja","title":"練習: バウンスボールのデモに機能を追加する","native":"日本語"},{"locale":"pt-BR","title":"Adicionando recursos à nossa demonstração de bolas pulantes","native":"Português (do Brasil)"},{"locale":"ru","title":"Добавление функций в нашу демонстрацию отбойных шаров","native":"Русский"},{"locale":"zh-CN","title":"为“弹球”示例添加新功能","native":"中文 (简体)"},{"locale":"zh-TW","title":"為彈跳彩球添增其他功能","native":"正體中文 (繁體)"}],"pageTitle":"Challenge: Adding features to our bouncing balls demo - Learn web development | MDN","parents":[{"uri":"/en-US/docs/Learn_web_development","title":"Learn"},{"uri":"/en-US/docs/Learn_web_development/Extensions","title":"Extension modules"},{"uri":"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects","title":"Advanced JavaScript objects"},{"uri":"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features","title":"Challenge: Adding features to our bouncing balls demo"}],"popularity":null,"short_title":"Challenge: Adding features to our bouncing balls demo","sidebarHTML":"<ol><li class=\"section\"><a href=\"/en-US/docs/Learn_web_development/Getting_started\">Getting started modules</a></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup\">Environment setup</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Installing_software\">Installing basic software</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Browsing_the_web\">Browsing the web</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Code_editors\">Code editors</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Dealing_with_files\">Dealing with files</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Command_line\">Command line crash course</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website\">Your first website</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/What_will_your_website_look_like\">What will your website look like?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Creating_the_content\">HTML: Creating the content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Styling_the_content\">CSS: Styling the content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Adding_interactivity\">JavaScript: Adding interactivity</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Publishing_your_website\">Publishing your website</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards\">Web standards</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards/How_the_web_works\">How the web works</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards/The_web_standards_model\">The web standards model</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards/How_browsers_load_websites\">How browsers load websites</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills\">Soft skills</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Research_and_learning\">Research and learning</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Collaboration_and_teamwork\">Collaboration and teamwork</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Workflows_and_processes\">Workflows and processes</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Job_interviews\">Succeeding in job interviews</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn_web_development/Core\">Core modules</a></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content\">Structuring content with HTML</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Basic_HTML_syntax\">Basic HTML syntax</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Webpage_metadata\">What's in the head? Webpage metadata</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Headings_and_paragraphs\">Headings and paragraphs in HTML</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Emphasis_and_importance\">Emphasis and importance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Lists\">Lists</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_documents\">Structuring documents</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Advanced_text_features\">Advanced text features</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Creating_links\">Creating links</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Marking_up_a_letter\">Challenge: Marking up a letter</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_a_page_of_content\">Challenge: Structuring a page of content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_images\">HTML images</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_video_and_audio\">HTML video and audio</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Mozilla_splash_page\">Challenge: Mozilla splash page</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_table_basics\">HTML table basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Table_accessibility\">HTML table accessibility</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Planet_data_table\">Challenge: Structuring a planet data table</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_forms\">Forms and buttons in HTML</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Debugging_HTML\">Debugging HTML</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics\">CSS styling basics</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/What_is_CSS\">What is CSS?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Getting_started\">Getting started with CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Styling_a_bio_page\">Challenge: Styling a biography page</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Basic_selectors\">Basic CSS selectors</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Attribute_selectors\">Attribute selectors</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Pseudo_classes_and_elements\">Pseudo-classes and pseudo-elements</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Combinators\">Combinators</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Box_model\">The box model</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Handling_conflicts\">Handling conflicts</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Values_and_units\">CSS values and units</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Sizing\">Sizing items in CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Backgrounds_and_borders\">Backgrounds and borders</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Overflow\">Overflowing content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Images_media_forms\">Images, media, and form elements</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Tables\">Styling tables</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Debugging_CSS\">Debugging CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Fundamental_CSS_comprehension\">Challenge: Fundamental CSS comprehension</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Fancy_letterheaded_paper\">Challenge: Creating fancy letterheaded paper</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Cool-looking_box\">Challenge: A cool-looking box</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling\">CSS text styling</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Fundamentals\">Fundamental text and font styling</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Styling_lists\">Styling lists</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Styling_links\">Styling links</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Web_fonts\">Web fonts</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Typesetting_a_homepage\">Challenge: Typesetting a community school homepage</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout\">CSS layout</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Introduction\">Introduction to CSS layout</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Floats\">Floats</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Positioning\">Positioning</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Flexbox\">Flexbox</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Grids\">CSS grid layout</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Responsive_Design\">Responsive design</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Media_queries\">Media query fundamentals</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Fundamental_Layout_Comprehension\">Challenge: Fundamental layout comprehension</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Scripting\">Dynamic scripting with JavaScript</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript\">What is JavaScript?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/A_first_splash\">A first splash into JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/What_went_wrong\">What went wrong? Troubleshooting JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Variables\">Storing the information you need — Variables</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Math\">Basic math in JavaScript — numbers and operators</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Strings\">Handling text — strings in JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Useful_string_methods\">Useful string methods</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Arrays\">Arrays</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Silly_story_generator\">Challenge: Silly story generator</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Conditionals\">Making decisions in your code — conditionals</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Loops\">Looping code</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Functions\">Functions — reusable blocks of code</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Build_your_own_function\">Build your own function</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Return_values\">Function return values</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Events\">Introduction to events</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Event_bubbling\">Event bubbling</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Image_gallery\">Challenge: Image gallery</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Object_basics\">JavaScript object basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/DOM_scripting\">DOM scripting introduction</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Network_requests\">Making network requests with JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/JSON\">Working with JSON</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Debugging_JavaScript\">Debugging JavaScript and handling errors</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries\">JavaScript frameworks and libraries</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Introduction\">Introduction to client-side frameworks</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Main_features\">Framework main features</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_getting_started\">Getting started with React</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_todo_list_beginning\">Beginning our React todo list</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_components\">Componentizing our React app</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_events_state\">React interactivity: Events and state</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_filtering_conditional_rendering\">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_accessibility\">Accessibility in React</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_resources\">React resources</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility\">Accessibility</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/What_is_accessibility\">What is accessibility?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Tooling\">Accessibility tooling and assistive technology</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/HTML\">HTML: A good basis for accessibility</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/CSS_and_JavaScript\">CSS and JavaScript accessibility best practices</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/WAI-ARIA_basics\">WAI-ARIA basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Multimedia\">Accessible multimedia</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Mobile\">Mobile accessibility</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Accessibility_troubleshooting\">Challenge: Accessibility troubleshooting</a></li></ol></details></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Design_for_developers\">Design for developers</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Version_control\">Version control</a></li><li class=\"section\"><a href=\"/en-US/docs/Learn_web_development/Extensions\">Extension modules</a></li><li class=\"toggle\"><details open=\"\"><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects\">Advanced JavaScript objects</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_prototypes\">Object prototypes</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object-oriented_programming\">Object-oriented programming</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Classes_in_JavaScript\">Classes in JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice\">Object building practice</a></li><li><em><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features\" aria-current=\"page\">Challenge: Adding features to our bouncing balls demo</a></em></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs\">Client-side web APIs</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Introduction\">Introduction to web APIs</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Video_and_audio_APIs\">Video and Audio APIs</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Drawing_graphics\">Drawing graphics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Client-side_storage\">Client-side storage</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Third_party_APIs\">Third-party APIs</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS\">Asynchronous JavaScript</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Introducing\">Introducing asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Promises\">How to use promises</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Implementing_a_promise-based_API\">How to implement a promise-based API</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Introducing_workers\">Introducing workers</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Sequencing_animations\">Challenge: Sequencing animations</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms\">Web forms</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Your_first_form\">Your first form</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/How_to_structure_a_web_form\">How to structure a web form</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Basic_native_form_controls\">Basic native form controls</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/HTML5_input_types\">The HTML5 input types</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Other_form_controls\">Other form controls</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Styling_web_forms\">Styling web forms</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Advanced_form_styling\">Advanced form styling</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/UI_pseudo-classes\">UI pseudo-classes</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Form_validation\">Client-side form validation</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Sending_and_retrieving_form_data\">Sending form data</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools\">Understanding client-side tools</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Overview\">Client-side tooling overview</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Package_management\">Package management basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Introducing_complete_toolchain\">Introducing a complete toolchain</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Deployment\">Deploying our app</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side\">Server-side websites</a></summary><ol><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps\">Server-side first steps</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Introduction\">Introduction to the server side</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Client-Server_overview\">Client-Server Overview</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Web_frameworks\">Server-side web frameworks</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Website_security\">Website security</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django\">Django web framework (Python)</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Introduction\">Django introduction</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/development_environment\">Setting up a Django development environment</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Tutorial_local_library_website\">Django Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/skeleton_website\">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Models\">Django Tutorial Part 3: Using models</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Admin_site\">Django Tutorial Part 4: Django admin site</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Home_page\">Django Tutorial Part 5: Creating our home page</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Generic_views\">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Sessions\">Django Tutorial Part 7: Sessions framework</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Authentication\">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Forms\">Django Tutorial Part 9: Working with forms</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Testing\">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Deployment\">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/web_application_security\">Django web application security</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/django_assessment_blog\">Assessment: DIY Django mini blog</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs\">Express web framework (Node.js)</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Introduction\">Express/Node introduction</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/development_environment\">Setting up a Node development environment</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Tutorial_local_library_website\">Express Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/skeleton_website\">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/mongoose\">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/routes\">Express Tutorial Part 4: Routes and controllers</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Displaying_data\">Express Tutorial Part 5: Displaying library data</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/forms\">Express Tutorial Part 6: Working with forms</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/deployment\">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance\">Web performance</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/why_web_performance\">The \"why\" of web performance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/What_is_web_performance\">What is web performance?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Perceived_performance\">Perceived performance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Measuring_performance\">Measuring performance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia\">Multimedia: Images</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/video\">Multimedia: video</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/JavaScript\">JavaScript performance optimization</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/HTML\">HTML performance optimization</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/CSS\">CSS performance optimization</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/business_case_for_performance\">The business case for web performance</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing\">Testing</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Introduction\">Introduction to cross-browser testing</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Testing_strategies\">Strategies for carrying out testing</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/HTML_and_CSS\">Handling common HTML and CSS problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Feature_detection\">Implementing feature detection</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Automated_testing\">Introduction to automated testing</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Your_own_automation_environment\">Setting up your own test automation environment</a></li></ol></details></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Transform_animate\">Transform and animate CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Security_privacy\">Security and privacy</a></li><li class=\"section\">Further resources</li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Howto\">How to solve common problems</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Howto/Solve_HTML_problems\">Solve common HTML problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Solve_CSS_problems\">Solve common CSS problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Solve_JavaScript_problems\">Solve common JavaScript problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Web_mechanics\">Web mechanics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Tools_and_setup\">Tools and setup</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Design_and_accessibility\">Design and accessibility</a></li></ol></details></li><li><a href=\"/en-US/docs/Learn_web_development/About\">About</a></li><li><a href=\"/en-US/docs/Learn_web_development/Educators\">Resources for educators</a></li><li><a href=\"/en-US/docs/Learn_web_development/Changelog\">Changelog</a></li></ol>","source":{"folder":"en-us/learn_web_development/extensions/advanced_javascript_objects/adding_bouncing_balls_features","github_url":"https://github.com/mdn/content/blob/main/files/en-us/learn_web_development/extensions/advanced_javascript_objects/adding_bouncing_balls_features/index.md","last_commit_url":"https://github.com/mdn/content/commit/5b20f5f4265f988f80f513db0e4b35c7e0cd70dc","filename":"index.md"},"summary":"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.","title":"Challenge: Adding features to our bouncing balls demo","toc":[{"text":"Starting point","id":"starting_point"},{"text":"Hints and tips","id":"hints_and_tips"},{"text":"Project brief","id":"project_brief"},{"text":"Steps to complete","id":"steps_to_complete"}],"pageType":"learn-module-assessment"}}</script></body></html>