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started</a></li><li class="document-toc-item "><a class="document-toc-link" href="#modeling_a_ball_in_our_program">Modeling a ball in our program</a></li><li class="document-toc-item "><a class="document-toc-link" href="#animating_the_ball">Animating the ball</a></li><li class="document-toc-item "><a class="document-toc-link" href="#adding_collision_detection">Adding collision detection</a></li><li class="document-toc-item "><a class="document-toc-link" href="#summary">Summary</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></div><div class="sidebar-body"><ol><li class="section"><a href="/en-US/docs/Learn/Getting_started_with_the_web">Complete beginners start here!</a></li><li><details><summary>Getting started with the web</summary><ol><li><a href="/en-US/docs/Learn/Getting_started_with_the_web">Getting started with the web</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/Installing_basic_software">Installing basic software</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/What_will_your_website_look_like">What will your website look like?</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/Dealing_with_files">Dealing with files</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/HTML_basics">HTML basics</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/CSS_basics">CSS basics</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/JavaScript_basics">JavaScript basics</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/Publishing_your_website">Publishing your website</a></li><li><a href="/en-US/docs/Learn/Getting_started_with_the_web/How_the_Web_works">How the web works</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/HTML">HTML — Structuring the web</a></li><li><details><summary>Introduction to HTML</summary><ol><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML">Introduction to HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Getting_started">Getting started with HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/The_head_metadata_in_HTML">What's in the head? Metadata in HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/HTML_text_fundamentals">HTML text fundamentals</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks">Creating hyperlinks</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Advanced_text_formatting">Advanced text formatting</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure">Document and website structure</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Debugging_HTML">Debugging HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Marking_up_a_letter">Marking up a letter</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Structuring_a_page_of_content">Structuring a page of content</a></li></ol></details></li><li><details><summary>Multimedia and embedding</summary><ol><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding">Multimedia and embedding</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Images_in_HTML">Images in HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Video_and_audio_content">Video and audio content</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Other_embedding_technologies">From object to iframe — other embedding technologies</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Adding_vector_graphics_to_the_Web">Adding vector graphics to the web</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images">Responsive images</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Mozilla_splash_page">Mozilla splash page</a></li></ol></details></li><li><details><summary>HTML tables</summary><ol><li><a href="/en-US/docs/Learn/HTML/Tables">HTML tables</a></li><li><a href="/en-US/docs/Learn/HTML/Tables/Basics">HTML table basics</a></li><li><a href="/en-US/docs/Learn/HTML/Tables/Advanced">HTML table advanced features and accessibility</a></li><li><a href="/en-US/docs/Learn/HTML/Tables/Structuring_planet_data">Structuring planet data</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/CSS">CSS — Styling the web</a></li><li><details><summary>CSS first steps</summary><ol><li><a href="/en-US/docs/Learn/CSS/First_steps">CSS first steps</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/What_is_CSS">What is CSS?</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/Getting_started">Getting started with CSS</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/How_CSS_is_structured">How CSS is structured</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/How_CSS_works">How CSS works</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/Styling_a_biography_page">Styling a biography page</a></li></ol></details></li><li><details><summary>CSS building blocks</summary><ol><li><a href="/en-US/docs/Learn/CSS/Building_blocks">CSS building blocks</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors">CSS selectors</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Type_Class_and_ID_Selectors">Type, class, and ID selectors</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Attribute_selectors">Attribute selectors</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Pseudo-classes_and_pseudo-elements">Pseudo-classes and pseudo-elements</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Combinators">Combinators</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance">Cascade, specificity, and inheritance</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Cascade_layers">Cascade layers</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/The_box_model">The box model</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Backgrounds_and_borders">Backgrounds and borders</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Handling_different_text_directions">Handling different text directions</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Overflowing_content">Overflowing content</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Values_and_units">CSS values and units</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Sizing_items_in_CSS">Sizing items in CSS</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Images_media_form_elements">Images, media, and form elements</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Styling_tables">Styling tables</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Advanced_styling_effects">Advanced styling effects</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Debugging_CSS">Debugging CSS</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Organizing">Organizing your CSS</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Fundamental_CSS_comprehension">Fundamental CSS comprehension</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Creating_fancy_letterheaded_paper">Creating fancy letterheaded paper</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/A_cool_looking_box">A cool-looking box</a></li></ol></details></li><li><details><summary>Styling text</summary><ol><li><a href="/en-US/docs/Learn/CSS/Styling_text">CSS styling text</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Fundamentals">Fundamental text and font styling</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Styling_lists">Styling lists</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Styling_links">Styling links</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Web_fonts">Web fonts</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Typesetting_a_homepage">Typesetting a community school homepage</a></li></ol></details></li><li><details><summary>CSS layout</summary><ol><li><a href="/en-US/docs/Learn/CSS/CSS_layout">CSS layout</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Introduction">Introduction to CSS layout</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Normal_Flow">Normal Flow</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Flexbox">Flexbox</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Grids">Grids</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Floats">Floats</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Positioning">Positioning</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Multiple-column_Layout">Multiple-column layout</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Responsive_Design">Responsive design</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Media_queries">Beginner's guide to media queries</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Legacy_Layout_Methods">Legacy layout methods</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Supporting_Older_Browsers">Supporting older browsers</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Fundamental_Layout_Comprehension">Fundamental layout comprehension</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/JavaScript">JavaScript — Dynamic client-side scripting</a></li><li><details><summary>JavaScript first steps</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/First_steps">JavaScript first steps</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/What_is_JavaScript">What is JavaScript?</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/A_first_splash">A first splash into JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/What_went_wrong">What went wrong? Troubleshooting JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Variables">Storing the information you need — Variables</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Math">Basic math in JavaScript — numbers and operators</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Strings">Handling text — strings in JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Useful_string_methods">Useful string methods</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Arrays">Arrays</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Silly_story_generator">Silly story generator</a></li></ol></details></li><li><details><summary>JavaScript building blocks</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks">JavaScript building blocks</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/conditionals">Making decisions in your code — conditionals</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Looping_code">Looping code</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Functions">Functions — reusable blocks of code</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Build_your_own_function">Build your own function</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Return_values">Function return values</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Events">Introduction to events</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Event_bubbling">Event bubbling</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Image_gallery">Image gallery</a></li></ol></details></li><li><details open=""><summary>Introducing JavaScript objects</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Objects">Introducing JavaScript objects</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Basics">JavaScript object basics</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Object_prototypes">Object prototypes</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Object-oriented_programming">Object-oriented programming</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Classes_in_JavaScript">Classes in JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/JSON">Working with JSON</a></li><li><em><a href="/en-US/docs/Learn/JavaScript/Objects/Object_building_practice" aria-current="page">Object building practice</a></em></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features">Adding features to our bouncing balls demo</a></li></ol></details></li><li><details><summary>Asynchronous JavaScript</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous">Asynchronous JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Introducing">Introducing asynchronous JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Promises">How to use promises</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Implementing_a_promise-based_API">How to implement a promise-based API</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Introducing_workers">Introducing workers</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Sequencing_animations">Sequencing animations</a></li></ol></details></li><li><details><summary>Client-side web APIs</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs">Client-side web APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Introduction">Introduction to web APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Manipulating_documents">Manipulating documents</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data">Fetching data from the server</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Third_party_APIs">Third-party APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics">Drawing graphics</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Video_and_audio_APIs">Video and Audio APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Client-side_storage">Client-side storage</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Forms">Web forms — Working with user data</a></li><li><details><summary>Web form building blocks</summary><ol><li><a href="/en-US/docs/Learn/Forms">Web form building blocks</a></li><li><a href="/en-US/docs/Learn/Forms/Your_first_form">Your first form</a></li><li><a href="/en-US/docs/Learn/Forms/How_to_structure_a_web_form">How to structure a web form</a></li><li><a href="/en-US/docs/Learn/Forms/Basic_native_form_controls">Basic native form controls</a></li><li><a href="/en-US/docs/Learn/Forms/HTML5_input_types">The HTML5 input types</a></li><li><a href="/en-US/docs/Learn/Forms/Other_form_controls">Other form controls</a></li><li><a href="/en-US/docs/Learn/Forms/Styling_web_forms">Styling web forms</a></li><li><a href="/en-US/docs/Learn/Forms/Advanced_form_styling">Advanced form styling</a></li><li><a href="/en-US/docs/Learn/Forms/UI_pseudo-classes">UI pseudo-classes</a></li><li><a href="/en-US/docs/Learn/Forms/Form_validation">Client-side form validation</a></li><li><a href="/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data">Sending form data</a></li></ol></details></li><li><details><summary>Advanced web form techniques</summary><ol><li><a href="/en-US/docs/Learn/Forms/How_to_build_custom_form_controls">How to build custom form controls</a></li><li><a href="/en-US/docs/Learn/Forms/Sending_forms_through_JavaScript">Sending forms through JavaScript</a></li><li><a href="/en-US/docs/Learn/Forms/Property_compatibility_table_for_form_controls">CSS property compatibility table for form controls</a></li><li><a href="/en-US/docs/Learn/Forms/HTML_forms_in_legacy_browsers">HTML forms in legacy browsers</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Accessibility">Accessibility — Make the web usable by everyone</a></li><li><details><summary>Accessibility guides</summary><ol><li><a href="/en-US/docs/Learn/Accessibility">Accessibility</a></li><li><a href="/en-US/docs/Learn/Accessibility/What_is_accessibility">What is accessibility?</a></li><li><a href="/en-US/docs/Learn/Accessibility/HTML">HTML: A good basis for accessibility</a></li><li><a href="/en-US/docs/Learn/Accessibility/CSS_and_JavaScript">CSS and JavaScript accessibility best practices</a></li><li><a href="/en-US/docs/Learn/Accessibility/WAI-ARIA_basics">WAI-ARIA basics</a></li><li><a href="/en-US/docs/Learn/Accessibility/Multimedia">Accessible multimedia</a></li><li><a href="/en-US/docs/Learn/Accessibility/Mobile">Mobile accessibility</a></li><li><a href="/en-US/docs/Learn/Accessibility/Accessibility_troubleshooting">Assessment: Accessibility troubleshooting</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Performance">Performance — Making websites fast and responsive</a></li><li><details><summary>Performance guides</summary><ol><li><a href="/en-US/docs/Learn/Performance">Web performance</a></li><li><a href="/en-US/docs/Learn/Performance/why_web_performance">The "why" of web performance</a></li><li><a href="/en-US/docs/Learn/Performance/What_is_web_performance">What is web performance?</a></li><li><a href="/en-US/docs/Learn/Performance/Perceived_performance">Perceived performance</a></li><li><a href="/en-US/docs/Learn/Performance/Measuring_performance">Measuring performance</a></li><li><a href="/en-US/docs/Learn/Performance/Multimedia">Multimedia: Images</a></li><li><a href="/en-US/docs/Learn/Performance/video">Multimedia: video</a></li><li><a href="/en-US/docs/Learn/Performance/JavaScript">JavaScript performance optimization</a></li><li><a href="/en-US/docs/Learn/Performance/HTML">HTML performance optimization</a></li><li><a href="/en-US/docs/Learn/Performance/CSS">CSS performance optimization</a></li><li><a href="/en-US/docs/Learn/Performance/business_case_for_performance">The business case for web performance</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/MathML">MathML — Writing mathematics with MathML</a></li><li><details><summary>MathML first steps</summary><ol><li><a href="/en-US/docs/Learn/MathML/First_steps">MathML first steps</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Getting_started">Getting started with MathML</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Text_containers">MathML Text Containers</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Fractions_and_roots">MathML fractions and roots</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Scripts">MathML scripted elements</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Tables">MathML tables</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Three_famous_mathematical_formulas">Three famous mathematical formulas</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/../Games">Games — Developing games for the web</a></li><li><details><summary>Guides and tutorials</summary><ol><li><a href="/en-US/docs/Games/Introduction">Introduction to game development for the Web</a></li><li><a href="/en-US/docs/Games/Techniques">Techniques for game development</a></li><li><a href="/en-US/docs/Games/Tutorials">Tutorials</a></li><li><a href="/en-US/docs/Games/Publishing_games">Publishing games</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Tools_and_testing">Tools and testing</a></li><li><details><summary>Client-side web development tools</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools">Understanding client-side web development tools</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Overview">Client-side tooling overview</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Command_line">Command line crash course</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Package_management">Package management basics</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Introducing_complete_toolchain">Introducing a complete toolchain</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Deployment">Deploying our app</a></li></ol></details></li><li><details><summary>Introduction to client-side frameworks</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Introduction">Introduction to client-side frameworks</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Main_features">Framework main features</a></li></ol></details></li><li><details><summary>React</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_getting_started">Getting started with React</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_todo_list_beginning">Beginning our React todo list</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_components">Componentizing our React app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_events_state">React interactivity: Events and state</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_filtering_conditional_rendering">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_accessibility">Accessibility in React</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_resources">React resources</a></li></ol></details></li><li><details><summary>Ember</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_getting_started">Getting started with Ember</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_structure_componentization">Ember app structure and componentization</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_interactivity_events_state">Ember interactivity: Events, classes and state</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_conditional_footer">Ember Interactivity: Footer functionality, conditional rendering</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_routing">Routing in Ember</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_resources">Ember resources and troubleshooting</a></li></ol></details></li><li><details><summary>Vue</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_getting_started">Getting started with Vue</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_first_component">Creating our first Vue component</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_rendering_lists">Rendering a list of Vue components</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_methods_events_models">Adding a new todo form: Vue events, methods, and models</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_styling">Styling Vue components with CSS</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_computed_properties">Using Vue computed properties</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_conditional_rendering">Vue conditional rendering: editing existing todos</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_refs_focus_management">Vue refs and lifecycle methods for focus management</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_resources">Vue resources</a></li></ol></details></li><li><details><summary>Svelte</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_getting_started">Getting started with Svelte</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_Todo_list_beginning">Starting our Svelte to-do list app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_variables_props">Dynamic behavior in Svelte: working with variables and props</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_components">Componentizing our Svelte app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_reactivity_lifecycle_accessibility">Advanced Svelte: Reactivity, lifecycle, accessibility</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_stores">Working with Svelte stores</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_TypeScript">TypeScript support in Svelte</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_deployment_next">Deployment and next steps</a></li></ol></details></li><li><details><summary>Angular</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_getting_started">Getting started with Angular</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_todo_list_beginning">Beginning our Angular todo list app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_styling">Styling our Angular app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_item_component">Creating an item component</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_filtering">Filtering our to-do items</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_building">Building Angular applications and further resources</a></li></ol></details></li><li><details><summary>Git and GitHub</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/GitHub">Git and GitHub</a></li></ol></details></li><li><details><summary>Cross browser testing</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing">Cross browser testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Introduction">Introduction to cross-browser testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Testing_strategies">Strategies for carrying out testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/HTML_and_CSS">Handling common HTML and CSS problems</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/JavaScript">Handling common JavaScript problems</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility">Handling common accessibility problems</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection">Implementing feature detection</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Automated_testing">Introduction to automated testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Your_own_automation_environment">Setting up your own test automation environment</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Server-side">Server-side website programming</a></li><li><details><summary>First steps</summary><ol><li><a href="/en-US/docs/Learn/Server-side/First_steps">Server-side website programming first steps</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Introduction">Introduction to the server side</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Client-Server_overview">Client-Server Overview</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Web_frameworks">Server-side web frameworks</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Website_security">Website security</a></li></ol></details></li><li><details><summary>Django web framework (Python)</summary><ol><li><a href="/en-US/docs/Learn/Server-side/Django">Django Web Framework (Python)</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Introduction">Django introduction</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/development_environment">Setting up a Django development environment</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Tutorial_local_library_website">Django Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/skeleton_website">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Models">Django Tutorial Part 3: Using models</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Admin_site">Django Tutorial Part 4: Django admin site</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Home_page">Django Tutorial Part 5: Creating our home page</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Generic_views">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Sessions">Django Tutorial Part 7: Sessions framework</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Authentication">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Forms">Django Tutorial Part 9: Working with forms</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Testing">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Deployment">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/web_application_security">Django web application security</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/django_assessment_blog">Assessment: DIY Django mini blog</a></li></ol></details></li><li><details><summary>Express Web Framework (Node.js/JavaScript)</summary><ol><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs">Express web framework (Node.js/JavaScript)</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/Introduction">Express/Node introduction</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/development_environment">Setting up a Node development environment</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/Tutorial_local_library_website">Express Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/skeleton_website">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/mongoose">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/routes">Express Tutorial Part 4: Routes and controllers</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/Displaying_data">Express Tutorial Part 5: Displaying library data</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/forms">Express Tutorial Part 6: Working with forms</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/deployment">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Common_questions">Further resources</a></li><li><details><summary>Common questions</summary><ol><li><a href="/en-US/docs/Learn/Common_questions">Common questions</a></li><li><a href="/en-US/docs/Learn/HTML/Howto">Use HTML to solve common problems</a></li><li><a href="/en-US/docs/Learn/CSS/Howto">Use CSS to solve common problems</a></li><li><a href="/en-US/docs/Learn/JavaScript/Howto">Solve common problems in your JavaScript code</a></li><li><a href="/en-US/docs/Learn/Common_questions/Web_mechanics">Web mechanics</a></li><li><a href="/en-US/docs/Learn/Common_questions/Tools_and_setup">Tools and setup</a></li><li><a href="/en-US/docs/Learn/Common_questions/Design_and_accessibility">Design and accessibility</a></li></ol></details></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="#lets_bounce_some_balls">Let's bounce some balls</a></li><li class="document-toc-item "><a class="document-toc-link" href="#getting_started">Getting started</a></li><li class="document-toc-item "><a class="document-toc-link" href="#modeling_a_ball_in_our_program">Modeling a ball in our program</a></li><li class="document-toc-item "><a class="document-toc-link" href="#animating_the_ball">Animating the ball</a></li><li class="document-toc-item "><a class="document-toc-link" href="#adding_collision_detection">Adding collision detection</a></li><li class="document-toc-item "><a class="document-toc-link" href="#summary">Summary</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</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>Object building practice</h1></header><div class="section-content"><ul class="prev-next"> <li><a class="button secondary" href="/en-US/docs/Learn/JavaScript/Objects/JSON"><span class="button-wrap"> Previous </span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/JavaScript/Objects"><span class="button-wrap"> Overview: Introducing JavaScript objects</span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features"><span class="button-wrap"> Next </span></a></li> </ul> <p>In previous articles we looked at all the essential JavaScript object theory and syntax details, giving you a solid base to start from. In this article we dive into a practical exercise, giving you some more practice in building custom JavaScript objects, with a fun and colorful result.</p> <figure class="table-container"><table> <tbody> <tr> <th scope="row">Prerequisites:</th> <td> A basic understanding of HTML and CSS, familiarity with JavaScript basics (see <a href="/en-US/docs/Learn/JavaScript/First_steps">First steps</a> and <a href="/en-US/docs/Learn/JavaScript/Building_blocks">Building blocks</a>) and OOJS basics (see <a href="/en-US/docs/Learn/JavaScript/Objects/Basics">Introduction to objects</a>). </td> </tr> <tr> <th scope="row">Objective:</th> <td> To get some practice with using objects and object-oriented techniques in a real-world context. </td> </tr> </tbody> </table></figure></div><section aria-labelledby="lets_bounce_some_balls"><h2 id="lets_bounce_some_balls"><a href="#lets_bounce_some_balls">Let's bounce some balls</a></h2><div class="section-content"><p>In this article we will write a classic "bouncing balls" demo, to show you how useful objects can be in JavaScript. Our little balls will bounce around on the screen, and change color when they touch each other. The finished example will look a little something like this:</p> <p> <img src="/en-US/docs/Learn/JavaScript/Objects/Object_building_practice/bouncing-balls.png" alt="Screenshot of a webpage titled "Bouncing balls". 23 balls of various pastel colors and sizes are visible across a black screen with long trails behind them indicating motion." width="800" height="614" loading="lazy"> </p> <p>This example will make use of the <a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics">Canvas API</a> for drawing the balls to the screen, and the <a href="/en-US/docs/Web/API/Window/requestAnimationFrame"><code>requestAnimationFrame</code></a> API for animating the whole display — you don't need to have any previous knowledge of these APIs, and we hope that by the time you've finished this article you'll be interested in exploring them more. Along the way, we'll make use of some nifty objects, and show you a couple of nice techniques like bouncing balls off walls, and checking whether they have hit each other (otherwise known as <em>collision detection</em>).</p></div></section><section aria-labelledby="getting_started"><h2 id="getting_started"><a href="#getting_started">Getting started</a></h2><div class="section-content"><p>To begin with, make local copies of our <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/index.html" class="external" target="_blank"><code>index.html</code></a>, <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/style.css" class="external" target="_blank"><code>style.css</code></a>, and <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/main.js" class="external" target="_blank"><code>main.js</code></a> files. These contain the following, respectively:</p> <ol> <li>A very simple HTML document featuring an <a href="/en-US/docs/Web/HTML/Element/Heading_Elements">h1</a> element, a <a href="/en-US/docs/Web/HTML/Element/canvas"><code><canvas></code></a> element to draw our balls on, and elements to apply our CSS and JavaScript to our HTML.</li> <li>Some very simple styles, which mainly serve to style and position the <code><h1></code>, and get rid of any scrollbars or margin around the edge of the page (so that it looks nice and neat).</li> <li>Some JavaScript that serves to set up the <code><canvas></code> element and provide a general function that we're going to use.</li> </ol> <p>The first part of the script looks like so:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>const canvas = document.querySelector("canvas"); const ctx = canvas.getContext("2d"); const width = (canvas.width = window.innerWidth); const height = (canvas.height = window.innerHeight); </code></pre></div> <p>This script gets a reference to the <code><canvas></code> element, then calls the <a href="/en-US/docs/Web/API/HTMLCanvasElement/getContext"><code>getContext()</code></a> method on it to give us a context on which we can start to draw. The resulting constant (<code>ctx</code>) is the object that directly represents the drawing area of the canvas and allows us to draw 2D shapes on it.</p> <p>Next, we set constants called <code>width</code> and <code>height</code>, and the width and height of the canvas element (represented by the <code>canvas.width</code> and <code>canvas.height</code> properties) to equal the width and height of the browser viewport (the area which the webpage appears on — this can be gotten from the <a href="/en-US/docs/Web/API/Window/innerWidth"><code>Window.innerWidth</code></a> and <a href="/en-US/docs/Web/API/Window/innerHeight"><code>Window.innerHeight</code></a> properties).</p> <p>Note that we are chaining multiple assignments together, to get the variables all set quicker — this is perfectly OK.</p> <p>Then we have two helper functions:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function random(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } function randomRGB() { return `rgb(${random(0, 255)} ${random(0, 255)} ${random(0, 255)})`; } </code></pre></div> <p>The <code>random()</code> function takes two numbers as arguments, and returns a random number in the range between the two. The <code>randomRGB()</code> function generates a random color represented as an <a href="/en-US/docs/Web/CSS/color_value/rgb"><code>rgb()</code></a> string.</p></div></section><section aria-labelledby="modeling_a_ball_in_our_program"><h2 id="modeling_a_ball_in_our_program"><a href="#modeling_a_ball_in_our_program">Modeling a ball in our program</a></h2><div class="section-content"><p>Our program will feature lots of balls bouncing around the screen. Since these balls will all behave in the same way, it makes sense to represent them with an object. Let's start by adding the following class definition to the bottom of our code.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>class Ball { constructor(x, y, velX, velY, color, size) { this.x = x; this.y = y; this.velX = velX; this.velY = velY; this.color = color; this.size = size; } } </code></pre></div> <p>So far this class only contains a constructor, in which we can initialize the properties each ball needs in order to function in our program:</p> <ul> <li><code>x</code> and <code>y</code> coordinates — the horizontal and vertical coordinates where the ball starts on the screen. This can range between 0 (top left hand corner) to the width and height of the browser viewport (bottom right-hand corner).</li> <li>horizontal and vertical velocity (<code>velX</code> and <code>velY</code>) — each ball is given a horizontal and vertical velocity; in real terms these values are regularly added to the <code>x</code>/<code>y</code> coordinate values when we animate the balls, to move them by this much on each frame.</li> <li><code>color</code> — each ball gets a color.</li> <li><code>size</code> — each ball gets a size — this is its radius, in pixels.</li> </ul> <p>This handles the properties, but what about the methods? We want to get our balls to actually do something in our program.</p></div></section><section aria-labelledby="drawing_the_ball"><h3 id="drawing_the_ball"><a href="#drawing_the_ball">Drawing the ball</a></h3><div class="section-content"><p>First add the following <code>draw()</code> method to the <code>Ball</code> class:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>draw() { ctx.beginPath(); ctx.fillStyle = this.color; ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI); ctx.fill(); } </code></pre></div> <p>Using this function, we can tell the ball to draw itself onto the screen, by calling a series of members of the 2D canvas context we defined earlier (<code>ctx</code>). The context is like the paper, and now we want to command our pen to draw something on it:</p> <ul> <li> <p>First, we use <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/beginPath"><code>beginPath()</code></a> to state that we want to draw a shape on the paper.</p> </li> <li> <p>Next, we use <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/fillStyle"><code>fillStyle</code></a> to define what color we want the shape to be — we set it to our ball's <code>color</code> property.</p> </li> <li> <p>Next, we use the <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/arc"><code>arc()</code></a> method to trace an arc shape on the paper. Its parameters are:</p> <ul> <li>The <code>x</code> and <code>y</code> position of the arc's center — we are specifying the ball's <code>x</code> and <code>y</code> properties.</li> <li>The radius of the arc — in this case, the ball's <code>size</code> property.</li> <li>The last two parameters specify the start and end number of degrees around the circle that the arc is drawn between. Here we specify 0 degrees, and <code>2 * PI</code>, which is the equivalent of 360 degrees in radians (annoyingly, you have to specify this in radians). That gives us a complete circle. If you had specified only <code>1 * PI</code>, you'd get a semi-circle (180 degrees).</li> </ul> </li> <li> <p>Last of all, we use the <a href="/en-US/docs/Web/API/CanvasRenderingContext2D/fill"><code>fill()</code></a> method, which basically states "finish drawing the path we started with <code>beginPath()</code>, and fill the area it takes up with the color we specified earlier in <code>fillStyle</code>."</p> </li> </ul> <p>You can start testing your object out already.</p> <ol> <li> <p>Save the code so far, and load the HTML file in a browser.</p> </li> <li> <p>Open the browser's JavaScript console, and then refresh the page so that the canvas size changes to the smaller visible viewport that remains when the console opens.</p> </li> <li> <p>Type in the following to create a new ball instance:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>const testBall = new Ball(50, 100, 4, 4, "blue", 10); </code></pre></div> </li> <li> <p>Try calling its members:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>testBall.x; testBall.size; testBall.color; testBall.draw(); </code></pre></div> </li> <li> <p>When you enter the last line, you should see the ball draw itself somewhere on the canvas.</p> </li> </ol></div></section><section aria-labelledby="updating_the_balls_data"><h3 id="updating_the_balls_data"><a href="#updating_the_balls_data">Updating the ball's data</a></h3><div class="section-content"><p>We can draw the ball in position, but to actually move the ball, we need an update function of some kind. Add the following code inside the class definition for <code>Ball</code>:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>update() { if ((this.x + this.size) >= width) { this.velX = -(this.velX); } if ((this.x - this.size) <= 0) { this.velX = -(this.velX); } if ((this.y + this.size) >= height) { this.velY = -(this.velY); } if ((this.y - this.size) <= 0) { this.velY = -(this.velY); } this.x += this.velX; this.y += this.velY; } </code></pre></div> <p>The first four parts of the function check whether the ball has reached the edge of the canvas. If it has, we reverse the polarity of the relevant velocity to make the ball travel in the opposite direction. So for example, if the ball was traveling upwards (negative <code>velY</code>), then the vertical velocity is changed so that it starts to travel downwards instead (positive <code>velY</code>).</p> <p>In the four cases, we are checking to see:</p> <ul> <li>if the <code>x</code> coordinate is greater than the width of the canvas (the ball is going off the right edge).</li> <li>if the <code>x</code> coordinate is smaller than 0 (the ball is going off the left edge).</li> <li>if the <code>y</code> coordinate is greater than the height of the canvas (the ball is going off the bottom edge).</li> <li>if the <code>y</code> coordinate is smaller than 0 (the ball is going off the top edge).</li> </ul> <p>In each case, we include the <code>size</code> of the ball in the calculation because the <code>x</code>/<code>y</code> coordinates are in the center of the ball, but we want the edge of the ball to bounce off the perimeter — we don't want the ball to go halfway off the screen before it starts to bounce back.</p> <p>The last two lines add the <code>velX</code> value to the <code>x</code> coordinate, and the <code>velY</code> value to the <code>y</code> coordinate — the ball is in effect moved each time this method is called.</p> <p>This will do for now; let's get on with some animation!</p></div></section><section aria-labelledby="animating_the_ball"><h2 id="animating_the_ball"><a href="#animating_the_ball">Animating the ball</a></h2><div class="section-content"><p>Now let's make this fun. We are now going to start adding balls to the canvas, and animating them.</p> <p>First, we need to create somewhere to store all our balls and then populate it. The following will do this job — add it to the bottom of your code now:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>const balls = []; while (balls.length < 25) { const size = random(10, 20); const ball = new Ball( // ball position always drawn at least one ball width // away from the edge of the canvas, to avoid drawing errors random(0 + size, width - size), random(0 + size, height - size), random(-7, 7), random(-7, 7), randomRGB(), size, ); balls.push(ball); } </code></pre></div> <p>The <code>while</code> loop creates a new instance of our <code>Ball()</code> using random values generated with our <code>random()</code> and <code>randomRGB()</code> functions, then <code>push()</code>es it onto the end of our balls array, but only while the number of balls in the array is less than 25. So when we have 25 balls in the array, no more balls will be pushed. You can try varying the number in <code>balls.length < 25</code> to get more or fewer balls in the array. Depending on how much processing power your computer/browser has, specifying several thousand balls might slow down the animation rather a lot!</p> <p>Next, add the following to the bottom of your code:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function loop() { ctx.fillStyle = "rgb(0 0 0 / 25%)"; ctx.fillRect(0, 0, width, height); for (const ball of balls) { ball.draw(); ball.update(); } requestAnimationFrame(loop); } </code></pre></div> <p>All programs that animate things generally involve an animation loop, which serves to update the information in the program and then render the resulting view on each frame of the animation; this is the basis for most games and other such programs. Our <code>loop()</code> function does the following:</p> <ul> <li>Sets the canvas fill color to semi-transparent black, then draws a rectangle of the color across the whole width and height of the canvas, using <code>fillRect()</code> (the four parameters provide a start coordinate, and a width and height for the rectangle drawn). This serves to cover up the previous frame's drawing before the next one is drawn. If you don't do this, you'll just see long snakes worming their way around the canvas instead of balls moving! The color of the fill is set to semi-transparent, <code>rgb(0 0 0 / 25%)</code>, to allow the previous few frames to shine through slightly, producing the little trails behind the balls as they move. If you changed 0.25 to 1, you won't see them at all any more. Try varying this number to see the effect it has.</li> <li>Loops through all the balls in the <code>balls</code> array, and runs each ball's <code>draw()</code> and <code>update()</code> function to draw each one on the screen, then do the necessary updates to position and velocity in time for the next frame.</li> <li>Runs the function again using the <code>requestAnimationFrame()</code> method — when this method is repeatedly run and passed the same function name, it runs that function a set number of times per second to create a smooth animation. This is generally done recursively — which means that the function is calling itself every time it runs, so it runs over and over again.</li> </ul> <p>Finally, add the following line to the bottom of your code — we need to call the function once to get the animation started.</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>loop(); </code></pre></div> <p>That's it for the basics — try saving and refreshing to test your bouncing balls out!</p></div></section><section aria-labelledby="adding_collision_detection"><h2 id="adding_collision_detection"><a href="#adding_collision_detection">Adding collision detection</a></h2><div class="section-content"><p>Now for a bit of fun, let's add some collision detection to our program, so our balls know when they have hit another ball.</p> <p>First, add the following method definition to your <code>Ball</code> class.</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) { 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>This method is a little complex, so don't worry if you don't understand exactly how it works for now. An explanation follows:</p> <ul> <li>For each ball, we need to check every other ball to see if it has collided with the current ball. To do this, we start another <code>for...of</code> loop to loop through all the balls in the <code>balls[]</code> array.</li> <li>Immediately inside the for loop, we use an <code>if</code> statement to check whether the current ball being looped through is the same ball as the one we are currently checking. We don't want to check whether a ball has collided with itself! To do this, we check whether the current ball (i.e., the ball whose collisionDetect method is being invoked) is the same as the loop ball (i.e., the ball that is being referred to by the current iteration of the for loop in the collisionDetect method). We then use <code>!</code> to negate the check, so that the code inside the <code>if</code> statement only runs if they are <strong>not</strong> the same.</li> <li>We then use a common algorithm to check the collision of two circles. We are basically checking whether any of the two circle's areas overlap. This is explained further in <a href="/en-US/docs/Games/Techniques/2D_collision_detection">2D collision detection</a>.</li> <li>If a collision is detected, the code inside the inner <code>if</code> statement is run. In this case, we only set the <code>color</code> property of both the circles to a new random color. We could have done something far more complex, like get the balls to bounce off each other realistically, but that would have been far more complex to implement. For such physics simulations, developers tend to use a games or physics libraries such as <a href="https://wellcaffeinated.net/PhysicsJS/" class="external" target="_blank">PhysicsJS</a>, <a href="https://brm.io/matter-js/" class="external" target="_blank">matter.js</a>, <a href="https://phaser.io/" class="external" target="_blank">Phaser</a>, etc.</li> </ul> <p>You also need to call this method in each frame of the animation. Update your <code>loop()</code> function to call <code>ball.collisionDetect()</code> after <code>ball.update()</code>:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function loop() { ctx.fillStyle = "rgb(0 0 0 / 25%)"; ctx.fillRect(0, 0, width, height); for (const ball of balls) { ball.draw(); ball.update(); ball.collisionDetect(); } requestAnimationFrame(loop); } </code></pre></div> <p>Save and refresh the demo again, and you'll see your balls change color when they collide!</p> <div class="notecard note"> <p><strong>Note:</strong> If you have trouble getting this example to work, try comparing your JavaScript code against our <a href="https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/main-finished.js" class="external" target="_blank">finished version</a> (also see it <a href="https://mdn.github.io/learning-area/javascript/oojs/bouncing-balls/index-finished.html" class="external" target="_blank">running live</a>).</p> </div></div></section><section aria-labelledby="summary"><h2 id="summary"><a href="#summary">Summary</a></h2><div class="section-content"><p>We hope you had fun writing your own real-world random bouncing balls example, using various object and object-oriented techniques from throughout the module! This should have given you some useful practice in using objects, and good real-world context.</p> <p>That's it for object articles — all that remains now is for you to test your skills in the object assessment.</p></div></section><section aria-labelledby="see_also"><h2 id="see_also"><a href="#see_also">See also</a></h2><div class="section-content"><ul> <li><a href="/en-US/docs/Web/API/Canvas_API/Tutorial">Canvas tutorial</a> — a beginner's guide to using 2D canvas.</li> <li><a href="/en-US/docs/Web/API/Window/requestAnimationFrame">requestAnimationFrame()</a></li> <li><a href="/en-US/docs/Games/Techniques/2D_collision_detection">2D collision detection</a></li> <li><a href="/en-US/docs/Games/Techniques/3D_collision_detection">3D collision detection</a></li> <li><a href="/en-US/docs/Games/Tutorials/2D_Breakout_game_pure_JavaScript">2D breakout game using pure JavaScript</a> — a great beginner's tutorial showing how to build a 2D game.</li> <li><a href="/en-US/docs/Games/Tutorials/2D_breakout_game_Phaser">2D breakout game using Phaser</a> — explains the basics of building a 2D game using a JavaScript game library.</li> </ul><ul class="prev-next"> <li><a class="button secondary" href="/en-US/docs/Learn/JavaScript/Objects/JSON"><span class="button-wrap"> Previous </span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/JavaScript/Objects"><span class="button-wrap"> Overview: Introducing JavaScript objects</span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features"><span class="button-wrap"> Next </span></a></li> </ul></div></section></article><aside class="article-footer"><div class="article-footer-inner"><div class="svg-container"><svg xmlns="http://www.w3.org/2000/svg" width="162" height="162" viewBox="0 0 162 162" fill="none" role="none"><mask id="b" fill="#fff"><path d="M97.203 47.04c8.113-7.886 18.004-13.871 28.906-17.492a78 78 0 0 1 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Metadata in HTML</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/HTML_text_fundamentals\">HTML text fundamentals</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks\">Creating hyperlinks</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Advanced_text_formatting\">Advanced text formatting</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure\">Document and website structure</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Debugging_HTML\">Debugging HTML</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Marking_up_a_letter\">Marking up a letter</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Structuring_a_page_of_content\">Structuring a page of content</a></li></ol></details></li><li><details><summary>Multimedia and embedding</summary><ol><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding\">Multimedia and embedding</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Images_in_HTML\">Images in HTML</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Video_and_audio_content\">Video and audio content</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Other_embedding_technologies\">From object to iframe — other embedding technologies</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Adding_vector_graphics_to_the_Web\">Adding vector graphics to the web</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images\">Responsive images</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Mozilla_splash_page\">Mozilla splash page</a></li></ol></details></li><li><details><summary>HTML tables</summary><ol><li><a href=\"/en-US/docs/Learn/HTML/Tables\">HTML tables</a></li><li><a href=\"/en-US/docs/Learn/HTML/Tables/Basics\">HTML table basics</a></li><li><a href=\"/en-US/docs/Learn/HTML/Tables/Advanced\">HTML table advanced features and accessibility</a></li><li><a href=\"/en-US/docs/Learn/HTML/Tables/Structuring_planet_data\">Structuring planet data</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/CSS\">CSS — Styling the web</a></li><li><details><summary>CSS first steps</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/First_steps\">CSS first steps</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/What_is_CSS\">What is CSS?</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/Getting_started\">Getting started with CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/How_CSS_is_structured\">How CSS is structured</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/How_CSS_works\">How CSS works</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/Styling_a_biography_page\">Styling a biography page</a></li></ol></details></li><li><details><summary>CSS building blocks</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks\">CSS building blocks</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors\">CSS selectors</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Type_Class_and_ID_Selectors\">Type, class, and ID selectors</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Attribute_selectors\">Attribute selectors</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Pseudo-classes_and_pseudo-elements\">Pseudo-classes and pseudo-elements</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Combinators\">Combinators</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance\">Cascade, specificity, and inheritance</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Cascade_layers\">Cascade layers</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/The_box_model\">The box model</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Backgrounds_and_borders\">Backgrounds and borders</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Handling_different_text_directions\">Handling different text directions</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Overflowing_content\">Overflowing content</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Values_and_units\">CSS values and units</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Sizing_items_in_CSS\">Sizing items in CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Images_media_form_elements\">Images, media, and form elements</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Styling_tables\">Styling tables</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Advanced_styling_effects\">Advanced styling effects</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Debugging_CSS\">Debugging CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Organizing\">Organizing your CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Fundamental_CSS_comprehension\">Fundamental CSS comprehension</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Creating_fancy_letterheaded_paper\">Creating fancy letterheaded paper</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/A_cool_looking_box\">A cool-looking box</a></li></ol></details></li><li><details><summary>Styling text</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/Styling_text\">CSS styling text</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Fundamentals\">Fundamental text and font styling</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Styling_lists\">Styling lists</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Styling_links\">Styling links</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Web_fonts\">Web fonts</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Typesetting_a_homepage\">Typesetting a community school homepage</a></li></ol></details></li><li><details><summary>CSS layout</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout\">CSS layout</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Introduction\">Introduction to CSS layout</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Normal_Flow\">Normal Flow</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Flexbox\">Flexbox</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Grids\">Grids</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Floats\">Floats</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Positioning\">Positioning</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Multiple-column_Layout\">Multiple-column layout</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Responsive_Design\">Responsive design</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Media_queries\">Beginner's guide to media queries</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Legacy_Layout_Methods\">Legacy layout methods</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Supporting_Older_Browsers\">Supporting older browsers</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Fundamental_Layout_Comprehension\">Fundamental layout comprehension</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/JavaScript\">JavaScript — Dynamic client-side scripting</a></li><li><details><summary>JavaScript first steps</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps\">JavaScript first steps</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/What_is_JavaScript\">What is JavaScript?</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/A_first_splash\">A first splash into JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/What_went_wrong\">What went wrong? Troubleshooting JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Variables\">Storing the information you need — Variables</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Math\">Basic math in JavaScript — numbers and operators</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Strings\">Handling text — strings in JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Useful_string_methods\">Useful string methods</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Arrays\">Arrays</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Silly_story_generator\">Silly story generator</a></li></ol></details></li><li><details><summary>JavaScript building blocks</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks\">JavaScript building blocks</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/conditionals\">Making decisions in your code — conditionals</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Looping_code\">Looping code</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Functions\">Functions — reusable blocks of code</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Build_your_own_function\">Build your own function</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Return_values\">Function return values</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Events\">Introduction to events</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Event_bubbling\">Event bubbling</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Image_gallery\">Image gallery</a></li></ol></details></li><li><details open=\"\"><summary>Introducing JavaScript objects</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Objects\">Introducing JavaScript objects</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Basics\">JavaScript object basics</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Object_prototypes\">Object prototypes</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Object-oriented_programming\">Object-oriented programming</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Classes_in_JavaScript\">Classes in JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/JSON\">Working with JSON</a></li><li><em><a href=\"/en-US/docs/Learn/JavaScript/Objects/Object_building_practice\" aria-current=\"page\">Object building practice</a></em></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features\">Adding features to our bouncing balls demo</a></li></ol></details></li><li><details><summary>Asynchronous JavaScript</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous\">Asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Introducing\">Introducing asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Promises\">How to use promises</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Implementing_a_promise-based_API\">How to implement a promise-based API</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Introducing_workers\">Introducing workers</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Sequencing_animations\">Sequencing animations</a></li></ol></details></li><li><details><summary>Client-side web APIs</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs\">Client-side web APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Introduction\">Introduction to web APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Manipulating_documents\">Manipulating documents</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data\">Fetching data from the server</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Third_party_APIs\">Third-party APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics\">Drawing graphics</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Video_and_audio_APIs\">Video and Audio APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Client-side_storage\">Client-side storage</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Forms\">Web forms — Working with user data</a></li><li><details><summary>Web form building blocks</summary><ol><li><a href=\"/en-US/docs/Learn/Forms\">Web form building blocks</a></li><li><a href=\"/en-US/docs/Learn/Forms/Your_first_form\">Your first form</a></li><li><a href=\"/en-US/docs/Learn/Forms/How_to_structure_a_web_form\">How to structure a web form</a></li><li><a href=\"/en-US/docs/Learn/Forms/Basic_native_form_controls\">Basic native form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/HTML5_input_types\">The HTML5 input types</a></li><li><a href=\"/en-US/docs/Learn/Forms/Other_form_controls\">Other form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/Styling_web_forms\">Styling web forms</a></li><li><a href=\"/en-US/docs/Learn/Forms/Advanced_form_styling\">Advanced form styling</a></li><li><a href=\"/en-US/docs/Learn/Forms/UI_pseudo-classes\">UI pseudo-classes</a></li><li><a href=\"/en-US/docs/Learn/Forms/Form_validation\">Client-side form validation</a></li><li><a href=\"/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data\">Sending form data</a></li></ol></details></li><li><details><summary>Advanced web form techniques</summary><ol><li><a href=\"/en-US/docs/Learn/Forms/How_to_build_custom_form_controls\">How to build custom form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/Sending_forms_through_JavaScript\">Sending forms through JavaScript</a></li><li><a href=\"/en-US/docs/Learn/Forms/Property_compatibility_table_for_form_controls\">CSS property compatibility table for form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/HTML_forms_in_legacy_browsers\">HTML forms in legacy browsers</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Accessibility\">Accessibility — Make the web usable by everyone</a></li><li><details><summary>Accessibility guides</summary><ol><li><a href=\"/en-US/docs/Learn/Accessibility\">Accessibility</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/What_is_accessibility\">What is accessibility?</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/HTML\">HTML: A good basis for accessibility</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/CSS_and_JavaScript\">CSS and JavaScript accessibility best practices</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/WAI-ARIA_basics\">WAI-ARIA basics</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/Multimedia\">Accessible multimedia</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/Mobile\">Mobile accessibility</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/Accessibility_troubleshooting\">Assessment: Accessibility troubleshooting</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Performance\">Performance — Making websites fast and responsive</a></li><li><details><summary>Performance guides</summary><ol><li><a href=\"/en-US/docs/Learn/Performance\">Web performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/why_web_performance\">The \"why\" of web performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/What_is_web_performance\">What is web performance?</a></li><li><a href=\"/en-US/docs/Learn/Performance/Perceived_performance\">Perceived performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/Measuring_performance\">Measuring performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/Multimedia\">Multimedia: Images</a></li><li><a href=\"/en-US/docs/Learn/Performance/video\">Multimedia: video</a></li><li><a href=\"/en-US/docs/Learn/Performance/JavaScript\">JavaScript performance optimization</a></li><li><a href=\"/en-US/docs/Learn/Performance/HTML\">HTML performance optimization</a></li><li><a href=\"/en-US/docs/Learn/Performance/CSS\">CSS performance optimization</a></li><li><a href=\"/en-US/docs/Learn/Performance/business_case_for_performance\">The business case for web performance</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/MathML\">MathML — Writing mathematics with MathML</a></li><li><details><summary>MathML first steps</summary><ol><li><a href=\"/en-US/docs/Learn/MathML/First_steps\">MathML first steps</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Getting_started\">Getting started with MathML</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Text_containers\">MathML Text Containers</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Fractions_and_roots\">MathML fractions and roots</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Scripts\">MathML scripted elements</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Tables\">MathML tables</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Three_famous_mathematical_formulas\">Three famous mathematical formulas</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/../Games\">Games — Developing games for the web</a></li><li><details><summary>Guides and tutorials</summary><ol><li><a href=\"/en-US/docs/Games/Introduction\">Introduction to game development for the Web</a></li><li><a href=\"/en-US/docs/Games/Techniques\">Techniques for game development</a></li><li><a href=\"/en-US/docs/Games/Tutorials\">Tutorials</a></li><li><a href=\"/en-US/docs/Games/Publishing_games\">Publishing games</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Tools_and_testing\">Tools and testing</a></li><li><details><summary>Client-side web development tools</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools\">Understanding client-side web development tools</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Overview\">Client-side tooling overview</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Command_line\">Command line crash course</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Package_management\">Package management basics</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Introducing_complete_toolchain\">Introducing a complete toolchain</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Deployment\">Deploying our app</a></li></ol></details></li><li><details><summary>Introduction to client-side frameworks</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Introduction\">Introduction to client-side frameworks</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Main_features\">Framework main features</a></li></ol></details></li><li><details><summary>React</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_getting_started\">Getting started with React</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_todo_list_beginning\">Beginning our React todo list</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_components\">Componentizing our React app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_events_state\">React interactivity: Events and state</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_filtering_conditional_rendering\">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_accessibility\">Accessibility in React</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_resources\">React resources</a></li></ol></details></li><li><details><summary>Ember</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_getting_started\">Getting started with Ember</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_structure_componentization\">Ember app structure and componentization</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_interactivity_events_state\">Ember interactivity: Events, classes and state</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_conditional_footer\">Ember Interactivity: Footer functionality, conditional rendering</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_routing\">Routing in Ember</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_resources\">Ember resources and troubleshooting</a></li></ol></details></li><li><details><summary>Vue</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_getting_started\">Getting started with Vue</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_first_component\">Creating our first Vue component</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_rendering_lists\">Rendering a list of Vue components</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_methods_events_models\">Adding a new todo form: Vue events, methods, and models</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_styling\">Styling Vue components with CSS</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_computed_properties\">Using Vue computed properties</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_conditional_rendering\">Vue conditional rendering: editing existing todos</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_refs_focus_management\">Vue refs and lifecycle methods for focus management</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_resources\">Vue resources</a></li></ol></details></li><li><details><summary>Svelte</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_getting_started\">Getting started with Svelte</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_Todo_list_beginning\">Starting our Svelte to-do list app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_variables_props\">Dynamic behavior in Svelte: working with variables and props</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_components\">Componentizing our Svelte app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_reactivity_lifecycle_accessibility\">Advanced Svelte: Reactivity, lifecycle, accessibility</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_stores\">Working with Svelte stores</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_TypeScript\">TypeScript support in Svelte</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_deployment_next\">Deployment and next steps</a></li></ol></details></li><li><details><summary>Angular</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_getting_started\">Getting started with Angular</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_todo_list_beginning\">Beginning our Angular todo list app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_styling\">Styling our Angular app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_item_component\">Creating an item component</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_filtering\">Filtering our to-do items</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_building\">Building Angular applications and further resources</a></li></ol></details></li><li><details><summary>Git and GitHub</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/GitHub\">Git and GitHub</a></li></ol></details></li><li><details><summary>Cross browser testing</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing\">Cross browser testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Introduction\">Introduction to cross-browser testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Testing_strategies\">Strategies for carrying out testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/HTML_and_CSS\">Handling common HTML and CSS problems</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/JavaScript\">Handling common JavaScript problems</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility\">Handling common accessibility problems</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection\">Implementing feature detection</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Automated_testing\">Introduction to automated testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Your_own_automation_environment\">Setting up your own test automation environment</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Server-side\">Server-side website programming</a></li><li><details><summary>First steps</summary><ol><li><a href=\"/en-US/docs/Learn/Server-side/First_steps\">Server-side website programming first steps</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Introduction\">Introduction to the server side</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Client-Server_overview\">Client-Server Overview</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Web_frameworks\">Server-side web frameworks</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Website_security\">Website security</a></li></ol></details></li><li><details><summary>Django web framework (Python)</summary><ol><li><a href=\"/en-US/docs/Learn/Server-side/Django\">Django Web Framework (Python)</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Introduction\">Django introduction</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/development_environment\">Setting up a Django development environment</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Tutorial_local_library_website\">Django Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/skeleton_website\">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Models\">Django Tutorial Part 3: Using models</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Admin_site\">Django Tutorial Part 4: Django admin site</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Home_page\">Django Tutorial Part 5: Creating our home page</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Generic_views\">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Sessions\">Django Tutorial Part 7: Sessions framework</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Authentication\">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Forms\">Django Tutorial Part 9: Working with forms</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Testing\">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Deployment\">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/web_application_security\">Django web application security</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/django_assessment_blog\">Assessment: DIY Django mini blog</a></li></ol></details></li><li><details><summary>Express Web Framework (Node.js/JavaScript)</summary><ol><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs\">Express web framework (Node.js/JavaScript)</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/Introduction\">Express/Node introduction</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/development_environment\">Setting up a Node development environment</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/Tutorial_local_library_website\">Express Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/skeleton_website\">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/mongoose\">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/routes\">Express Tutorial Part 4: Routes and controllers</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/Displaying_data\">Express Tutorial Part 5: Displaying library data</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/forms\">Express Tutorial Part 6: Working with forms</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/deployment\">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Common_questions\">Further resources</a></li><li><details><summary>Common questions</summary><ol><li><a href=\"/en-US/docs/Learn/Common_questions\">Common questions</a></li><li><a href=\"/en-US/docs/Learn/HTML/Howto\">Use HTML to solve common problems</a></li><li><a href=\"/en-US/docs/Learn/CSS/Howto\">Use CSS to solve common problems</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Howto\">Solve common problems in your JavaScript code</a></li><li><a href=\"/en-US/docs/Learn/Common_questions/Web_mechanics\">Web mechanics</a></li><li><a href=\"/en-US/docs/Learn/Common_questions/Tools_and_setup\">Tools and setup</a></li><li><a href=\"/en-US/docs/Learn/Common_questions/Design_and_accessibility\">Design and accessibility</a></li></ol></details></li></ol>","sidebarMacro":"LearnSidebar","body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<ul class=\"prev-next\">\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/JavaScript/Objects/JSON\"><span class=\"button-wrap\"> Previous </span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/JavaScript/Objects\"><span class=\"button-wrap\"> Overview: Introducing JavaScript objects</span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features\"><span class=\"button-wrap\"> Next </span></a></li>\n</ul>\n<p>In previous articles we looked at all the essential JavaScript object theory and syntax details, giving you a solid base to start from. In this article we dive into a practical exercise, giving you some more practice in building custom JavaScript objects, with a fun and colorful result.</p>\n<figure class=\"table-container\"><table>\n <tbody>\n <tr>\n <th scope=\"row\">Prerequisites:</th>\n <td>\n A basic understanding of HTML and CSS,\n familiarity with JavaScript basics (see\n <a href=\"/en-US/docs/Learn/JavaScript/First_steps\">First steps</a> and\n <a href=\"/en-US/docs/Learn/JavaScript/Building_blocks\">Building blocks</a>) and OOJS basics (see\n <a href=\"/en-US/docs/Learn/JavaScript/Objects/Basics\">Introduction to objects</a>).\n </td>\n </tr>\n <tr>\n <th scope=\"row\">Objective:</th>\n <td>\n To get some practice with using objects and object-oriented techniques\n in a real-world context.\n </td>\n </tr>\n </tbody>\n</table></figure>"}},{"type":"prose","value":{"id":"lets_bounce_some_balls","title":"Let's bounce some balls","isH3":false,"content":"<p>In this article we will write a classic \"bouncing balls\" demo, to show you how useful objects can be in JavaScript. Our little balls will bounce around on the screen, and change color when they touch each other. The finished example will look a little something like this:</p>\n<p>\n <img src=\"/en-US/docs/Learn/JavaScript/Objects/Object_building_practice/bouncing-balls.png\" alt=\"Screenshot of a webpage titled "Bouncing balls". 23 balls of various pastel colors and sizes are visible across a black screen with long trails behind them indicating motion.\" width=\"800\" height=\"614\" loading=\"lazy\">\n</p>\n<p>This example will make use of the <a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics\">Canvas API</a> for drawing the balls to the screen, and the <a href=\"/en-US/docs/Web/API/Window/requestAnimationFrame\"><code>requestAnimationFrame</code></a> API for animating the whole display — you don't need to have any previous knowledge of these APIs, and we hope that by the time you've finished this article you'll be interested in exploring them more. Along the way, we'll make use of some nifty objects, and show you a couple of nice techniques like bouncing balls off walls, and checking whether they have hit each other (otherwise known as <em>collision detection</em>).</p>"}},{"type":"prose","value":{"id":"getting_started","title":"Getting started","isH3":false,"content":"<p>To begin with, make local copies of our <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/index.html\" class=\"external\" target=\"_blank\"><code>index.html</code></a>, <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/style.css\" class=\"external\" target=\"_blank\"><code>style.css</code></a>, and <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/main.js\" class=\"external\" target=\"_blank\"><code>main.js</code></a> files. These contain the following, respectively:</p>\n<ol>\n <li>A very simple HTML document featuring an <a href=\"/en-US/docs/Web/HTML/Element/Heading_Elements\">h1</a> element, a <a href=\"/en-US/docs/Web/HTML/Element/canvas\"><code><canvas></code></a> element to draw our balls on, and elements to apply our CSS and JavaScript to our HTML.</li>\n <li>Some very simple styles, which mainly serve to style and position the <code><h1></code>, and get rid of any scrollbars or margin around the edge of the page (so that it looks nice and neat).</li>\n <li>Some JavaScript that serves to set up the <code><canvas></code> element and provide a general function that we're going to use.</li>\n</ol>\n<p>The first part of the script looks like so:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>const canvas = document.querySelector(\"canvas\");\nconst ctx = canvas.getContext(\"2d\");\n\nconst width = (canvas.width = window.innerWidth);\nconst height = (canvas.height = window.innerHeight);\n</code></pre></div>\n<p>This script gets a reference to the <code><canvas></code> element, then calls the <a href=\"/en-US/docs/Web/API/HTMLCanvasElement/getContext\"><code>getContext()</code></a> method on it to give us a context on which we can start to draw. The resulting constant (<code>ctx</code>) is the object that directly represents the drawing area of the canvas and allows us to draw 2D shapes on it.</p>\n<p>Next, we set constants called <code>width</code> and <code>height</code>, and the width and height of the canvas element (represented by the <code>canvas.width</code> and <code>canvas.height</code> properties) to equal the width and height of the browser viewport (the area which the webpage appears on — this can be gotten from the <a href=\"/en-US/docs/Web/API/Window/innerWidth\"><code>Window.innerWidth</code></a> and <a href=\"/en-US/docs/Web/API/Window/innerHeight\"><code>Window.innerHeight</code></a> properties).</p>\n<p>Note that we are chaining multiple assignments together, to get the variables all set quicker — this is perfectly OK.</p>\n<p>Then we have two helper functions:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function random(min, max) {\n return Math.floor(Math.random() * (max - min + 1)) + min;\n}\n\nfunction randomRGB() {\n return `rgb(${random(0, 255)} ${random(0, 255)} ${random(0, 255)})`;\n}\n</code></pre></div>\n<p>The <code>random()</code> function takes two numbers as arguments, and returns a random number in the range between the two. The <code>randomRGB()</code> function generates a random color represented as an <a href=\"/en-US/docs/Web/CSS/color_value/rgb\"><code>rgb()</code></a> string.</p>"}},{"type":"prose","value":{"id":"modeling_a_ball_in_our_program","title":"Modeling a ball in our program","isH3":false,"content":"<p>Our program will feature lots of balls bouncing around the screen. Since these balls will all behave in the same way, it makes sense to represent them with an object. Let's start by adding the following class definition to the bottom of our code.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>class Ball {\n constructor(x, y, velX, velY, color, size) {\n this.x = x;\n this.y = y;\n this.velX = velX;\n this.velY = velY;\n this.color = color;\n this.size = size;\n }\n}\n</code></pre></div>\n<p>So far this class only contains a constructor, in which we can initialize the properties each ball needs in order to function in our program:</p>\n<ul>\n <li><code>x</code> and <code>y</code> coordinates — the horizontal and vertical coordinates where the ball starts on the screen. This can range between 0 (top left hand corner) to the width and height of the browser viewport (bottom right-hand corner).</li>\n <li>horizontal and vertical velocity (<code>velX</code> and <code>velY</code>) — each ball is given a horizontal and vertical velocity; in real terms these values are regularly added to the <code>x</code>/<code>y</code> coordinate values when we animate the balls, to move them by this much on each frame.</li>\n <li><code>color</code> — each ball gets a color.</li>\n <li><code>size</code> — each ball gets a size — this is its radius, in pixels.</li>\n</ul>\n<p>This handles the properties, but what about the methods? We want to get our balls to actually do something in our program.</p>"}},{"type":"prose","value":{"id":"drawing_the_ball","title":"Drawing the ball","isH3":true,"content":"<p>First add the following <code>draw()</code> method to the <code>Ball</code> class:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>draw() {\n ctx.beginPath();\n ctx.fillStyle = this.color;\n ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI);\n ctx.fill();\n}\n</code></pre></div>\n<p>Using this function, we can tell the ball to draw itself onto the screen, by calling a series of members of the 2D canvas context we defined earlier (<code>ctx</code>). The context is like the paper, and now we want to command our pen to draw something on it:</p>\n<ul>\n <li>\n <p>First, we use <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/beginPath\"><code>beginPath()</code></a> to state that we want to draw a shape on the paper.</p>\n </li>\n <li>\n <p>Next, we use <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/fillStyle\"><code>fillStyle</code></a> to define what color we want the shape to be — we set it to our ball's <code>color</code> property.</p>\n </li>\n <li>\n <p>Next, we use the <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/arc\"><code>arc()</code></a> method to trace an arc shape on the paper. Its parameters are:</p>\n <ul>\n <li>The <code>x</code> and <code>y</code> position of the arc's center — we are specifying the ball's <code>x</code> and <code>y</code> properties.</li>\n <li>The radius of the arc — in this case, the ball's <code>size</code> property.</li>\n <li>The last two parameters specify the start and end number of degrees around the circle that the arc is drawn between. Here we specify 0 degrees, and <code>2 * PI</code>, which is the equivalent of 360 degrees in radians (annoyingly, you have to specify this in radians). That gives us a complete circle. If you had specified only <code>1 * PI</code>, you'd get a semi-circle (180 degrees).</li>\n </ul>\n </li>\n <li>\n <p>Last of all, we use the <a href=\"/en-US/docs/Web/API/CanvasRenderingContext2D/fill\"><code>fill()</code></a> method, which basically states \"finish drawing the path we started with <code>beginPath()</code>, and fill the area it takes up with the color we specified earlier in <code>fillStyle</code>.\"</p>\n </li>\n</ul>\n<p>You can start testing your object out already.</p>\n<ol>\n <li>\n <p>Save the code so far, and load the HTML file in a browser.</p>\n </li>\n <li>\n <p>Open the browser's JavaScript console, and then refresh the page so that the canvas size changes to the smaller visible viewport that remains when the console opens.</p>\n </li>\n <li>\n <p>Type in the following to create a new ball instance:</p>\n <div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>const testBall = new Ball(50, 100, 4, 4, \"blue\", 10);\n</code></pre></div>\n </li>\n <li>\n <p>Try calling its members:</p>\n <div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>testBall.x;\ntestBall.size;\ntestBall.color;\ntestBall.draw();\n</code></pre></div>\n </li>\n <li>\n <p>When you enter the last line, you should see the ball draw itself somewhere on the canvas.</p>\n </li>\n</ol>"}},{"type":"prose","value":{"id":"updating_the_balls_data","title":"Updating the ball's data","isH3":true,"content":"<p>We can draw the ball in position, but to actually move the ball, we need an update function of some kind. Add the following code inside the class definition for <code>Ball</code>:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>update() {\n if ((this.x + this.size) >= width) {\n this.velX = -(this.velX);\n }\n\n if ((this.x - this.size) <= 0) {\n this.velX = -(this.velX);\n }\n\n if ((this.y + this.size) >= height) {\n this.velY = -(this.velY);\n }\n\n if ((this.y - this.size) <= 0) {\n this.velY = -(this.velY);\n }\n\n this.x += this.velX;\n this.y += this.velY;\n}\n</code></pre></div>\n<p>The first four parts of the function check whether the ball has reached the edge of the canvas. If it has, we reverse the polarity of the relevant velocity to make the ball travel in the opposite direction. So for example, if the ball was traveling upwards (negative <code>velY</code>), then the vertical velocity is changed so that it starts to travel downwards instead (positive <code>velY</code>).</p>\n<p>In the four cases, we are checking to see:</p>\n<ul>\n <li>if the <code>x</code> coordinate is greater than the width of the canvas (the ball is going off the right edge).</li>\n <li>if the <code>x</code> coordinate is smaller than 0 (the ball is going off the left edge).</li>\n <li>if the <code>y</code> coordinate is greater than the height of the canvas (the ball is going off the bottom edge).</li>\n <li>if the <code>y</code> coordinate is smaller than 0 (the ball is going off the top edge).</li>\n</ul>\n<p>In each case, we include the <code>size</code> of the ball in the calculation because the <code>x</code>/<code>y</code> coordinates are in the center of the ball, but we want the edge of the ball to bounce off the perimeter — we don't want the ball to go halfway off the screen before it starts to bounce back.</p>\n<p>The last two lines add the <code>velX</code> value to the <code>x</code> coordinate, and the <code>velY</code> value to the <code>y</code> coordinate — the ball is in effect moved each time this method is called.</p>\n<p>This will do for now; let's get on with some animation!</p>"}},{"type":"prose","value":{"id":"animating_the_ball","title":"Animating the ball","isH3":false,"content":"<p>Now let's make this fun. We are now going to start adding balls to the canvas, and animating them.</p>\n<p>First, we need to create somewhere to store all our balls and then populate it. The following will do this job — add it to the bottom of your code now:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>const balls = [];\n\nwhile (balls.length < 25) {\n const size = random(10, 20);\n const ball = new Ball(\n // ball position always drawn at least one ball width\n // away from the edge of the canvas, to avoid drawing errors\n random(0 + size, width - size),\n random(0 + size, height - size),\n random(-7, 7),\n random(-7, 7),\n randomRGB(),\n size,\n );\n\n balls.push(ball);\n}\n</code></pre></div>\n<p>The <code>while</code> loop creates a new instance of our <code>Ball()</code> using random values generated with our <code>random()</code> and <code>randomRGB()</code> functions, then <code>push()</code>es it onto the end of our balls array, but only while the number of balls in the array is less than 25. So when we have 25 balls in the array, no more balls will be pushed. You can try varying the number in <code>balls.length < 25</code> to get more or fewer balls in the array. Depending on how much processing power your computer/browser has, specifying several thousand balls might slow down the animation rather a lot!</p>\n<p>Next, add the following to the bottom of your code:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function loop() {\n ctx.fillStyle = \"rgb(0 0 0 / 25%)\";\n ctx.fillRect(0, 0, width, height);\n\n for (const ball of balls) {\n ball.draw();\n ball.update();\n }\n\n requestAnimationFrame(loop);\n}\n</code></pre></div>\n<p>All programs that animate things generally involve an animation loop, which serves to update the information in the program and then render the resulting view on each frame of the animation; this is the basis for most games and other such programs. Our <code>loop()</code> function does the following:</p>\n<ul>\n <li>Sets the canvas fill color to semi-transparent black, then draws a rectangle of the color across the whole width and height of the canvas, using <code>fillRect()</code> (the four parameters provide a start coordinate, and a width and height for the rectangle drawn). This serves to cover up the previous frame's drawing before the next one is drawn. If you don't do this, you'll just see long snakes worming their way around the canvas instead of balls moving! The color of the fill is set to semi-transparent, <code>rgb(0 0 0 / 25%)</code>, to allow the previous few frames to shine through slightly, producing the little trails behind the balls as they move. If you changed 0.25 to 1, you won't see them at all any more. Try varying this number to see the effect it has.</li>\n <li>Loops through all the balls in the <code>balls</code> array, and runs each ball's <code>draw()</code> and <code>update()</code> function to draw each one on the screen, then do the necessary updates to position and velocity in time for the next frame.</li>\n <li>Runs the function again using the <code>requestAnimationFrame()</code> method — when this method is repeatedly run and passed the same function name, it runs that function a set number of times per second to create a smooth animation. This is generally done recursively — which means that the function is calling itself every time it runs, so it runs over and over again.</li>\n</ul>\n<p>Finally, add the following line to the bottom of your code — we need to call the function once to get the animation started.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>loop();\n</code></pre></div>\n<p>That's it for the basics — try saving and refreshing to test your bouncing balls out!</p>"}},{"type":"prose","value":{"id":"adding_collision_detection","title":"Adding collision detection","isH3":false,"content":"<p>Now for a bit of fun, let's add some collision detection to our program, so our balls know when they have hit another ball.</p>\n<p>First, add the following method definition to your <code>Ball</code> class.</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) {\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>This method is a little complex, so don't worry if you don't understand exactly how it works for now. An explanation follows:</p>\n<ul>\n <li>For each ball, we need to check every other ball to see if it has collided with the current ball. To do this, we start another <code>for...of</code> loop to loop through all the balls in the <code>balls[]</code> array.</li>\n <li>Immediately inside the for loop, we use an <code>if</code> statement to check whether the current ball being looped through is the same ball as the one we are currently checking. We don't want to check whether a ball has collided with itself! To do this, we check whether the current ball (i.e., the ball whose collisionDetect method is being invoked) is the same as the loop ball (i.e., the ball that is being referred to by the current iteration of the for loop in the collisionDetect method). We then use <code>!</code> to negate the check, so that the code inside the <code>if</code> statement only runs if they are <strong>not</strong> the same.</li>\n <li>We then use a common algorithm to check the collision of two circles. We are basically checking whether any of the two circle's areas overlap. This is explained further in <a href=\"/en-US/docs/Games/Techniques/2D_collision_detection\">2D collision detection</a>.</li>\n <li>If a collision is detected, the code inside the inner <code>if</code> statement is run. In this case, we only set the <code>color</code> property of both the circles to a new random color. We could have done something far more complex, like get the balls to bounce off each other realistically, but that would have been far more complex to implement. For such physics simulations, developers tend to use a games or physics libraries such as <a href=\"https://wellcaffeinated.net/PhysicsJS/\" class=\"external\" target=\"_blank\">PhysicsJS</a>, <a href=\"https://brm.io/matter-js/\" class=\"external\" target=\"_blank\">matter.js</a>, <a href=\"https://phaser.io/\" class=\"external\" target=\"_blank\">Phaser</a>, etc.</li>\n</ul>\n<p>You also need to call this method in each frame of the animation. Update your <code>loop()</code> function to call <code>ball.collisionDetect()</code> after <code>ball.update()</code>:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function loop() {\n ctx.fillStyle = \"rgb(0 0 0 / 25%)\";\n ctx.fillRect(0, 0, width, height);\n\n for (const ball of balls) {\n ball.draw();\n ball.update();\n ball.collisionDetect();\n }\n\n requestAnimationFrame(loop);\n}\n</code></pre></div>\n<p>Save and refresh the demo again, and you'll see your balls change color when they collide!</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> If you have trouble getting this example to work, try comparing your JavaScript code against our <a href=\"https://github.com/mdn/learning-area/blob/main/javascript/oojs/bouncing-balls/main-finished.js\" class=\"external\" target=\"_blank\">finished version</a> (also see it <a href=\"https://mdn.github.io/learning-area/javascript/oojs/bouncing-balls/index-finished.html\" class=\"external\" target=\"_blank\">running live</a>).</p>\n</div>"}},{"type":"prose","value":{"id":"summary","title":"Summary","isH3":false,"content":"<p>We hope you had fun writing your own real-world random bouncing balls example, using various object and object-oriented techniques from throughout the module! This should have given you some useful practice in using objects, and good real-world context.</p>\n<p>That's it for object articles — all that remains now is for you to test your skills in the object assessment.</p>"}},{"type":"prose","value":{"id":"see_also","title":"See also","isH3":false,"content":"<ul>\n <li><a href=\"/en-US/docs/Web/API/Canvas_API/Tutorial\">Canvas tutorial</a> — a beginner's guide to using 2D canvas.</li>\n <li><a href=\"/en-US/docs/Web/API/Window/requestAnimationFrame\">requestAnimationFrame()</a></li>\n <li><a href=\"/en-US/docs/Games/Techniques/2D_collision_detection\">2D collision detection</a></li>\n <li><a href=\"/en-US/docs/Games/Techniques/3D_collision_detection\">3D collision detection</a></li>\n <li><a href=\"/en-US/docs/Games/Tutorials/2D_Breakout_game_pure_JavaScript\">2D breakout game using pure JavaScript</a> — a great beginner's tutorial showing how to build a 2D game.</li>\n <li><a href=\"/en-US/docs/Games/Tutorials/2D_breakout_game_Phaser\">2D breakout game using Phaser</a> — explains the basics of building a 2D game using a JavaScript game library.</li>\n</ul><ul class=\"prev-next\">\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/JavaScript/Objects/JSON\"><span class=\"button-wrap\"> Previous </span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/JavaScript/Objects\"><span class=\"button-wrap\"> Overview: Introducing JavaScript objects</span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features\"><span class=\"button-wrap\"> Next </span></a></li>\n</ul>"}}],"toc":[{"text":"Let's bounce some balls","id":"lets_bounce_some_balls"},{"text":"Getting started","id":"getting_started"},{"text":"Modeling a ball in our program","id":"modeling_a_ball_in_our_program"},{"text":"Animating the ball","id":"animating_the_ball"},{"text":"Adding collision detection","id":"adding_collision_detection"},{"text":"Summary","id":"summary"},{"text":"See also","id":"see_also"}],"summary":"We hope you had fun writing your own real-world random bouncing balls example, using various object and object-oriented techniques from throughout the module! This should have given you some useful practice in using objects, and good real-world context.","popularity":0.0095,"modified":"2024-07-25T21:20:22.000Z","other_translations":[{"locale":"de","title":"Objekt-Baupraxis","native":"Deutsch"},{"locale":"es","title":"Ejercicio práctico de construcción de objetos","native":"Español"},{"locale":"fr","title":"La construction d'objet en pratique","native":"Français"},{"locale":"ja","title":"オブジェクト構築の練習","native":"日本語"},{"locale":"pt-BR","title":"Prática de construção de objetos","native":"Português (do Brasil)"},{"locale":"ru","title":"Практика построения объектов","native":"Русский"},{"locale":"zh-CN","title":"实践对象构造","native":"中文 (简体)"},{"locale":"zh-TW","title":"物件建構實作","native":"正體中文 (繁體)"}],"pageType":"learn-module-chapter","source":{"folder":"en-us/learn/javascript/objects/object_building_practice","github_url":"https://github.com/mdn/content/blob/main/files/en-us/learn/javascript/objects/object_building_practice/index.md","last_commit_url":"https://github.com/mdn/content/commit/bc0d0d1ef796435e969f6d65c7e5d3c08f4023aa","filename":"index.md"},"short_title":"Object building practice","parents":[{"uri":"/en-US/docs/Learn","title":"Guides"},{"uri":"/en-US/docs/Learn/JavaScript","title":"JavaScript — Dynamic client-side scripting"},{"uri":"/en-US/docs/Learn/JavaScript/Objects","title":"Introducing JavaScript objects"},{"uri":"/en-US/docs/Learn/JavaScript/Objects/Object_building_practice","title":"Object building practice"}],"pageTitle":"Object building practice - Learn web development | MDN","noIndexing":false}}</script></body></html>