How to Build an Interactive FAQ System with HTML, CSS and JavaScript

CSS Design & Responsive Behavior

• Adaptable layout using @media queries for mobile and desktop
• Slide-in animation for FAQ panel on mobile via .faq-items--slide-in
• Dark overlay activated by .faq-overlay--is-visible on small screens
• Hover and focus styling for sidebar links with .faq-category
• Mobile close button using .faq-close-item styled with pseudo-elements

On desktop, all topic groups appear statically. On mobile, interactions are animated with CSS transforms and an overlay-based interface.

JavaScript Explained Step by Step

The JavaScript powering this FAQ system is self-invoked and modular, structured into two main components:

• FAQ – an object with utility functions
• FAQComponent – a constructor managing all interactions

Utility Functions: Class Control & Animation

• FAQ.hasClass() – checks if an element has a specific class
• FAQ.addClass() / removeClass() – adds or removes one or more classes from an element
• FAQ.toggleClass() – toggles class based on a boolean condition

These helpers simplify DOM manipulation and visual state control throughout the interaction flow.

Height Animation: FAQ.setHeight()

• Animates an element's height smoothly using requestAnimationFrame
• Used when expanding or collapsing answer blocks upon question click

Animated Scrolling: FAQ.scrollTo()

On desktop, clicking a category smoothly scrolls the page to the corresponding topic group:

• It starts from window.scrollY or document.documentElement.scrollTop
• Applies a natural transition curve via Math.easeInOutQuad()
• Supports a custom duration (default: 200ms)

Device Detection: getMode()

This function checks whether the FAQ system is rendered on desktop or mobile:

• Uses getComputedStyle() to read the ::before pseudo-element
• Returns 'mobile' or 'desktop' depending on layout
• Determines how interactions behave (slide-in or scroll)

Component Constructor: FAQComponent & Initialization

FAQComponent orchestrates everything:

• Defines all key elements: questions, answers, overlay, container, links
• init() sets up event listeners for category clicks, question toggles, closing, and scrolling
• closePanel() hides the mobile panel and background overlay

Scroll Tracking Behavior

On desktop, the script continuously updates the active category based on scroll position:

• Checks which .faq-group is currently in the viewport
• Applies .faq-category-selected on the related menu link
• Defaults to the first category if no group is active

Extensibility & Customization

• Easily integrated into CMS platforms or UI libraries
• Modular CSS styling ready for theming (light/dark)
• Expandable with live search, filtering, or animations
• JavaScript logic can be ported to ES Modules or TypeScript for larger applications

Its flexibility makes this system suitable for scalable web projects and reusable components across multiple pages or frameworks.

Conclusion

This FAQ system showcases how to build an elegant and responsive web component using pure HTML, CSS and vanilla JavaScript. Its semantic structure, modular styling, and smooth interactivity deliver a professional experience without relying on external libraries. Download the archive