Scroll Reveal Libraries

Overview

This skill covers AOS (Animate On Scroll), a lightweight CSS-driven library for scroll-triggered animations. AOS excels at simple fade, slide, and zoom effects activated when elements enter the viewport.

Key Features:

• Minimal Setup: Single JavaScript file + CSS
• Data Attribute API: Configure animations in HTML
• Performance: CSS-driven, GPU-accelerated animations
• 50+ Built-in Animations: Fades, slides, zooms, flips
• Framework Agnostic: Works with vanilla JS, React, Vue, etc.

When to Use:

• Marketing/landing pages with simple scroll effects
• Content-heavy sites (blogs, documentation)
• Quick prototypes requiring scroll animations
• Projects where GSAP/Framer Motion complexity isn't needed

When NOT to Use:

• Complex animation timelines or orchestration → Use GSAP ScrollTrigger
• Physics-based animations → Use React Spring or Framer Motion
• Precise scroll-synced animations → Use GSAP ScrollTrigger
• Heavy interactive animations → Use Framer Motion

Core Concepts

Installation

CDN (Quickest):

<head>
  <link rel="stylesheet" href="https://unpkg.com/aos@next/dist/aos.css" />
</head>
<body>
  <!-- Content with data-aos attributes -->

  <script src="https://unpkg.com/aos@next/dist/aos.js"></script>
  <script>
    AOS.init();
  </script>
</body>

NPM/Yarn (Recommended):

npm install aos@next
# or
yarn add aos@next

import AOS from 'aos';
import 'aos/dist/aos.css';

AOS.init();

Basic Usage

Apply animations using the data-aos attribute:

<!-- Fade in -->
<div data-aos="fade-in">Content</div>

<!-- Fade up -->
<div data-aos="fade-up">Content</div>

<!-- Slide from right -->
<div data-aos="slide-left">Content</div>

<!-- Zoom in -->
<div data-aos="zoom-in">Content</div>

Configuration Options

Global Configuration:

AOS.init({
  // Animation settings
  duration: 800,  // Animation duration (ms): 0-3000
  delay: 0,       // Delay before animation (ms): 0-3000
  offset: 120,    // Offset from trigger point (px)
  easing: 'ease', // Easing function
  once: false,    // Animate only once (true) or every time (false)
  mirror: false,  // Animate out when scrolling past

  // Placement
  anchorPlacement: 'top-bottom', // Which position triggers animation

  // Performance
  disable: false,                // Disable on mobile/tablet
  startEvent: 'DOMContentLoaded', // Initialization event
  debounceDelay: 50,             // Window resize debounce
  throttleDelay: 99              // Scroll throttle
});

Per-Element Overrides:

<div
  data-aos="fade-up"
  data-aos-duration="1000"
  data-aos-delay="200"
  data-aos-offset="50"
  data-aos-easing="ease-in-out"
  data-aos-once="true"
  data-aos-mirror="true"
  data-aos-anchor-placement="center-bottom"
>
  Custom configured element
</div>

Common Patterns

1. Landing Page Hero Section

<section class="hero">
  <!-- Staggered heading words -->
  <h1
    data-aos="fade-down"
    data-aos-duration="800"
  >
    Welcome to the Future
  </h1>

  <!-- Delayed subheading -->
  <p
    data-aos="fade-up"
    data-aos-delay="200"
    data-aos-duration="600"
  >
    Transform your ideas into reality
  </p>

  <!-- CTA button -->
  <button
    data-aos="zoom-in"
    data-aos-delay="400"
    data-aos-duration="500"
  >
    Get Started
  </button>
</section>

2. Feature Cards Grid

<div class="features-grid">
  <!-- Stagger cards with increasing delays -->
  <div
    class="feature-card"
    data-aos="fade-up"
    data-aos-duration="600"
    data-aos-delay="0"
  >
    <h3>Feature 1</h3>
    <p>Description...</p>
  </div>

  <div
    class="feature-card"
    data-aos="fade-up"
    data-aos-duration="600"
    data-aos-delay="100"
  >
    <h3>Feature 2</h3>
    <p>Description...</p>
  </div>

  <div
    class="feature-card"
    data-aos="fade-up"
    data-aos-duration="600"
    data-aos-delay="200"
  >
    <h3>Feature 3</h3>
    <p>Description...</p>
  </div>
</div>

3. Alternating Content Sections

<!-- Content from left -->
<div class="section">
  <div
    class="content"
    data-aos="slide-right"
    data-aos-duration="800"
  >
    <h2>Section Title</h2>
    <p>Content slides in from left...</p>
  </div>
  <img
    src="image1.jpg"
    data-aos="fade-left"
    data-aos-delay="200"
  />
</div>

<!-- Content from right -->
<div class="section reverse">
  <img
    src="image2.jpg"
    data-aos="fade-right"
  />
  <div
    class="content"
    data-aos="slide-left"
    data-aos-duration="800"
    data-aos-delay="200"
  >
    <h2>Section Title</h2>
    <p>Content slides in from right...</p>
  </div>
</div>

4. Scroll-Triggered Testimonials

<div class="testimonials">
  <div
    class="testimonial"
    data-aos="zoom-in"
    data-aos-duration="500"
  >
    <blockquote>"Amazing product!"</blockquote>
    <cite>- John Doe</cite>
  </div>

  <div
    class="testimonial"
    data-aos="zoom-in"
    data-aos-duration="500"
    data-aos-delay="100"
  >
    <blockquote>"Exceeded expectations"</blockquote>
    <cite>- Jane Smith</cite>
  </div>
</div>

5. Custom Anchor Triggers

Trigger animations based on a different element's scroll position:

<!-- Fixed sidebar animates based on main content scroll -->
<div class="main-content">
  <div id="trigger-point" data-aos-id="sidebar-trigger">
    <!-- Content -->
  </div>
</div>

<aside
  class="sidebar"
  data-aos="fade-left"
  data-aos-anchor="#trigger-point"
>
  Sidebar content
</aside>

6. Sequential Animation Chain

<div class="animation-sequence">
  <!-- Step 1: Heading -->
  <h2
    data-aos="fade-down"
    data-aos-duration="600"
    data-aos-delay="0"
  >
    Our Process
  </h2>

  <!-- Step 2: Description -->
  <p
    data-aos="fade-up"
    data-aos-duration="600"
    data-aos-delay="200"
  >
    Follow these simple steps
  </p>

  <!-- Step 3-5: Process cards -->
  <div
    class="process-step"
    data-aos="flip-left"
    data-aos-delay="400"
  >
    Step 1
  </div>

  <div
    class="process-step"
    data-aos="flip-left"
    data-aos-delay="600"
  >
    Step 2
  </div>

  <div
    class="process-step"
    data-aos="flip-left"
    data-aos-delay="800"
  >
    Step 3
  </div>
</div>

7. Image Gallery with Zoom Effects

<div class="gallery">
  <img
    src="photo1.jpg"
    data-aos="zoom-in-up"
    data-aos-duration="800"
  />
  <img
    src="photo2.jpg"
    data-aos="zoom-in-up"
    data-aos-duration="800"
    data-aos-delay="100"
  />
  <img
    src="photo3.jpg"
    data-aos="zoom-in-up"
    data-aos-duration="800"
    data-aos-delay="200"
  />
</div>

Integration Patterns

React Integration

Basic Setup:

import { useEffect } from 'react';
import AOS from 'aos';
import 'aos/dist/aos.css';

function App() {
  useEffect(() => {
    AOS.init({
      duration: 800,
      once: true,
      offset: 100
    });
  }, []);

  return (
    <div>
      <h1 data-aos="fade-down">Welcome</h1>
      <p data-aos="fade-up">Content here</p>
    </div>
  );
}

Refreshing on Route Changes:

import { useEffect } from 'react';
import { useLocation } from 'react-router-dom';
import AOS from 'aos';

function App() {
  const location = useLocation();

  useEffect(() => {
    AOS.init({ duration: 800 });
  }, []);

  // Refresh AOS on route change
  useEffect(() => {
    AOS.refresh();
  }, [location]);

  return <Routes>{/* routes */}</Routes>;
}

Dynamic Content Updates:

import { useState, useEffect } from 'react';
import AOS from 'aos';

function DynamicList() {
  const [items, setItems] = useState([]);

  useEffect(() => {
    AOS.init();
  }, []);

  const addItem = () => {
    setItems([...items, { id: Date.now(), text: 'New Item' }]);

    // Refresh AOS to detect new elements
    setTimeout(() => AOS.refresh(), 50);
  };

  return (
    <div>
      <button onClick={addItem}>Add Item</button>
      <ul>
        {items.map((item) => (
          <li key={item.id} data-aos="fade-in">
            {item.text}
          </li>
        ))}
      </ul>
    </div>
  );
}

Component Wrapper Pattern:

import AOS from 'aos';
import 'aos/dist/aos.css';

function AnimatedSection({ children, animation = "fade-up", delay = 0, ...props }) {
  return (
    <div
      data-aos={animation}
      data-aos-delay={delay}
      {...props}
    >
      {children}
    </div>
  );
}

// Usage
<AnimatedSection animation="slide-right" delay={200}>
  <h2>Animated Content</h2>
</AnimatedSection>

Vue.js Integration

<template>
  <div>
    <h1 data-aos="fade-down">Vue + AOS</h1>
    <div
      v-for="(item, index) in items"
      :key="item.id"
      data-aos="fade-up"
      :data-aos-delay="index * 100"
    >
      {{ item.text }}
    </div>
  </div>
</template>

<script>
import AOS from 'aos';
import 'aos/dist/aos.css';

export default {
  mounted() {
    AOS.init({ duration: 800 });
  },
  updated() {
    // Refresh when component updates
    this.$nextTick(() => {
      AOS.refresh();
    });
  },
  data() {
    return {
      items: [/*...*/]
    };
  }
};
</script>

Next.js Integration

// pages/_app.js
import { useEffect } from 'react';
import AOS from 'aos';
import 'aos/dist/aos.css';

function MyApp({ Component, pageProps }) {
  useEffect(() => {
    AOS.init({
      duration: 800,
      once: true
    });
  }, []);

  return <Component {...pageProps} />;
}

export default MyApp;

// pages/index.js
export default function Home() {
  return (
    <main>
      <h1 data-aos="fade-down">Next.js + AOS</h1>
      <p data-aos="fade-up">Server-side rendered content with animations</p>
    </main>
  );
}

Performance Optimization

1. Disable on Mobile Devices

AOS.init({
  disable: 'mobile', // Disable on mobile
  // Or use function for custom logic
  disable: function() {
    return window.innerWidth < 768;
  }
});

2. Use Once for Better Performance

AOS.init({
  once: true, // Animate only once (better performance)
  mirror: false // Don't animate out
});

3. Optimize Throttle and Debounce

AOS.init({
  throttleDelay: 99,  // Scroll event throttle (default)
  debounceDelay: 50   // Resize event debounce (default)
});

4. Disable Mutation Observer for Static Content

AOS.init({
  disableMutationObserver: true // Disable for fully static content
});

5. Reduce Animation Complexity

<!-- Simpler animations perform better -->
<div data-aos="fade-in">Simple fade</div>

<!-- Complex animations may cause jank -->
<div data-aos="flip-left">Complex flip</div>

6. Use RequestIdleCallback for Initialization

if ('requestIdleCallback' in window) {
  requestIdleCallback(() => {
    AOS.init({ duration: 800 });
  });
} else {
  AOS.init({ duration: 800 });
}

Common Pitfalls

1. Forgetting to Refresh After DOM Changes

Problem: New elements don't animate after being added dynamically.

Solution: Call AOS.refresh() or AOS.refreshHard():

// After adding elements to DOM
const newElement = document.createElement('div');
newElement.setAttribute('data-aos', 'fade-in');
container.appendChild(newElement);

// Refresh AOS
AOS.refresh(); // Recalculate positions
// or
AOS.refreshHard(); // Reinitialize completely

2. Animations Not Working in React

Problem: AOS doesn't detect elements on first render or route changes.

Solution: Initialize in useEffect and refresh on route/content changes:

useEffect(() => {
  AOS.init();
  return () => AOS.refresh(); // Cleanup
}, []);

useEffect(() => {
  AOS.refresh(); // Refresh on route change
}, [location.pathname]);

3. Scroll Performance Issues

Problem: Page scrolling feels janky with many animated elements.

Solution: Reduce animated elements and use once: true:

AOS.init({
  once: true, // Animate only once
  disable: window.innerWidth < 768 // Disable on mobile
});

4. CSS Conflicts

Problem: Custom CSS interferes with AOS animations.

Solution: Use more specific selectors and avoid !important:

/* Bad: Conflicts with AOS */
div {
  opacity: 1 !important;
}

/* Good: Specific selector */
.my-content > div {
  /* styles */
}

5. Anchor Placement Confusion

Problem: Animations trigger at unexpected scroll positions.

Solution: Understand anchor placement options:

// Triggers when element's top hits viewport bottom
data-aos-anchor-placement="top-bottom"

// Triggers when element's center hits viewport center
data-aos-anchor-placement="center-center"

// Triggers when element's bottom hits viewport top
data-aos-anchor-placement="bottom-top"

6. Duration/Delay Limits

Problem: Values above 3000ms don't work.

Solution: Add custom CSS for extended durations:

body[data-aos-duration='4000'] [data-aos],
[data-aos][data-aos][data-aos-duration='4000'] {
  transition-duration: 4000ms;
}

<div data-aos="fade-in" data-aos-duration="4000">
  Long animation
</div>

Built-in Animations

Fade Animations

• fade-in - Simple fade in
• fade-up - Fade in from bottom
• fade-down - Fade in from top
• fade-left - Fade in from right
• fade-right - Fade in from left
• fade-up-right - Diagonal fade
• fade-up-left - Diagonal fade
• fade-down-right - Diagonal fade
• fade-down-left - Diagonal fade

Slide Animations

• slide-up - Slide from bottom
• slide-down - Slide from top
• slide-left - Slide from right
• slide-right - Slide from left

Zoom Animations

• zoom-in - Zoom in
• zoom-in-up - Zoom in from bottom
• zoom-in-down - Zoom in from top
• zoom-in-left - Zoom in from right
• zoom-in-right - Zoom in from left
• zoom-out - Zoom out
• zoom-out-up - Zoom out to top
• zoom-out-down - Zoom out to bottom
• zoom-out-left - Zoom out to left
• zoom-out-right - Zoom out to right

Flip Animations

• flip-up - Flip from bottom
• flip-down - Flip from top
• flip-left - Flip from right
• flip-right - Flip from left

Custom Animations

Create custom animations with CSS:

[data-aos="custom-slide-bounce"] {
  opacity: 0;
  transform: translateY(100px);
  transition-property: transform, opacity;
}

[data-aos="custom-slide-bounce"].aos-animate {
  opacity: 1;
  transform: translateY(0);
  animation: bounce 0.5s;
}

@keyframes bounce {
  0%, 20%, 50%, 80%, 100% {
    transform: translateY(0);
  }
  40% {
    transform: translateY(-10px);
  }
  60% {
    transform: translateY(-5px);
  }
}

<div data-aos="custom-slide-bounce">
  Custom animation
</div>

Comparison with Alternatives

AOS vs GSAP ScrollTrigger

Feature	AOS	GSAP ScrollTrigger
Complexity	Simple, data-attribute based	Advanced, JavaScript API
Use Case	Simple reveals	Complex timelines
File Size	~13KB	~27KB (GSAP) + ScrollTrigger
Performance	CSS-driven	JavaScript-driven
Learning Curve	Minutes	Hours
Customization	Limited	Extensive
Best For	Marketing pages	Interactive experiences

Use AOS when:

• Simple fade/slide/zoom effects
• Quick implementation needed
• Minimal JavaScript preferred
• Basic scroll reveals sufficient

Use GSAP ScrollTrigger when:

• Complex animation sequences
• Precise scroll-synced animations
• Timeline orchestration needed
• Advanced easing/physics required

Resources

Official Documentation

• AOS: https://michalsnik.github.io/aos/
• GitHub: https://github.com/michalsnik/aos

Key Scripts

• scripts/aos_generator.py - Generate AOS HTML boilerplate
• scripts/config_builder.py - Build AOS configuration

References

• references/aos_api.md - Complete AOS API reference
• references/animation_catalog.md - All built-in animations with demos
• references/integration_patterns.md - Framework integration guides

Starter Assets

• assets/starter_aos/ - Complete AOS starter template
• assets/examples/ - Production-ready patterns

Related Skills

• gsap-scrolltrigger: For complex scroll-driven animations
• motion-framer: For React-specific animations with physics
• locomotive-scroll: For smooth scrolling with parallax
• animated-component-libraries: For pre-built animated React components