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1042---
title: "Intersection Observer in JavaScript"
sidebarTitle: "Intersection Observer"
description: "Learn the Intersection Observer API in JavaScript. Detect element visibility, implement lazy loading, infinite scroll, and scroll animations efficiently without scroll events."
---
How do you know when an element scrolls into view? How can you lazy-load images only when they're about to be seen? How do infinite-scroll feeds know when to load more content? And how can you trigger animations at just the right moment as users scroll through your page?
```javascript
// Lazy load images when they come into view
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
const img = entry.target;
img.src = img.dataset.src;
observer.unobserve(img);
}
});
});
document.querySelectorAll('img[data-src]').forEach(img => observer.observe(img));
```
The **[Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API)** provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or the viewport. It's the modern, performant solution for detecting element visibility, replacing expensive scroll event listeners with browser-optimized callbacks.
<Info>
**What you'll learn in this guide:**
- What Intersection Observer is and why it's better than scroll events
- How to create and configure observers with options
- Understanding thresholds, root, and rootMargin
- Implementing lazy loading for images and content
- Building infinite scroll functionality
- Creating scroll-triggered animations
- Common mistakes and best practices
</Info>
---
## What is Intersection Observer in JavaScript?
The **[Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API)** is a browser API that lets you detect when an element enters, exits, or crosses a certain visibility threshold within a viewport or container element. Instead of constantly checking element positions during scroll events, the browser efficiently notifies your code only when visibility actually changes. **In short: Intersection Observer tells you when elements become visible or hidden, without the performance cost of scroll listeners.**
---
## Why Not Just Use Scroll Events?
Before Intersection Observer, developers used scroll event listeners with `getBoundingClientRect()` to detect element visibility:
```javascript
// The OLD way: scroll events (DON'T do this!)
window.addEventListener('scroll', () => {
const elements = document.querySelectorAll('.lazy-image');
elements.forEach(el => {
const rect = el.getBoundingClientRect();
if (rect.top < window.innerHeight && rect.bottom > 0) {
// Element is visible - load it
el.src = el.dataset.src;
}
});
});
```
**Problems with this approach:**
| Issue | Why It's Bad |
|-------|--------------|
| **Main thread blocking** | Scroll fires 60+ times per second, blocking other JavaScript |
| **Layout thrashing** | `getBoundingClientRect()` forces browser to recalculate layout |
| **Battery drain** | Constant calculations drain mobile device batteries |
| **No throttling built-in** | You must manually debounce/throttle |
| **iframe limitations** | Can't detect visibility in cross-origin iframes |
```
┌─────────────────────────────────────────────────────────────────────────────┐
│ SCROLL EVENTS vs INTERSECTION OBSERVER │
├─────────────────────────────────────────────────────────────────────────────┤
│ │
│ SCROLL EVENTS (Old Way) INTERSECTION OBSERVER (Modern Way) │
│ ───────────────────────── ───────────────────────────────── │
│ │
│ User scrolls User scrolls │
│ │ │ │
│ ▼ ▼ │
│ scroll event fires Browser calculates │
│ (60+ times/sec) intersections internally │
│ │ │ │
│ ▼ ▼ │
│ YOUR CODE runs Callback fires ONLY when │
│ on EVERY scroll visibility ACTUALLY changes │
│ │ │ │
│ ▼ ▼ │
│ getBoundingClientRect() Entry object with all │
│ forces layout data pre-calculated │
│ │ │ │
│ ▼ ▼ │
│ 🐌 SLOW, janky scrolling 🚀 SMOOTH, 60fps scrolling │
│ │
└─────────────────────────────────────────────────────────────────────────────┘
```
**Intersection Observer runs off the main thread** and is optimized by the browser, making it dramatically more efficient.
---
## How to Create an Intersection Observer
Creating an observer involves two steps: instantiate the observer with a callback, then tell it what elements to observe.
### Basic Syntax
```javascript
// Step 1: Create the observer with a callback
const observer = new IntersectionObserver((entries, observer) => {
entries.forEach(entry => {
console.log(entry.target, 'isIntersecting:', entry.isIntersecting);
});
});
// Step 2: Tell it what to observe
const element = document.querySelector('.my-element');
observer.observe(element);
```
The **[`IntersectionObserver`](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/IntersectionObserver)** constructor takes two arguments:
1. **Callback function** — Called whenever observed elements cross visibility thresholds
2. **Options object** (optional) — Configures when and how intersections are detected
### The Callback Function
The callback receives two parameters:
```javascript
const callback = (entries, observer) => {
// entries: Array of IntersectionObserverEntry objects
// observer: The IntersectionObserver instance (useful for unobserving)
entries.forEach(entry => {
// entry.target — The observed element
// entry.isIntersecting — Is it currently visible?
// entry.intersectionRatio — How much is visible (0 to 1)
// entry.boundingClientRect — Element's size and position
// entry.intersectionRect — The visible portion's rectangle
// entry.rootBounds — The root element's rectangle
// entry.time — Timestamp when intersection was recorded
});
};
```
<Note>
**Important:** The callback fires once immediately when you call `observe()` on an element, reporting its current intersection state. This is intentional, so you know the initial visibility.
</Note>
### IntersectionObserverEntry Properties
Each entry in the callback provides detailed intersection data:
| Property | Type | Description |
|----------|------|-------------|
| `target` | Element | The element being observed |
| `isIntersecting` | boolean | `true` if element is currently intersecting root |
| `intersectionRatio` | number | Percentage visible (0.0 to 1.0) |
| `boundingClientRect` | DOMRect | Target element's bounding rectangle |
| `intersectionRect` | DOMRect | The visible portion's rectangle |
| `rootBounds` | DOMRect | Root element's bounding rectangle (or viewport) |
| `time` | number | Timestamp when intersection change was recorded |
```javascript
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
console.log('Element:', entry.target.id);
console.log('Is visible:', entry.isIntersecting);
console.log('Visibility %:', Math.round(entry.intersectionRatio * 100) + '%');
console.log('Position:', entry.boundingClientRect.top, 'px from top');
});
});
```
---
## Intersection Observer Options
The options object customizes when the callback fires:
```javascript
const options = {
root: null, // The viewport (or a specific container element)
rootMargin: '0px', // Margin around the root
threshold: 0 // When to trigger (0 = any pixel, 1 = fully visible)
};
const observer = new IntersectionObserver(callback, options);
```
### The `root` Option
The **root** defines the container used for checking visibility. It defaults to `null` (the browser viewport).
```javascript
// Observe visibility relative to the viewport (default)
const observer1 = new IntersectionObserver(callback, { root: null });
// Observe visibility relative to a scrollable container
const scrollContainer = document.querySelector('.scroll-container');
const observer2 = new IntersectionObserver(callback, { root: scrollContainer });
```
<Warning>
When using a custom root, the observed elements **must be descendants** of that root element. Otherwise, the observer won't detect intersections.
</Warning>
### The `rootMargin` Option
The **rootMargin** grows or shrinks the root's detection area. It works like CSS margins:
```javascript
// Start detecting 100px BEFORE element enters viewport (for preloading)
const observer = new IntersectionObserver(callback, {
rootMargin: '100px 0px' // top/bottom: 100px, left/right: 0px
});
// Shrink the detection area (element must be 50px inside viewport)
const observer2 = new IntersectionObserver(callback, {
rootMargin: '-50px' // All sides shrink by 50px
});
```
```
┌─────────────────────────────────────────────────────────────────────────────┐
│ rootMargin EXPLAINED │
├─────────────────────────────────────────────────────────────────────────────┤
│ │
│ rootMargin: '100px 0px 100px 0px' │
│ │
│ ┌──────────────────────┐ │
│ │ +100px margin (top) │ ← Elements detected HERE │
│ ├──────────────────────┤ │
│ │ │ │
│ │ VIEWPORT │ ← Actual visible area │
│ │ │ │
│ ├──────────────────────┤ │
│ │ +100px margin (bottom)│ ← Elements detected HERE │
│ └──────────────────────┘ │
│ │
│ Use positive margins for PRELOADING (lazy load before visible) │
│ Use negative margins for DELAYING (wait until fully in view) │
│ │
└─────────────────────────────────────────────────────────────────────────────┘
```
**Common rootMargin patterns:**
| Value | Use Case |
|-------|----------|
| `'100px'` | Preload images 100px before they enter viewport |
| `'-50px'` | Wait until element is 50px inside viewport |
| `'0px 0px -50%'` | Trigger when top half of element is visible |
| `'200px 0px 200px 0px'` | Large buffer for slow networks |
### The `threshold` Option
The **threshold** determines at what visibility percentage the callback fires. It can be a single number or an array:
```javascript
// Fire when ANY pixel becomes visible (default)
const observer1 = new IntersectionObserver(callback, { threshold: 0 });
// Fire when element is 50% visible
const observer2 = new IntersectionObserver(callback, { threshold: 0.5 });
// Fire when element is FULLY visible
const observer3 = new IntersectionObserver(callback, { threshold: 1.0 });
// Fire at multiple points (0%, 25%, 50%, 75%, 100%)
const observer4 = new IntersectionObserver(callback, {
threshold: [0, 0.25, 0.5, 0.75, 1.0]
});
```
```javascript
// Practical example: Track how much of an ad is viewed
const adObserver = new IntersectionObserver((entries) => {
entries.forEach(entry => {
const percentVisible = Math.round(entry.intersectionRatio * 100);
console.log(`Ad is ${percentVisible}% visible`);
if (entry.intersectionRatio >= 0.5) {
trackAdImpression(entry.target); // Count as "viewed" when 50%+ visible
}
});
}, { threshold: [0, 0.25, 0.5, 0.75, 1.0] });
```
---
## Observer Methods
The **[`IntersectionObserver`](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver)** instance has four methods:
### observe(element)
Start observing an element:
```javascript
const element = document.querySelector('.target');
observer.observe(element);
// Observe multiple elements
document.querySelectorAll('.lazy-image').forEach(img => {
observer.observe(img);
});
```
### unobserve(element)
Stop observing a specific element (useful after lazy loading):
```javascript
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
loadImage(entry.target);
observer.unobserve(entry.target); // Stop watching after loading
}
});
});
```
### disconnect()
Stop observing ALL elements:
```javascript
// Stop everything
observer.disconnect();
// Common pattern: cleanup when component unmounts
class LazyLoader {
constructor() {
this.observer = new IntersectionObserver(this.handleIntersect);
}
destroy() {
this.observer.disconnect();
}
}
```
### takeRecords()
Get any pending intersection records without waiting for the callback:
```javascript
// Rarely needed, but useful for synchronous access
const pendingEntries = observer.takeRecords();
pendingEntries.forEach(entry => {
// Process immediately
});
```
---
## Implementing Lazy Loading Images
Lazy loading is the most common use case for Intersection Observer. Here's a complete implementation:
### HTML Setup
```html
<!-- Use data-src instead of src for lazy images -->
<img class="lazy" data-src="hero-image.jpg" alt="Hero image">
<img class="lazy" data-src="product-1.jpg" alt="Product 1">
<img class="lazy" data-src="product-2.jpg" alt="Product 2">
<!-- Optional: Add a placeholder or low-quality preview -->
<img class="lazy"
src="placeholder.svg"
data-src="high-res-image.jpg"
alt="High resolution image">
```
### JavaScript Implementation
```javascript
// Create the lazy loading observer
const lazyImageObserver = new IntersectionObserver((entries, observer) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
const img = entry.target;
// Swap data-src to src
img.src = img.dataset.src;
// Optional: Handle srcset for responsive images
if (img.dataset.srcset) {
img.srcset = img.dataset.srcset;
}
// Remove lazy class (for CSS transitions)
img.classList.remove('lazy');
img.classList.add('loaded');
// Stop observing this image
observer.unobserve(img);
}
});
}, {
// Start loading 100px before image enters viewport
rootMargin: '100px 0px',
threshold: 0
});
// Observe all lazy images
document.querySelectorAll('img.lazy').forEach(img => {
lazyImageObserver.observe(img);
});
```
### CSS for Smooth Loading
```css
.lazy {
opacity: 0;
transition: opacity 0.3s ease-in;
}
.lazy.loaded {
opacity: 1;
}
/* Optional: Blur-up effect */
.lazy {
filter: blur(10px);
transition: filter 0.3s ease-in, opacity 0.3s ease-in;
}
.lazy.loaded {
filter: blur(0);
opacity: 1;
}
```
<Tip>
**Native lazy loading:** Modern browsers support `<img loading="lazy">` which handles basic lazy loading automatically. Use Intersection Observer when you need more control (custom thresholds, animations, or loading indicators).
</Tip>
---
## Building Infinite Scroll
Infinite scroll loads more content as the user approaches the bottom of the page:
```javascript
// The sentinel element sits at the bottom of your content
// <div id="sentinel"></div>
const sentinel = document.querySelector('#sentinel');
const contentContainer = document.querySelector('#content');
let page = 1;
let isLoading = false;
const infiniteScrollObserver = new IntersectionObserver(async (entries) => {
const entry = entries[0];
if (entry.isIntersecting && !isLoading) {
isLoading = true;
try {
// Fetch more content
const response = await fetch(`/api/posts?page=${++page}`);
const posts = await response.json();
if (posts.length === 0) {
// No more content - stop observing
infiniteScrollObserver.unobserve(sentinel);
sentinel.textContent = 'No more posts';
return;
}
// Append new content
posts.forEach(post => {
const article = createPostElement(post);
contentContainer.appendChild(article);
});
} catch (error) {
console.error('Failed to load more posts:', error);
} finally {
isLoading = false;
}
}
}, {
// Load more when sentinel is 200px from viewport
rootMargin: '200px'
});
infiniteScrollObserver.observe(sentinel);
function createPostElement(post) {
const article = document.createElement('article');
article.innerHTML = `
<h2>${post.title}</h2>
<p>${post.excerpt}</p>
`;
return article;
}
```
### Infinite Scroll HTML Structure
```html
<div id="content">
<!-- Initial posts loaded here -->
<article>...</article>
<article>...</article>
</div>
<!-- Sentinel must be AFTER all content -->
<div id="sentinel">Loading more...</div>
```
---
## Scroll-Triggered Animations
Trigger animations when elements scroll into view:
```javascript
const animatedElements = document.querySelectorAll('.animate-on-scroll');
const animationObserver = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
// Add animation class
entry.target.classList.add('animated');
// Optional: Only animate once
animationObserver.unobserve(entry.target);
}
});
}, {
threshold: 0.2, // Trigger when 20% visible
rootMargin: '0px 0px -50px 0px' // Trigger slightly before fully in view
});
animatedElements.forEach(el => animationObserver.observe(el));
```
### CSS Animations
```css
.animate-on-scroll {
opacity: 0;
transform: translateY(30px);
transition: opacity 0.6s ease-out, transform 0.6s ease-out;
}
.animate-on-scroll.animated {
opacity: 1;
transform: translateY(0);
}
/* Staggered animations */
.animate-on-scroll:nth-child(1) { transition-delay: 0.1s; }
.animate-on-scroll:nth-child(2) { transition-delay: 0.2s; }
.animate-on-scroll:nth-child(3) { transition-delay: 0.3s; }
```
### Reusable Animation Observer
```javascript
function createScrollAnimator(options = {}) {
const {
threshold = 0.2,
rootMargin = '0px',
animateOnce = true,
animatedClass = 'animated'
} = options;
return new IntersectionObserver((entries, observer) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
entry.target.classList.add(animatedClass);
if (animateOnce) {
observer.unobserve(entry.target);
}
} else if (!animateOnce) {
entry.target.classList.remove(animatedClass);
}
});
}, { threshold, rootMargin });
}
// Usage
const animator = createScrollAnimator({ threshold: 0.3, animateOnce: false });
document.querySelectorAll('[data-animate]').forEach(el => animator.observe(el));
```
---
## Sticky Header Detection
Detect when a header becomes sticky:
```javascript
// CSS: header { position: sticky; top: 0; }
const header = document.querySelector('header');
// Create a sentinel element just before the header
const sentinel = document.createElement('div');
sentinel.style.height = '1px';
header.before(sentinel);
const stickyObserver = new IntersectionObserver(([entry]) => {
// When sentinel is NOT intersecting, header is stuck
header.classList.toggle('is-stuck', !entry.isIntersecting);
}, {
threshold: 0,
rootMargin: '-1px 0px 0px 0px' // Trigger right at the top
});
stickyObserver.observe(sentinel);
```
```css
header {
position: sticky;
top: 0;
background: white;
transition: box-shadow 0.3s ease;
}
header.is-stuck {
box-shadow: 0 2px 10px rgba(0, 0, 0, 0.1);
}
```
---
## Section-Based Navigation
Highlight navigation links based on which section is visible:
```javascript
const sections = document.querySelectorAll('section[id]');
const navLinks = document.querySelectorAll('nav a');
const sectionObserver = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
// Remove active from all links
navLinks.forEach(link => link.classList.remove('active'));
// Add active to corresponding link
const activeLink = document.querySelector(`nav a[href="#${entry.target.id}"]`);
if (activeLink) {
activeLink.classList.add('active');
}
}
});
}, {
threshold: 0.5, // Section is "active" when 50% visible
rootMargin: '-20% 0px -20% 0px' // Focus on center of viewport
});
sections.forEach(section => sectionObserver.observe(section));
```
---
## The #1 Intersection Observer Mistake: Not Cleaning Up
The most common mistake is forgetting to disconnect observers, leading to memory leaks:
```javascript
// ❌ BAD: Observer keeps running forever
function setupLazyLoading() {
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
entry.target.src = entry.target.dataset.src;
// Forgot to unobserve!
}
});
});
document.querySelectorAll('.lazy').forEach(img => observer.observe(img));
}
// ✅ GOOD: Unobserve after loading
function setupLazyLoading() {
const observer = new IntersectionObserver((entries, obs) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
entry.target.src = entry.target.dataset.src;
obs.unobserve(entry.target); // Stop watching after load
}
});
});
document.querySelectorAll('.lazy').forEach(img => observer.observe(img));
// Return cleanup function for frameworks
return () => observer.disconnect();
}
```
### Framework Cleanup Patterns
```javascript
// React
useEffect(() => {
const observer = new IntersectionObserver(callback);
observer.observe(elementRef.current);
return () => observer.disconnect(); // Cleanup on unmount
}, []);
// Vue 3 Composition API
onMounted(() => {
observer = new IntersectionObserver(callback);
observer.observe(element.value);
});
onUnmounted(() => {
observer?.disconnect();
});
```
---
## Common Mistakes
<AccordionGroup>
<Accordion title="Mistake 1: Using scroll events for visibility detection">
```javascript
// ❌ WRONG: Scroll events are expensive
window.addEventListener('scroll', () => {
const rect = element.getBoundingClientRect();
if (rect.top < window.innerHeight) {
loadContent();
}
});
// ✅ RIGHT: Use Intersection Observer
const observer = new IntersectionObserver((entries) => {
if (entries[0].isIntersecting) {
loadContent();
observer.unobserve(entries[0].target);
}
});
observer.observe(element);
```
Scroll events fire constantly and block the main thread. Intersection Observer is optimized by the browser.
</Accordion>
<Accordion title="Mistake 2: Creating multiple observers for the same options">
```javascript
// ❌ WRONG: Creating a new observer for each element
images.forEach(img => {
const observer = new IntersectionObserver(callback);
observer.observe(img);
});
// ✅ RIGHT: One observer can watch many elements
const observer = new IntersectionObserver(callback);
images.forEach(img => observer.observe(img));
```
A single observer can efficiently track many elements with the same options.
</Accordion>
<Accordion title="Mistake 3: Forgetting the callback fires immediately">
```javascript
// ❌ WRONG: Assuming callback only fires on scroll
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
// This fires IMMEDIATELY for current state!
loadImage(entry.target);
});
});
// ✅ RIGHT: Check isIntersecting before acting
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
loadImage(entry.target);
}
});
});
```
The callback fires once immediately when you call `observe()` to report current state.
</Accordion>
<Accordion title="Mistake 4: Using threshold: 1 without accounting for partial visibility">
```javascript
// ❌ WRONG: threshold: 1 may never trigger for tall elements
const observer = new IntersectionObserver(callback, {
threshold: 1.0 // Requires 100% visibility
});
// If element is taller than viewport, it can NEVER be 100% visible!
// ✅ RIGHT: Use appropriate threshold or check intersectionRatio
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
// Use intersectionRatio for flexible visibility checking
if (entry.intersectionRatio >= 0.8 || entry.isIntersecting) {
handleVisibility(entry.target);
}
});
}, { threshold: [0, 0.25, 0.5, 0.75, 1.0] });
```
</Accordion>
<Accordion title="Mistake 5: Not handling the root element requirement">
```javascript
// ❌ WRONG: Observed element must be a descendant of root
const container = document.querySelector('.sidebar');
const observer = new IntersectionObserver(callback, { root: container });
// This element is NOT inside .sidebar - won't work!
observer.observe(document.querySelector('.main-content .item'));
// ✅ RIGHT: Observe elements inside the root
observer.observe(container.querySelector('.sidebar-item'));
```
</Accordion>
</AccordionGroup>
---
## Browser Support and Polyfill
Intersection Observer has excellent browser support (available since March 2019 in all major browsers):
```javascript
// Feature detection
if ('IntersectionObserver' in window) {
// Use Intersection Observer
const observer = new IntersectionObserver(callback);
} else {
// Fallback for very old browsers
// Load polyfill or use scroll events
}
```
For legacy browser support, use the [official polyfill](https://github.com/w3c/IntersectionObserver/tree/main/polyfill):
```html
<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
```
---
## Key Takeaways
<Info>
**The key things to remember:**
1. **Intersection Observer replaces scroll events** — It's more performant and runs off the main thread
2. **The callback fires immediately** — When you call `observe()`, it reports current visibility state
3. **Use `isIntersecting` to check visibility** — Don't assume the callback means "now visible"
4. **One observer, many elements** — A single observer can efficiently watch multiple targets
5. **Clean up with `unobserve()` or `disconnect()`** — Prevent memory leaks, especially after lazy loading
6. **`rootMargin` enables preloading** — Use positive margins to detect elements before they're visible
7. **`threshold` controls precision** — Use arrays for fine-grained visibility tracking
8. **Always handle the null root** — Defaults to viewport, but custom roots must contain observed elements
9. **Combine with CSS for smooth animations** — Observer triggers classes, CSS handles transitions
10. **Consider native `loading="lazy"`** — For simple image lazy loading, the native attribute may suffice
</Info>
---
## Test Your Knowledge
<AccordionGroup>
<Accordion title="Question 1: Why is Intersection Observer better than scroll events for visibility detection?">
**Answer:** Intersection Observer is better because:
1. **Runs off the main thread** — Doesn't block JavaScript execution
2. **Browser-optimized** — Efficiently batches calculations
3. **No layout thrashing** — Doesn't force `getBoundingClientRect()` recalculations
4. **Built-in throttling** — Fires only when visibility actually changes
5. **Works with iframes** — Can detect visibility in cross-origin contexts
Scroll events fire 60+ times per second and require manual throttling, while Intersection Observer only fires when relevant visibility changes occur.
</Accordion>
<Accordion title="Question 2: What does rootMargin: '-50px' do?">
**Answer:** `rootMargin: '-50px'` shrinks the detection area by 50px on all sides.
This means an element must be at least 50px inside the viewport before it's considered "intersecting." It's useful for:
- Triggering animations when elements are fully in view
- Ensuring content is clearly visible before acting
- Avoiding edge-case flickering near viewport boundaries
```javascript
// Element must be 50px inside viewport to trigger
const observer = new IntersectionObserver(callback, {
rootMargin: '-50px'
});
```
</Accordion>
<Accordion title="Question 3: When would you use threshold: [0, 0.25, 0.5, 0.75, 1]?">
**Answer:** Use multiple thresholds when you need to track progressive visibility, such as:
- **Ad viewability tracking** — Count impressions at different visibility levels
- **Video playback** — Pause at 25% visible, play at 75% visible
- **Progress indicators** — Show how much of an article has been read
- **Parallax effects** — Adjust animations based on scroll position
```javascript
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
const percent = Math.round(entry.intersectionRatio * 100);
updateProgressBar(percent);
});
}, { threshold: [0, 0.25, 0.5, 0.75, 1] });
```
</Accordion>
<Accordion title="Question 4: Why should you call unobserve() after lazy loading an image?">
**Answer:** You should call `unobserve()` because:
1. **Memory efficiency** — The observer no longer needs to track this element
2. **Performance** — Fewer elements to check means faster intersection calculations
3. **Prevents double-loading** — Without unobserving, the image could be "loaded" multiple times
4. **Clean architecture** — Once lazy loading is complete, the observer's job is done
```javascript
const observer = new IntersectionObserver((entries, obs) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
entry.target.src = entry.target.dataset.src;
obs.unobserve(entry.target); // Clean up!
}
});
});
```
</Accordion>
<Accordion title="Question 5: What happens if you use a custom root that doesn't contain the observed element?">
**Answer:** The observer **won't detect any intersections**. The observed element must be a descendant of the root element.
```javascript
const sidebar = document.querySelector('.sidebar');
const observer = new IntersectionObserver(callback, { root: sidebar });
// ❌ This won't work - element is outside sidebar
observer.observe(document.querySelector('.main .card'));
// ✅ This works - element is inside sidebar
observer.observe(sidebar.querySelector('.sidebar-item'));
```
Always ensure observed elements are descendants of the root, or use `root: null` for viewport detection.
</Accordion>
</AccordionGroup>
---
## Related Concepts
<CardGroup cols={2}>
<Card title="Mutation Observer" icon="eye" href="/beyond/concepts/mutation-observer">
Watch for DOM changes like added/removed elements and attribute modifications.
</Card>
<Card title="Resize Observer" icon="expand" href="/beyond/concepts/resize-observer">
Detect when elements change size without polling or resize events.
</Card>
<Card title="Performance Observer" icon="gauge" href="/beyond/concepts/performance-observer">
Monitor performance metrics like Long Tasks, layout shifts, and resource timing.
</Card>
<Card title="Event Loop" icon="arrows-rotate" href="/concepts/event-loop">
Understand how JavaScript handles async operations and when callbacks fire.
</Card>
</CardGroup>
---
## Resources
### Reference
<CardGroup cols={2}>
<Card title="Intersection Observer API — MDN" icon="book" href="https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API">
Complete API documentation covering all options, methods, and the IntersectionObserverEntry interface. The authoritative reference for browser behavior and edge cases.
</Card>
<Card title="IntersectionObserver Interface — MDN" icon="book" href="https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver">
Detailed reference for the IntersectionObserver constructor, properties (root, rootMargin, thresholds), and methods (observe, unobserve, disconnect, takeRecords).
</Card>
</CardGroup>
### Articles
<CardGroup cols={2}>
<Card title="A Few Functional Uses for Intersection Observer — CSS-Tricks" icon="newspaper" href="https://css-tricks.com/a-few-functional-uses-for-intersection-observer-to-know-when-an-element-is-in-view/">
Practical walkthrough of real-world Intersection Observer use cases including lazy loading, content visibility tracking, and auto-pausing videos. Great code examples with detailed explanations.
</Card>
<Card title="Intersection Observer v2: Trust is good, observation is better — web.dev" icon="newspaper" href="https://web.dev/articles/intersectionobserver-v2">
Covers the advanced Intersection Observer v2 API for tracking actual visibility (not just intersection). Essential reading for ad viewability and fraud prevention use cases.
</Card>
<Card title="Scroll Animations with Intersection Observer — freeCodeCamp" icon="newspaper" href="https://www.freecodecamp.org/news/scroll-animations-with-javascript-intersection-observer-api/">
Step-by-step guide to implementing scroll-triggered animations. Covers reveal-on-scroll effects, CSS transitions, and best practices for performant animations.
</Card>
<Card title="The Complete Guide to Lazy Loading Images — CSS-Tricks" icon="newspaper" href="https://css-tricks.com/the-complete-guide-to-lazy-loading-images/">
Comprehensive guide covering all lazy loading approaches including Intersection Observer, native loading="lazy", and fallback strategies. Includes performance considerations.
</Card>
</CardGroup>
### Videos
<CardGroup cols={2}>
<Card title="Learn Intersection Observer In 15 Minutes — Web Dev Simplified" icon="video" href="https://www.youtube.com/watch?v=2IbRtjez6ag">
Clear, beginner-friendly introduction covering observer basics, all configuration options, and a practical infinite scroll implementation. Perfect starting point.
</Card>
<Card title="Introduction to Intersection Observer — Kevin Powell" icon="video" href="https://www.youtube.com/watch?v=T8EYosX4NOo">
Explains why Intersection Observer is better than scroll events, with visual demonstrations of how intersection detection works. Great for understanding the fundamentals.
</Card>
<Card title="Lazy Load Images with Intersection Observer — Fireship" icon="video" href="https://www.youtube.com/watch?v=aUjBvuUdkhg">
Quick, practical tutorial showing how to implement lazy-loaded images. Covers data attributes, viewport detection, and unobserving after load.
</Card>
<Card title="How to Lazy Load Images — Kevin Powell" icon="video" href="https://www.youtube.com/watch?v=mC93zsEsSrg">
Detailed lazy loading implementation with rootMargin for pre-loading and practical tips for production use. Great follow-up after learning the basics.
</Card>
</CardGroup>