auto-sizes

Automatic sizes attribute calculation for responsive images.

Note: Extracted from lazysizes by Alexander Farkas. This is a lightweight, focused version containing only the automatic sizes calculation feature (~3.3KB minified vs 7.8KB for full lazysizes).

Features

🎯 Automatic calculation of sizes attribute from element width
📦 Tiny size (~2.3KB minified, ~1.1KB gzipped)
🚀 High performance with RAF batching and debounced resize
🖼️ Full <picture> support — updates <source sizes="auto"> siblings too
🔄 Auto-updates on window resize
🎨 Customizable via events and CSS classes
📱 Works with any responsive image setup
✨ Modern ES6 code for current browsers

Quick Start

Recommended: Load the script in <head> or before your images for optimal performance.

<script type="module">
    import 'auto-sizes';
    // That's it! The library auto-initializes and calculates sizes.
</script>

<img
  class="autosizes"
  sizes="auto"
  srcset="image-300.jpg 300w, image-600.jpg 600w, image-900.jpg 900w"
  src="image-300.jpg"
  alt="Responsive image"
/>

Result:

<img
  class="autosizes autosized"
  sizes="450px"
  srcset="..."
  alt="..."
/>

Installation

npm

npm install auto-sizes

CDN

<!-- unpkg (recommended) -->
<script type="module" src="https://unpkg.com/auto-sizes"></script>

<!-- jsDelivr -->
<script type="module" src="https://cdn.jsdelivr.net/npm/auto-sizes"></script>

<!-- Minified version -->
<script src="https://unpkg.com/auto-sizes/autosizes.min.js"></script>

Import Methods

// ES modules (auto-executes)
import 'auto-sizes';

// CommonJS
require('auto-sizes');

// With explicit import for manual control
import autoSizes from 'auto-sizes';
autoSizes.init();
autoSizes.updateAll();

Usage

Basic Image

<img
  class="autosizes"
  sizes="auto"
  srcset="image-300.jpg 300w, image-600.jpg 600w, image-900.jpg 900w"
  src="image-300.jpg"
  alt="Responsive image"
/>

The library will:

Find elements with class="autosizes" and sizes="auto"
Calculate width based on rendered size
Set sizes="450px" (actual calculated value)
Add autosized class for styling hooks

Picture Element

Add autosizes class to the <img> and sizes="auto" to every <source> that needs automatic sizing. The library sets the calculated width on the <img> and on all <source sizes="auto"> siblings:

<picture>
  <source sizes="auto" srcset="desktop-800.jpg 800w, desktop-1200.jpg 1200w" media="(min-width: 768px)" />
  <source sizes="auto" srcset="mobile-400.jpg 400w, mobile-600.jpg 600w" />
  <img class="autosizes" sizes="auto" srcset="fallback-600.jpg 600w" src="fallback-600.jpg" alt="..." />
</picture>

After calculation:

<picture>
  <source sizes="450px" srcset="desktop-800.jpg 800w, desktop-1200.jpg 1200w" media="(min-width: 768px)" />
  <source sizes="450px" srcset="mobile-400.jpg 400w, mobile-600.jpg 600w" />
  <img class="autosizes autosized" sizes="450px" srcset="fallback-600.jpg 600w" src="fallback-600.jpg" alt="..." />
</picture>

How it works: When the browser selects a <source> based on its media query, it uses that source's own sizes attribute to pick a srcset variant — not the <img>'s sizes. Without sizes="auto" on <source>, browsers default to 100vw and download the largest available image regardless of the actual rendered width.

Sources with explicit hint values are left untouched — useful when you know the layout width at template time:

<picture>
  <!-- explicit: library does not override this -->
  <source sizes="(min-width: 1024px) 380px, 100vw" srcset="card-380.jpg 380w, card-760.jpg 760w" media="(min-width: 1024px)" />
  <!-- auto: library measures and sets the calculated width -->
  <source sizes="auto" srcset="mobile-320.jpg 320w, mobile-640.jpg 640w" />
  <img class="autosizes" sizes="auto" srcset="fallback.jpg 320w" src="fallback.jpg" alt="..." />
</picture>

Styling with `autosized` Class

/* Fade in after calculation */
img.autosizes:not(.autosized) {
  opacity: 0.3;
}

img.autosized {
  opacity: 1;
  transition: opacity 0.3s;
}

Configuration

Set window.autoSizesConfig before loading the library:

window.autoSizesConfig = {
  minSize: 40,                        // Traverse up DOM if element < 40px
  targetElementClass: 'autosizes',    // Class to identify elements
  processedElementClass: 'autosized', // Class added after processing
  sizesAttr: 'sizes',                 // Attribute to check for "auto"
  init: true,                         // Auto-initialize
  resizeDebounce: 99                  // Resize debounce delay (ms)
};

Attribute Prefix Support

Use prefixed attributes like data-sizes. Both the prefixed and base attributes will be set:

window.autoSizesConfig = {
  sizesAttr: 'data-sizes'
};

<!-- Input -->
<img class="autosizes" data-sizes="auto" srcset="..." />

<!-- Output - both attributes set -->
<img class="autosizes autosized" data-sizes="450px" sizes="450px" srcset="..." />

Useful for:

Framework compatibility (Vue, React data attributes)
Preserving original attribute for debugging
Tools that expect both attributes

Events

beforeSizesUpdate

Fired before sizes is set. Modify width or prevent update:

image.addEventListener('beforeSizesUpdate', (e) => {
  // Round to nearest 100px
  e.detail.width = Math.ceil(e.detail.width / 100) * 100;

  // Or prevent update
  // e.preventDefault();
});

Event detail:

width (number) - Calculated width in pixels (modifiable)
dataAttr (boolean) - Whether triggered by data attribute

afterSizesUpdate

Fired after sizes is set:

image.addEventListener('afterSizesUpdate', (e) => {
  console.log('Sizes set to:', e.detail.sizes); // "450px"
});

Event detail:

width (number) - Final width value
sizes (string) - The sizes value that was set

API

// Update all elements
autoSizes.updateAll();

// Update specific element
autoSizes.updateElem(imageElement);

// Initialize manually (if auto-init disabled)
autoSizes.init();

// Access config
autoSizes.cfg.minSize = 50;

Browser Support

Modern browsers with:

srcset and sizes attributes
classList API
requestAnimationFrame

Tested: Chrome/Edge 90+, Firefox 88+, Safari 14+, iOS Safari, Chrome Mobile

Differences from lazysizes

This is a focused extraction of only the sizes calculation feature from lazysizes.

What's Removed ❌

Removed	Why
Lazy loading	Use native `loading="lazy"` or your own solution
Intersection Observer	Only needed for lazy loading
MutationObserver	Reduced overhead; use `updateAll()` for dynamic content
Scroll events	Only resize monitored
IE11 support	Modern browsers only
Plugin system	Simplified to core functionality

What's Kept ✅

✅ Auto sizes calculation algorithm
✅ Picture element support
✅ Smart width detection with DOM traversal
✅ RAF batching for performance
✅ Debounced resize handling
✅ Customization events

New Features 🆕

sizes="auto" requirement - Explicit opt-in per element
Prefix support - data-sizes="auto" sets both attributes
Full <picture> handling - Calculates and sets sizes on <source sizes="auto"> siblings (browsers use source sizes, not <img> sizes, when a media condition matches)
Modern ES6 - Cleaner, more maintainable code
Simplified API - updateAll() and updateElem()

Size Comparison

Metric	lazysizes	auto-sizes	Savings
Source code	19.9 KB / 813 lines	8.6 KB / 371 lines	57%
Minified	7.8 KB	~2.3 KB	71%
Gzipped	~3.5 KB	~1.1 KB	69%

When to Use Each

Use lazysizes if:

Need lazy loading (images, iframes, scripts)
Need LQIP or progressive loading
Need IE11 support
Want extensive plugin ecosystem

Use auto-sizes if:

Only need sizes calculation
Already have lazy loading solution
Use native loading="lazy"
Want minimal bundle size
Target modern browsers only

Code Example

<!-- lazysizes: uses data-sizes with lazy loading -->
<img class="lazyload" data-sizes="auto" data-srcset="..." data-src="..." />

<!-- auto-sizes: uses sizes="auto" without lazy loading -->
<img class="autosizes" sizes="auto" srcset="..." src="..." />

Both share the same core algorithm, originally developed by Alexander Farkas for lazysizes.

Credits

This library extracts the automatic sizes calculation feature from lazysizes by Alexander Farkas.

If you need full lazy loading features, we highly recommend using lazysizes. It provides:

Lazy loading for images, iframes, and scripts
LQIP and progressive loading
Responsive image polyfills
Extensive plugin ecosystem
Battle-tested performance optimizations

License

MIT

Name		Name	Last commit message	Last commit date
Latest commit History 20 Commits
.gitignore		.gitignore
.npmignore		.npmignore
CHANGELOG.md		CHANGELOG.md
LICENSE		LICENSE
README.md		README.md
RELEASE_NOTES.md		RELEASE_NOTES.md
autosizes.js		autosizes.js
autosizes.min.js		autosizes.min.js
index.js		index.js
package.json		package.json
test-debug.html		test-debug.html
test-resize.html		test-resize.html
test-simple.html		test-simple.html
test.html		test.html

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auto-sizes

Features

Quick Start

Installation

npm

CDN

Import Methods

Usage

Basic Image

Picture Element

Styling with autosized Class

Configuration

Attribute Prefix Support

Events

beforeSizesUpdate

afterSizesUpdate

API

Browser Support

Differences from lazysizes

What's Removed ❌

What's Kept ✅

New Features 🆕

Size Comparison

When to Use Each

Code Example

Credits

License

About

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Styling with `autosized` Class

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