afarkas/lazysizes

★ 17,732JavaScriptAudience · developerComplexity · 2/5Setup · easy

Mindmap

mindmap
  root((repo))
    What it does
      Defer image loading
      Improve page speed
    How it works
      Watches scroll position
      Swaps in real image URL
    Features
      Responsive images
      Iframe support
      Dynamic content
    Setup
      Add script tag
      npm install
      Add lazyload class

mindmap root((repo)) What it does Defer image loading Improve page speed How it works Watches scroll position Swaps in real image URL Features Responsive images Iframe support Dynamic content Setup Add script tag npm install Add lazyload class

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Things people build with this

USE CASE 1

Speed up a page with many images by loading only what is visible on screen first, then fetching the rest as the user scrolls.

USE CASE 2

Add lazy loading to embedded video iframes so they do not slow down the initial page load.

USE CASE 3

Automatically pick the right image resolution for different screen sizes using lazysizes responsive image support.

USE CASE 4

Enable lazy loading for images that appear dynamically inside carousels or infinite scroll without extra configuration.

Tech stack

JavaScript

Getting it running

Difficulty · easy Time to first run · 5min

In plain English

lazysizes is a JavaScript library that delays loading images and iframes on a webpage until they are about to come into view, rather than downloading everything at once when the page first opens. This technique is called lazy loading, and it makes pages load faster because the browser skips images the user may never scroll to. The problem it solves is page performance: a page with dozens of images will spend bandwidth downloading images at the bottom that most visitors never see. By deferring those downloads until the user scrolls close to them, the page becomes usable much faster on slower connections. Setup requires almost no configuration. You add a script tag, then change image tags to use a class called "lazyload" and store the real image URL in a data-src attribute instead of the regular src. lazysizes automatically watches for when those elements approach the visible area and swaps in the real URL. It handles responsive images (different sizes served depending on screen width), iframes like embedded videos, and elements that become visible through JavaScript interactions like carousels or infinite scrolling, not just from scrolling the page. A notable feature is automatic calculation of the sizes attribute for responsive images, which tells the browser how large the image will be displayed so it can pick the right resolution. The library is also SEO-friendly: search engine crawlers that do not scroll will see all images immediately. You install it via npm or a script tag. It is written in JavaScript and extends via optional plugins. The full README is longer than what was provided.

Copy-paste prompts

Prompt 1

I want to add lazysizes to my HTML page to lazy-load all product images. Show me the exact script tag and the HTML changes needed to convert my img tags.

Prompt 2

My page uses srcset for responsive images. How do I set up lazysizes to handle them and auto-calculate the sizes attribute?

Prompt 3

I have a carousel that adds images dynamically with JavaScript. How do I configure lazysizes to detect and lazy-load those images too?

Prompt 4

How do I use lazysizes to lazy-load background images set in CSS rather than img tags?

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