lozad

Lozad.js

Highly performant, light ~0.7kb and configurable lazy loader in pure JS with no dependencies for images, iframes and more, using IntersectionObserver API

Lozad.js:

• lazy loads elements performantly using pure JavaScript,
• is a light-weight library, just minified & gzipped,
• has NO DEPENDENCIES :)
• allows lazy loading of dynamically added elements as well,
• supports responsive images and background images

It is written with an aim to lazy load images, iframes, ads, videos or any other element using the recently added Intersection Observer API with tremendous performance benefits.

Featured in:

• Product Hunt
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• David Walsh
• Codrops

Table of Contents

• Demo
• Background
• Install
• Usage
• Browser Support
• FAQs
• Contribute
• Changelog
• License

Yet another Lazy Loading JavaScript library, why?

Existing lazy loading libraries hook up to the scroll event or use a periodic timer and call getBoundingClientRect() on elements that need to be lazy loaded. This approach, however, is painfully slow as each call to getBoundingClientRect() forces the browser to re-layout the entire page and will introduce considerable jank to your website.

Making this more efficient and performant is what IntersectionObserver is designed for, and it’s landed in Chrome 51. IntersectionObservers let you know when an observed element enters or exits the browser’s viewport.

Install

# You can install lozad with npm
$ npm install --save lozad

# Alternatively you can use Yarn.
$ yarn add lozad

# Another option is to use Bower.
$ bower install lozad

Then with a module bundler like rollup or webpack, use as you would anything else:

// using ES6 modules
import lozad from 'lozad'

// using CommonJS modules
var lozad = require('lozad')

Or load via CDN and include in the head tag of your page.

<script type="text/javascript" src="https://cdn.jsdelivr.net/npm/lozad/dist/lozad.min.js"></script>

When loading from CDN, you can find the library on window.lozad.

Usage

In HTML, add an identifier to the element (default selector identified is lozad class):

<img class="lozad" data-src="image.png" />

All you need to do now is just instantiate Lozad as follows:

const observer = lozad(); // lazy loads elements with default selector as '.lozad'
observer.observe();

or with a DOM Element reference:

const el = document.querySelector('img');
const observer = lozad(el); // passing a `NodeList` (e.g. `document.querySelectorAll()`) is also valid
observer.observe();

or with custom options:

const observer = lozad('.lozad', {
    rootMargin: '10px 0px', // syntax similar to that of CSS Margin
    threshold: 0.1 // ratio of element convergence
});
observer.observe();

Reference:

• IntersectionObserver options: rootMargin
• IntersectionObserver options: thresholds

or if you want to give custom function definition to load element:

lozad('.lozad', {
    load: function(el) {
        console.log('loading element');

        // Custom implementation to load an element
        // e.g. el.src = el.getAttribute('data-src');
    }
});

If you want to lazy load dynamically added elements:

const observer = lozad();
observer.observe();

// ... code to dynamically add elements
observer.observe(); // observes newly added elements as well

for use with responsive images

<!-- responsive image example -->
<img class="lozad" data-src="image.png" data-srcset="image.png 1000w, image-2x.png 2000w" />

for use with background images

<!-- background image example -->
<div class="lozad" data-background-image="image.png">
</div>

If you want to load the images before they appear:

const observer = lozad();
observer.observe();

const coolImage = document.querySelector('.image-to-load-first')
// ... trigger the load of a image before it appears on the viewport
observer.triggerLoad(coolImage);

Browser Support

Available in latest browsers. If browser support is not available, then make use of this polyfill.

FAQs

Checkout the FAQ Wiki for some common gotchas to be aware of while using lozad.js

Sites using Lozad.js

• gis.utah.gov
• [Add your site] - See Contributing section

Support Lozad.js

Lozad.js is completely free and open-source. If you find it useful, you can show your support by starring it or sharing it in your social network or by simply letting me know how much you 💖 it by tweeting to @Apoorv_Saxena.

Contribute

Interested in contributing features and fixes?

Read more on contributing.

Changelog

See the Changelog

License

MIT © Apoorv Saxena