Highly performant, light 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 <img>, <picture>, iframes, videos, audios, responsive images, background images and multiple background images etc.
• even supports LQIP (Low Quality Image Placeholder)
• is completely free and open source.
• it will reload when the valid attributes change.

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

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• Demo
• Background
• Install
• Usage
• Example with picture tag
• Browser Support
• FAQs
• Contribute
• Changelog
• License

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.

# 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.

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
    enableAutoReload: true // it will reload the new image when validating attributes changes
});
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 would like to extend the loaded state of elements, you can add the loaded option:

Note: The "data-loaded"="true" attribute is used by lozad to determine if an element has been previously loaded.

lozad('.lozad', {
    loaded: function(el) {
        // Custom implementation on a loaded element
        el.classList.add('loaded');
    }
});

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>

for use with multiple background images

<!-- multiple background image example -->
<div class="lozad" data-background-image="path/to/first/image,path/to/second/image,path/to/third/image">
</div>

for use with responsive background images (image-set)

<!-- responsive background image-set example -->
<div class="lozad" data-background-image-set="url('photo.jpg') 1x, url('photo@2x.jpg') 2x">
</div>

To change the delimiter that splits background images:

<!-- custom delimiter for background images example -->
<div
  class="lozad"
  data-background-image="/first/custom,image,path/image.png-/second/custom,image,path/image.png"
  data-background-delimiter="-"
>
</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);

Sometimes image loading takes a long time. For this case, you can add a placeholder background:

<img class="lozad" data-placeholder-background="red" data-src="image.png">

Lozad sets a placeholder background color of img element and users will see the fallback till the image loads.

Create a broken picture element structure.

IE browser don't support picture tag! You need to set data-iesrc attribute (only for your picture tags) with source for IE browser

data-alt attribute can be added to picture tag for use in alt attribute of lazy-loaded images

<!-- For an element to be caught, add a block type that is different from the inline and some min-height for correct caught into view -->
<picture class="lozad" style="display: block; min-height: 1rem" data-iesrc="images/thumbs/04.jpg" data-alt="">
    <source srcset="images/thumbs/04.jpg" media="(min-width: 1280px)">
    <source srcset="images/thumbs/05.jpg" media="(min-width: 980px)">
    <source srcset="images/thumbs/06.jpg" media="(min-width: 320px)">
    <!-- NO img element -->
    <!-- instead of img element, there will be the last source with the minimum dimensions -->
    <!-- for disabled JS you can set <noscript><img src="images/thumbs/04.jpg" alt=""></noscript> -->
</picture>

When lozad loads this picture element, it will fix it.

If you want to use image placeholder (like low quality image placeholder), you can set a temporary img tag inside your picture tag. It will be removed when lozad loads the picture element.

<picture class="lozad" style="display: block; min-height: 1rem" data-iesrc="images/thumbs/04.jpg" data-alt="">
    <source srcset="images/thumbs/04.jpg" media="(min-width: 1280px)">
    <source srcset="images/thumbs/05.jpg" media="(min-width: 980px)">
    <source srcset="images/thumbs/06.jpg" media="(min-width: 320px)">
    <!-- you can define a low quality image placeholder that will be removed when the picture is loaded -->
    <img src="data:image/jpeg;base64,/some_lqip_in_base_64==">
</picture>

<video class="lozad" data-poster="images/backgrounds/video-poster.jpeg">
    <source data-src="video/mov_bbb.mp4" type="video/mp4">
    <source data-src="video/mov_bbb.ogg" type="video/ogg">
</video>

<iframe data-src="embed.html" class="lozad"></iframe>

That's all, just add the lozad class.

<div data-toggle-class="active" class="lozad">
    <!-- content -->
</div>

The active class will be toggled on the element when it enters the browser’s viewport.

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

For IE11 support, please make use of these polyfills.

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

Interested in contributing features and fixes?

Read more on contributing.

See the Changelog

MIT © Apoorv Saxena