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Highly performant, light ~0.5kb and configurable lazy loader in pure JS with no dependencies for images, iframes and more, using IntersectionObserver API

lozad.js lazy loading javascript library

Lozad.js:

  • lazy loads elements performantly using pure JavaScript,
  • is a light-weight library, just 535 bytes minified & gzipped,
  • has NO DEPENDENCIES :)
  • allows lazy loading of dynamically added elements as well.

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.

Table of Contents

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

Get the library using one of the following ways:

  1. GitHub

Full build

  1. Yarn : yarn add lozad

  2. Bower : bower install lozad

  3. npm: npm install --save lozad

  4. CDN: https://cdn.jsdelivr.net/npm/lozad

Now include the library in the HEAD tag of your page:

">
<script type="text/javascript" src="lozad.js"></script>

or

">
<script type="text/javascript" src="lozad.min.js"></script>

Note: lozad.js supports AMD and commonJS module pattern out of the box.

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 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:

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.dataset.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

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

Contribute

Interested in contributing features and fixes?

Read more on contributing.

Changelog

See the Changelog

License

MIT © Apoorv Saxena

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Highly performant, light ~0.5kb and configurable lazy loader in pure JS with no dependencies for images, iframes and more

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