300 lines
11 KiB
Markdown
Executable File
300 lines
11 KiB
Markdown
Executable File
# Lozad.js [](https://badge.fury.io/js/lozad) [](https://travis-ci.org/ApoorvSaxena/lozad.js) [](https://www.npmjs.com/package/lozad) [](https://www.jsdelivr.com/package/npm/lozad)
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> Highly performant, light and configurable lazy loader in pure JS with no dependencies for images, iframes and more, using IntersectionObserver API
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**Lozad.js**:
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- lazy loads elements performantly using pure JavaScript,
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- is a light-weight library, just [](https://cdn.jsdelivr.net/npm/lozad) minified & gzipped,
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- has NO DEPENDENCIES :)
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- allows lazy loading of dynamically added elements as well,
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- supports <img>, <picture>, iframes, videos, audios, responsive images, background images and multiple background images etc.
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- even supports LQIP (Low Quality Image Placeholder)
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- is completely free and open source.
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It is written with an aim to lazy load images, iframes, ads, videos or any other element using the recently added [Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) with tremendous performance benefits.
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## Featured in:
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- [Web | Google Developers](https://developers.google.com/web/fundamentals/performance/lazy-loading-guidance/images-and-video/)
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- [Product Hunt](https://www.producthunt.com/posts/lozad-js)
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- [Reddit](https://www.reddit.com/r/webdev/comments/6zg1x0/highly_performant_light_05kb_and_configurable/)
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- [CSS Tricks](https://css-tricks.com/lozad-js-performant-lazy-loading-images)
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- [David Walsh](https://twitter.com/davidwalshblog/status/915319510646829061)
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- [Codrops](https://twitter.com/Apoorv_Saxena/status/906586828265758720)
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- [SitePoint](https://www.sitepoint.com/five-techniques-lazy-load-images-website-performance/)
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## Brands using Lozad.js:
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and many more...
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## Table of Contents
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- [Demo](https://apoorv.pro/lozad.js/demo/)
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- [Background](#yet-another-lazy-loading-javascript-library-why)
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- [Install](#install)
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- [Usage](#usage)
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- [Example with picture tag](#example-with-picture-tag)
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- [Browser Support](#browser-support)
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- [FAQs](#faqs)
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- [Contribute](#contribute)
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- [Changelog](#changelog)
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- [License](#license)
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## Yet another Lazy Loading JavaScript library, why?
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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.
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Making this more efficient and performant is what [IntersectionObserver](https://developers.google.com/web/updates/2016/04/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.
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## Install
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```sh
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# You can install lozad with npm
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$ npm install --save lozad
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# Alternatively you can use Yarn
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$ yarn add lozad
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# Another option is to use Bower
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$ bower install lozad
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```
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Then with a module bundler like rollup or webpack, use as you would anything else:
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```javascript
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// using ES6 modules
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import lozad from 'lozad'
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// using CommonJS modules
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var lozad = require('lozad')
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```
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Or load via **CDN** and include in the `head` tag of your page.
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```html
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<script type="text/javascript" src="https://cdn.jsdelivr.net/npm/lozad/dist/lozad.min.js"></script>
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```
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When loading from CDN, you can find the library on `window.lozad`.
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---
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<a href="https://www.patreon.com/apoorvsaxena">
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<img src="https://c5.patreon.com/external/logo/become_a_patron_button@2x.png" width="160">
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</a>
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## Usage
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In HTML, add an identifier to the element (default selector identified is `lozad` class):
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```html
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<img class="lozad" data-src="image.png" />
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```
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All you need to do now is just instantiate Lozad as follows:
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```js
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const observer = lozad(); // lazy loads elements with default selector as '.lozad'
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observer.observe();
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```
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or with a DOM `Element` reference:
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```js
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const el = document.querySelector('img');
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const observer = lozad(el); // passing a `NodeList` (e.g. `document.querySelectorAll()`) is also valid
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observer.observe();
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```
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or with custom options:
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```js
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const observer = lozad('.lozad', {
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rootMargin: '10px 0px', // syntax similar to that of CSS Margin
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threshold: 0.1 // ratio of element convergence
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});
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observer.observe();
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```
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Reference:
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- [IntersectionObserver options: rootMargin](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/rootMargin)
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- [IntersectionObserver options: thresholds](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/thresholds)
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or if you want to give custom function definition to load element:
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```js
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lozad('.lozad', {
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load: function(el) {
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console.log('loading element');
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// Custom implementation to load an element
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// e.g. el.src = el.getAttribute('data-src');
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}
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});
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```
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If you would like to extend the `loaded` state of elements, you can add the loaded option:
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> **Note**: The `"data-loaded"="true"` attribute is used by lozad to determine if an element has been previously loaded.
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```js
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lozad('.lozad', {
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loaded: function(el) {
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// Custom implementation on a loaded element
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el.classList.add('loaded');
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}
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});
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```
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If you want to lazy load dynamically added elements:
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```js
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const observer = lozad();
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observer.observe();
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// ... code to dynamically add elements
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observer.observe(); // observes newly added elements as well
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```
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for use with responsive images
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```html
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<!-- responsive image example -->
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<img class="lozad" data-src="image.png" data-srcset="image.png 1000w, image-2x.png 2000w" />
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```
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for use with background images
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```html
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<!-- background image example -->
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<div class="lozad" data-background-image="image.png">
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</div>
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```
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for use with multiple background images
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```html
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<!-- multiple background image example -->
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<div class="lozad" data-background-image="path/to/first/image,path/to/second/image,path/to/third/image">
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</div>
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```
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for use with responsive background images (image-set)
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```html
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<!-- responsive background image-set example -->
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<div class="lozad" data-background-image-set="url('photo.jpg') 1x, url('photo@2x.jpg') 2x">
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</div>
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```
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To change the delimiter that splits background images:
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```html
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<!-- custom delimiter for background images example -->
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<div
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class="lozad"
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data-background-image="/first/custom,image,path/image.png-/second/custom,image,path/image.png"
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data-background-delimiter="-"
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>
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</div>
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```
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If you want to load the images before they appear:
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```js
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const observer = lozad();
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observer.observe();
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const coolImage = document.querySelector('.image-to-load-first')
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// ... trigger the load of a image before it appears on the viewport
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observer.triggerLoad(coolImage);
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```
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### Large image improvment
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Sometimes image loading takes a long time. For this case, you can add a placeholder background:
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```html
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<img class="lozad" data-placeholder-background="red" data-src="image.png" />
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```
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Lozad sets a placeholder background color of img element and users will see the fallback till the image loads.
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## Example with picture tag
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Create _a broken_ picture element structure.
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> IE browser don't support picture tag!
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> You need to set `data-iesrc` attribute (only for your picture tags) with source for IE browser
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> `data-alt` attribute can be added to picture tag for use in `alt` attribute of lazy-loaded images
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```html
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<!-- 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 -->
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<picture class="lozad" style="display: block; min-height: 1rem" data-iesrc="images/thumbs/04.jpg" data-alt="">
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<source srcset="images/thumbs/04.jpg" media="(min-width: 1280px)">
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<source srcset="images/thumbs/05.jpg" media="(min-width: 980px)">
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<source srcset="images/thumbs/06.jpg" media="(min-width: 320px)">
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<!-- NO img element -->
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<!-- instead of img element, there will be the last source with the minimum dimensions -->
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<!-- for disabled JS you can set <noscript><img src="images/thumbs/04.jpg" alt=""></noscript> -->
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</picture>
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```
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When _lozad_ loads this picture element, it will fix it.
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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.
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```html
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<picture class="lozad" style="display: block; min-height: 1rem" data-iesrc="images/thumbs/04.jpg" data-alt="">
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<source srcset="images/thumbs/04.jpg" media="(min-width: 1280px)">
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<source srcset="images/thumbs/05.jpg" media="(min-width: 980px)">
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<source srcset="images/thumbs/06.jpg" media="(min-width: 320px)">
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<!-- you can define a low quality image placeholder that will be removed when the picture is loaded -->
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<img src="data:image/jpeg;base64,/some_lqip_in_base_64==" />
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</picture>
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```
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## Example with video
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```html
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<video class="lozad" data-poster="images/backgrounds/video-poster.jpeg">
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<source data-src="video/mov_bbb.mp4" type="video/mp4">
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<source data-src="video/mov_bbb.ogg" type="video/ogg">
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</video>
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```
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## Example with iframe
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```html
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<iframe data-src="embed.html" class="lozad"></iframe>
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```
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That's all, just add the `lozad` class.
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## Example toggling class
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```html
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<div data-toggle-class="active" class="lozad">
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<!-- content -->
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</div>
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```
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The `active` class will be toggled on the element when it enters the browser’s viewport.
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## Browser Support
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Available in [latest browsers](http://caniuse.com/#feat=intersectionobserver). If browser support is not available, then make use of [polyfill](https://www.npmjs.com/package/intersection-observer).
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For IE11 support, please make use of these [polyfills](https://polyfill.io/v3/polyfill.min.js?flags=gated&features=Object.assign%2CIntersectionObserver).
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## FAQs
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Checkout the [FAQ Wiki](https://github.com/ApoorvSaxena/lozad.js/wiki/Frequently-Asked-Questions) for some common gotchas to be aware of while using **lozad.js**
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## Contribute
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Interested in contributing features and fixes?
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[Read more on contributing](./CONTRIBUTING.md).
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## Changelog
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See the [Changelog](https://github.com/ApoorvSaxena/lozad.js/wiki/Changelog)
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## License
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[MIT](LICENSE) © [Apoorv Saxena](https://apoorv.pro)
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