# Lozad.js [![npm version](https://badge.fury.io/js/lozad.svg)](https://badge.fury.io/js/lozad) [![Build Status](https://travis-ci.org/ApoorvSaxena/lozad.js.svg?branch=master)](https://travis-ci.org/ApoorvSaxena/lozad.js) [![npm](https://img.shields.io/npm/dm/lozad)](https://www.npmjs.com/package/lozad) [![](https://data.jsdelivr.com/v1/package/npm/lozad/badge)](https://www.jsdelivr.com/package/npm/lozad)

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

![lozad.js lazy loading javascript library](./banner/lozad-banner.jpg "lozad.js lazy loading javascript library")

**Lozad.js**:
- lazy loads elements performantly using pure JavaScript,
- is a light-weight library, just [![](http://img.badgesize.io/https://cdn.jsdelivr.net/npm/lozad?compression=gzip)](https://cdn.jsdelivr.net/npm/lozad) minified & gzipped,
- has NO DEPENDENCIES :)
- allows lazy loading of dynamically added elements as well,
- supports &lt;img&gt;, &lt;picture&gt;, 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](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) and [MutationObserver](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver) with tremendous performance benefits.

## Featured in:

- [Web | Google Developers](https://developers.google.com/web/fundamentals/performance/lazy-loading-guidance/images-and-video/)
- [Product Hunt](https://www.producthunt.com/posts/lozad-js)
- [Reddit](https://www.reddit.com/r/webdev/comments/6zg1x0/highly_performant_light_05kb_and_configurable/)
- [CSS Tricks](https://css-tricks.com/lozad-js-performant-lazy-loading-images)
- [David Walsh](https://twitter.com/davidwalshblog/status/915319510646829061)
- [Codrops](https://twitter.com/Apoorv_Saxena/status/906586828265758720)
- [SitePoint](https://www.sitepoint.com/five-techniques-lazy-load-images-website-performance/)

## Brands using Lozad.js:

![Tesla](./brands/tesla.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![Binance](./brands/binance.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![Domino's](./brands/dominos.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
![BNP Paribas](./brands/bnp-paribas.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![Mi](./brands/xiaomi.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
![Amway](./brands/amway.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![TATA](./brands/tata.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![Verizon](./brands/verizon.svg)
![Atlassian](./brands/atlassian.svg) ![BNP Paribas](./brands/livemint.svg)
![JLM Couture](./brands/jlm-couture.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![New Balance](./brands/new-balance.png)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ![BBC](./brands/bbc.png)

and many more...

## Table of Contents

- [Demo](https://apoorv.pro/lozad.js/demo/)
- [Background](#yet-another-lazy-loading-javascript-library-why)
- [Install](#install)
- [Usage](#usage)
- [Example with picture tag](#example-with-picture-tag)
- [Browser Support](#browser-support)
- [FAQs](#faqs)
- [Contribute](#contribute)
- [Changelog](#changelog)
- [License](#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](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.

## Install

```sh
# 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:

```javascript
// 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.

```html
<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`.

---

<a href="https://www.patreon.com/apoorvsaxena">
    <img src="https://c5.patreon.com/external/logo/become_a_patron_button@2x.png" width="160">
</a>

## Usage

In HTML, add an identifier to the element (default selector identified is `lozad` class):
```html
<img class="lozad" data-src="image.png">
```

All you need to do now is just instantiate Lozad as follows:
```js
const observer = lozad(); // lazy loads elements with default selector as '.lozad'
observer.observe();
```
or with a DOM `Element` reference:
```js
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:
```js
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](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/rootMargin)
 - [IntersectionObserver options: thresholds](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/thresholds)

or if you want to give custom function definition to load element:
```js
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.
```js
lozad('.lozad', {
    loaded: function(el) {
        // Custom implementation on a loaded element
        el.classList.add('loaded');
    }
});
```

If you want to lazy load dynamically added elements:

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

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

for use with responsive images

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

for use with background images
```html
<!-- background image example -->
<div class="lozad" data-background-image="image.png">
</div>
```

for use with multiple background images
```html
<!-- 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)
```html
<!-- 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:
```html
<!-- 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:

```js
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);
```

### Large image improvment

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

```html
<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.

## Example with picture tag

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

```html
<!-- 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.

```html
<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>
```

## Example with video

```html
<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>
```

## Example with iframe

```html
<iframe data-src="embed.html" class="lozad"></iframe>
```
That's all, just add the `lozad` class.

## Example toggling class

```html
<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.

## Browser Support

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).

For IE11 support, please make use of these [polyfills](https://polyfill.io/v3/polyfill.min.js?flags=gated&features=Object.assign%2CIntersectionObserver).

## FAQs

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**

## Contribute

Interested in contributing features and fixes?

[Read more on contributing](./CONTRIBUTING.md).

## Changelog

See the [Changelog](https://github.com/ApoorvSaxena/lozad.js/wiki/Changelog)

## License

[MIT](LICENSE) © [Apoorv Saxena](https://apoorv.pro)
