lazysizes

What is lazysizes?

lazysizes is a high-performance lazy loader for images (including responsive images), iframes, and scripts. It helps improve the performance of web pages by deferring the loading of non-critical resources until they are needed.

What are lazysizes's main functionalities?

Lazy Loading Images

This feature allows you to lazy load images by using the 'data-src' attribute instead of 'src'. The 'lazyload' class triggers the lazy loading functionality.

<img data-src='image.jpg' class='lazyload' alt='Lazy loaded image'>

Responsive Images

This feature enables lazy loading of responsive images using the 'data-srcset' and 'data-sizes' attributes. The 'lazyload' class ensures that the appropriate image is loaded based on the viewport size.

<img data-sizes='auto' data-srcset='image-320w.jpg 320w, image-640w.jpg 640w' data-src='image-640w.jpg' class='lazyload' alt='Responsive lazy loaded image'>

Lazy Loading Iframes

This feature allows you to lazy load iframes by using the 'data-src' attribute instead of 'src'. The 'lazyload' class triggers the lazy loading functionality.

<iframe data-src='https://www.example.com' class='lazyload'></iframe>

Lazy Loading Background Images

This feature allows you to lazy load background images by using the 'data-bg' attribute. The 'lazyload' class ensures that the background image is loaded only when the element is in the viewport.

<div class='lazyload' data-bg='image.jpg'></div>

Other packages similar to lazysizes

lozad

lozad.js is a highly performant, light-weight and configurable lazy loader in pure JavaScript with no dependencies. It is similar to lazysizes in terms of functionality but is often praised for its simplicity and ease of use.

react-lazyload

react-lazyload is a React component that provides lazy loading functionality for React applications. It is specifically designed for React and offers a more integrated solution for React developers compared to lazysizes.

vanilla-lazyload

vanilla-lazyload is a lightweight and flexible lazy loader for images, iframes, and more. It is similar to lazysizes but is often chosen for its smaller size and ease of customization.

README

lazysizes

lazysizes is a fast (jank-free), SEO-friendly and self-initializing lazyloader for images (including responsive images picture/srcset), iframes, scripts/widgets and much more. It also prioritizes resources by differentiating between crucial in view and near view elements to make perceived performance even faster.

It may become also your number one tool to integrate responsive images. It can automatically calculate the sizes attribute for your responsive images, it allows you to share media queries for your media attributes with your CSS, helping to separate layout (CSS) from content/structure (HTML) and it makes integrating responsive images into any environment really simple. It also includes a set of optional plugins to further extend its functionality.

How to

1. Download the lazysizes.min.js script and include lazysizes in your webpage. (Or install via npm: npm install lazysizes --save or bower bower install lazysizes --save)

   <script src="lazysizes.min.js" async=""></script>
   

   Or:

   import 'lazysizes';
   // import a plugin
   import 'lazysizes/plugins/parent-fit/ls.parent-fit';
   
   // Note: Never import/require the *.min.js files from the npm package.
   

   Note: For more information see here.

2. lazysizes does not need any JS configuration: Add the class "lazyload" to your images/iframes in conjunction with a data-src and/or data-srcset attribute. Optionally you can also add a src attribute with a low quality image:

   <!-- non-responsive: -->
   <img data-src="image.jpg" class="lazyload" />
   

   <!-- responsive example with automatic sizes calculation: -->
   <img
       data-sizes="auto"
       data-src="image2.jpg"
       data-srcset="image1.jpg 300w,
       image2.jpg 600w,
       image3.jpg 900w" class="lazyload" />
   

   <!-- iframe example -->
   <iframe frameborder="0"
   	class="lazyload"
       allowfullscreen=""
       data-src="//www.youtube.com/embed/ZfV-aYdU4uE">
   </iframe>
   

Demo with code examples

Can be seen here

Responsive image support (picture and/or srcset)

Lazysizes is built upon the Responsive image standard and extends it with additional functionality. For full cross browser responsive image support you must use either a full polyfill like picturefill or use the extreme lightweight partial respimg polyfill plugin or the responsive image on demand plugin. Alternatively, you can simply define a fallback src via the data-src attribute. If you want to learn more about the responsive image syntax read "The anatomy of responsive images".

What makes lazysizes so awesome:

lazysizes is different than other lazy image loaders.

1. Detects any visibility changes on current and future lazyload elements in any web environment automatically: The script works as an universal, self-initializing, self-configuring and self-destroying component and detects any changes to the visibility of any current and future image/iframe elements automatically no matter whether it becomes visible through a user scroll, a CSS animation triggered through :hover or through any kind of JS behavior (carousel, slider, infinite scroll, masonry, isotope/filtering/sorting, AJAX, SPAs...). It also works automatically in conjunction with any kind of JS-/CSS-/Frontend-Framework (jQuery mobile, Bootstrap, Backbone, Angular, React, Ember (see also the attrchange/re-initialization extension)).
2. Future-proof: It directly includes standard responsive image support (picture and srcset)
3. Separation of concerns: For responsive image support it adds an automatic sizes calculation as also alias names for media queries feature. There is also no JS change needed if you add a scrollable container with CSS (overflow: auto) or create a mega menu containing images.
4. Performance: It's based on highly efficient, best practice code (runtime and network) to work jank-free at 60fps and can be used with hundreds of images/iframes on CSS and JS-heavy pages or webapps.
5. Extendable: It provides JS and CSS hooks to extend lazysizes with any kind of lazy loading, lazy instantiation, in view callbacks or effects (see also the available plugins/snippets).
6. Intelligent prefetch/Intelligent resource prioritization: lazysizes prefetches/preloads near the view assets to improve user experience, but only while the browser network is idling (see also expand, expFactor and loadMode options). This way in view elements are loaded faster and near of view images are preloaded lazily before they come into view.
7. Lightweight, but mature solution: lazysizes has the right balance between a lightweight and a fast, reliable solution
8. SEO improved: lazysizes does not hide images/assets from Google. No matter what markup pattern you use. Google doesn't scroll/interact with your website. lazysizes detects, whether the user agent is capable to scroll and if not, reveals all images instantly.

More about the API

lazysizes comes with a simple markup and JS API. Normally you will only need to use the markup API.

Markup API

Add the class lazyload to all img and iframe elements, which should be loaded lazy. Instead of a src or srcset attribute use a data-src or data-srcset attribute:

<img data-src="image.jpg" class="lazyload" />
<!-- retina optimized image: -->
<img data-srcset="responsive-image1.jpg 1x, responsive-image2.jpg 2x" class="lazyload" />

Automatically setting the sizes attribute

lazysizes supports setting the sizes attribute automatically, corresponding to the current size of your image - just set the value of data-sizes to auto.

<img
	data-sizes="auto"
	data-srcset="responsive-image1.jpg 300w,
	    responsive-image2.jpg 600w,
	    responsive-image3.jpg 900w"
    class="lazyload" />

Important: How sizes is calculated: The automatic sizes calculation uses the display width of the image. This means that the width of the image has to be calculable at least approximately before the image itself is loaded (This means you can not use width: auto). Often the following general CSS rule might help: img[data-sizes="auto"] { display: block; width: 100%; } (see also specifying image/iframe dimensions with the recommended aspect ratio definition). If it is below 40 (can be configured through the minSize option), lazysizes traverses up the DOM tree until it finds a parent which is over 40 and uses this number.

The width auto-calculated by lazysizes can be modified using the lazybeforesizes event (lazybeforesizes documentation). Alternatively, the parent fit plugin can be used for sizing images to fit a parent / container, and is the only solution when an image's height needs to be taken into account when fitting it to its container (This also includes the use of object-fit).

The data-sizes="auto" feature only makes sense if you use the data-srcset attribute with width descriptors which allows the most appropriate image can be selected (It does not make sense if you use the x descriptor or only src.).

Recommended/possible markup patterns

lazysizes allows you to write an endless variety of different markup patterns. Find your own/best pattern or choose one of the following. (All of the following patterns can be also used for art direction using the picture element.)

Simple pattern

Add the class lazyload and simply omit the src attribute or add a data uri as fallback src.

<!--  responsive adaptive example -->

<img
	class="lazyload"
	data-srcset="image.jpg 1x, image2.jpg 2x"
    alt="my image" />
<!--  retina optimized example -->
<img class="lazyload"
	data-srcset="progressive-image.jpg 1x, progressive-image2.jpg 2x"
    alt="my image" />

<!-- or non-responsive: -->
<img
	data-src="image.jpg"
	class="lazyload" />

Note: In case you are using either srcset/data-srcset or picture, we recommend to extend this pattern with either a data-src (see next pattern: "Combine data-srcset with data-src") or with a suitable src attribute (see: "modern pattern" or "LQIP").

Combine data-srcset with data-src

In case you want to use responsive images for supporting browsers, but don't want to include a polyfill, simply combine your data-srcset with a data-src attribute.

<!-- responsive example: -->
<img
	data-sizes="auto"
    data-src="image3.jpg"
	data-srcset="image3.jpg 600w,
	    image1.jpg 220w,
	    image2.jpg 300w,
	    image3.jpg 600w,
	    image4.jpg 900w"
	class="lazyload" />

Note: Due to the fact that the data-src will also be picked up by "Read-Later" Apps and other tools (for example Pin it button), this pattern also makes sense if you use a polyfill. In case you don't use a polyfill it is recommended that the first image candidate matches the fallback src.

LQIP/blurry image placeholder/Blur up image technique

If you are using the LQIP (Low Quality Image Placeholder) pattern, simply add a low quality image as the src:

<!-- responsive example: -->
<img
	data-sizes="auto"
    src="lqip-src.jpg"
	data-srcset="lqip-src.jpg 220w,
    image2.jpg 300w,
    image3.jpg 600w,
    image4.jpg 900w" class="lazyload" />

<!-- or non-responsive: -->
<img src="lqip-src.jpg" data-src="image.jpg" class="lazyload" />

The LQIP technique can be enhanced by combining it with CSS transitions/animation to sharpen/unblur or overfade the LQIP image.

Please also have a look at our lazysizes Blur Up plugin (recommended).

<style>
	.blur-up {
		-webkit-filter: blur(5px);
		filter: blur(5px);
		transition: filter 400ms, -webkit-filter 400ms;
	}

	.blur-up.lazyloaded {
		-webkit-filter: blur(0);
		filter: blur(0);
	}
</style>

<img src="lqip-src.jpg" data-src="image.jpg" class="lazyload blur-up" />

<!-- ... -->

<style>
	.fade-box .lazyload,
	 .fade-box .lazyloading {
		opacity: 0;
		transition: opacity 400ms;
	}

	.fade-box img.lazyloaded {
		opacity: 1;
	}
</style>

<div class="ratio-box fade-box">
	<img src="lqip-src.jpg" />
	<img data-src="image.jpg" class="lazyload" />
</div>

modern transparent srcset pattern

Combine a normal src attribute with a transparent or low quality image as srcset value and a data-srcset attribute. This way modern browsers will lazy load without loading the src attribute and all others will simply fallback to the initial src attribute (without lazyload). (This nice pattern originated from @ivopetkov.)

<img
    src="image3.jpg"
    srcset="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
	data-srcset="image3.jpg 600w,
		image1.jpg 220w,
	    image2.jpg 300w,
	    image4.jpg 900w"
	data-sizes="auto"
	class="lazyload" />

The noscript pattern

In case disabled JavaScript is a concern you can combine this simple pattern with an image inside a noscript element.

<style>
	.no-js img.lazyload {
    	display: none;
    }
</style>

<!-- noscript pattern -->
<noscript>
	<img src="image.jpg" />
</noscript>
<img src="transparent.jpg" data-src="image.jpg" class="lazyload" />

Note: As an alternative to the noscript pattern also checkout the noscript extension.

[data-expand] attribute

Normally lazysizes will expand the viewport area to lazy preload images/iframes which might become visible soon. This value can be adjusted using the expand option.

Additionally, this general option can be overridden with the data-expand attribute for each element. Different than the general expand option the data-expand attribute also accepts negative values (All numbers but 0 are accepted!).

This becomes especially handy to add unveiling effects for teasers or other elements:

<style>
.lazyload,
.lazyloading {
	opacity: 0;
}
.lazyloaded {
	opacity: 1;
	transition: opacity 300ms;
}
</style>

<div class="teaser lazyload" data-expand="-20">
    <img data-src="image.jpg" class="lazyload" />
    <h1>Teaser Title</h1>
    <p>...</p>
</div>

CSS API

lazysizes adds the class lazyloading while the images are loading and the class lazyloaded as soon as the image is loaded. This can be used to add unveil effects:

/* fade image in after load */
.lazyload,
.lazyloading {
	opacity: 0;
}
.lazyloaded {
	opacity: 1;
	transition: opacity 300ms;
}

/* fade image in while loading and show a spinner as background image (good for progressive images) */

.lazyload {
	opacity: 0;
}

.lazyloading {
	opacity: 1;
	transition: opacity 300ms;
	background: #f7f7f7 url(loader.gif) no-repeat center;
}

Broken image symbol

In case you are using an alt attribute but do not declare a src/srcset attribute you will end up with a broken image symbol.

There are two easy ways to deal with it.

Either define a src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" or add the following CSS.

img.lazyload:not([src]) {
	visibility: hidden;
}

JS API

lazysizes automatically detects new elements with the class lazyload so you won't need to call or configure anything in most situations.

JS API - options

Options can be set by declaring a global configuration option object named lazySizesConfig. This object must be defined before the lazysizes script. A basic example:

window.lazySizesConfig = window.lazySizesConfig || {};

// use .lazy instead of .lazyload
window.lazySizesConfig.lazyClass = 'lazy';

// use data-original instead of data-src
lazySizesConfig.srcAttr = 'data-original';

//page is optimized for fast onload event
lazySizesConfig.loadMode = 1;

In case you are using a module bundler it is recommended to change the options directly after importing the lazysizes module:

import lazySizes from 'lazysizes';
// other imports ...

lazySizes.cfg.lazyClass = 'lazy';

Here the list of options:

• lazySizesConfig.lazyClass (default: "lazyload"): Marker class for all elements which should be lazy loaded (There can be only one class. In case you need to add some other element, without the defined class, simply add it per JS: $('.lazy-others').addClass('lazyload');)
• lazySizesConfig.preloadAfterLoad (default: false): Whether lazysizes should load all elements after the window onload event. Note: lazySizes will then still download those not-in-view images inside of a lazy queue, so that other downloads after onload are not blocked.)
• lazySizesConfig.preloadClass (default: "lazypreload"): Marker class for elements which should be lazy pre-loaded after onload. Those elements will be even preloaded, if the preloadAfterLoad option is set to false. Note: This class can be also dynamically set ($currentSlide.next().find('.lazyload').addClass('lazypreload');).
• lazySizesConfig.loadingClass (default: "lazyloading"): This class will be added to img element as soon as image loading starts. Can be used to add unveil effects.
• lazySizesConfig.loadedClass (default: "lazyloaded"): This class will be added to any element as soon as the image is loaded or the image comes into view. Can be used to add unveil effects or to apply styles.
• lazySizesConfig.expand (default: 370-500): The expand option expands the calculated visual viewport area in all directions, so that elements can be loaded before they become visible. The default value is calculated depending on the viewport size of the device. (Note: Reasonable values are between 300 and 1000 (depending on the expFactor option.) In case you have a lot of small images or you are using the LQIP pattern you can lower the value, in case you have larger images set it to a higher value. Also note, that lazySizes will dynamically shrink this value to 0 if the browser is currently downloading and expand it if the browser network is currently idling and the user not scrolling (by multiplying the expand option with 1.5 (expFactor)). This option can also be overridden with the [data-expand] attribute.
• lazySizesConfig.minSize (default: 40): For data-sizes="auto" feature. The minimum size of an image that is used to calculate the sizes attribute. In case it is under minSize the script traverses up the DOM tree until it finds a parent that is over minSize.
• lazySizesConfig.srcAttr (default: "data-src"): The attribute, which should be transformed to src.
• lazySizesConfig.srcsetAttr (default: "data-srcset"): The attribute, which should be transformed to srcset.
• lazySizesConfig.sizesAttr (default: "data-sizes"): The attribute, which should be transformed to sizes. Makes almost only makes sense with the value "auto". Otherwise, the sizes attribute should be used directly.
• lazySizesConfig.customMedia (default: {}): The customMedia option object is an alias map for different media queries. It can be used to separate/centralize your multiple specific media queries implementation (layout) from the source[media] attribute (content/structure) by creating labeled media queries. (See also the custommedia extension).
• lazySizesConfig.loadHidden (default: true): Whether to load visibility: hidden elements. Important: lazySizes will load hidden images always delayed. If you want them to be loaded as fast as possible you can use opacity: 0.001 but never visibility: hidden or opacity: 0.
• lazySizesConfig.ricTimeout (default: 0): The timeout option used for the requestIdleCallback. Reasonable values between: 0, 100 - 1000. (Values below 50 disable the requestIdleCallback feature.)
• lazySizesConfig.throttleDelay (default: 125): The timeout option used to throttle all listeners. Reasonable values between: 66 - 200.

<script>
window.lazySizesConfig = window.lazySizesConfig || {};
window.lazySizesConfig.customMedia = {
    '--small': '(max-width: 480px)',
    '--medium': '(max-width: 900px)',
    '--large': '(max-width: 1400px)',
};
</script>


<picture>
	<!--[if IE 9]><video style="display: none;><![endif]-->
	<source
		data-srcset="http://placehold.it/500x600/11e87f/fff"
		media="--small" />
	<source
		data-srcset="http://placehold.it/700x300"
		media="--medium" />
	<source
		data-srcset="http://placehold.it/1400x600/e8117f/fff"
		media="--large" />
	<source
        data-srcset="http://placehold.it/1800x900/117fe8/fff" />
    <!--[if IE 9]></video><![endif]-->
    <img

        data-src="http://placehold.it/1400x600/e8117f/fff"
        class="lazyload"
        alt="image with artdirection" />
</picture>

• lazySizesConfig.expFactor (default: 1.5): The expFactor is used to calculate the "preload expand", by multiplying the normal expand with the expFactor which is used to preload assets while the browser is idling (no important network traffic and no scrolling). (Reasonable values are between 1.5 and 4 depending on the expand option).
• lazySizesConfig.hFac (default: 0.8): The hFac (horizontal factor) modifies the horizontal expand by multiplying the expand value with the hFac value. Use case: In case of carousels there is often the wish to make the horizontal expand narrower than the normal vertical expand option. Reasonable values are between 0.4 - 1. In the unlikely case of a horizontal scrolling website also 1 - 1.5.
• lazySizesConfig.loadMode (default: 2): The loadMode can be used to constrain the allowed loading mode. Possible values are 0 = don't load anything, 1 = only load visible elements, 2 = load also very near view elements (expand option) and 3 = load also not so near view elements (expand * expFactor option). This value is automatically set to 3 after onload. Change this value to 1 if you (also) optimize for the onload event or change it to 3 if your onload event is already heavily delayed.
• lazySizesConfig.init (default: true): By default lazysizes initializes itself, to load in view assets as soon as possible. In the unlikely case you need to setup/configure something with a later script you can set this option to false and call lazySizes.init(); later explicitly.

JS API - events

lazysizes provides three events to modify or extend the behavior of lazysizes.

• lazybeforeunveil: This event will be fired on each lazyload element right before of the "unveil" transformation. This event can be used to extend the unveil functionality. In case the event is defaultPrevented the default transformation action will be prevented (see also the ls.unveilhooks.js plugin):

//add simple support for background images:
document.addEventListener('lazybeforeunveil', function(e){
    var bg = e.target.getAttribute('data-bg');
    if(bg){
        e.target.style.backgroundImage = 'url(' + bg + ')';
    }
});
//or add AJAX loading
//<div class="lazyload" data-ajax="my-url.html"></div>

$(document).on('lazybeforeunveil', function(){
	var ajax = $(e.target).data('ajax');
    if(ajax){
        $(e.target).load(ajax);
    }
});

The lazybeforeunveil can also be used for lazy initialization and due to the fact that lazysizes also detects new elements in the DOM automatically also for auto- and self-initialization of UI widgets:

<script>
document.addEventListener('lazybeforeunveil', function(e){
    $(e.target)
        .filter('.slider')
        .slider({
            sliderOption: true
        })
    ;
});

document.addEventListener('lazybeforeunveil', function(e){
    $(e.target)
        .filter('.chart')
        .chart({
            animate: true
        })
    ;
});
</script>

<div class="slider lazyload lazypreload"></div>

<div class="chart lazyload" data-expand="-10"></div>

• lazyloaded: After the image is fully loaded lazysizes dispatches a lazyloaded event. While this often duplicates the native load event it is often more convenient to use.

• lazybeforesizes: This event will be fired on each element with the data-sizes="auto" attribute right before the calculated sizes attribute will be set. The event.detail.width property is set to the calculated width of the element and can be changed to any number. In case the event is defaultPrevented the sizes attribute won't be set. See also the parent-fit extension.

$(document).on('lazybeforesizes', function(e){
    //use width of parent node instead of the image width itself
    e.detail.width = $(e.target).closest(':not(picture)').innerWidth() || e.detail.width;
});

JS API - methods

lazySizes.loader.unveil(DOMNode)

In case a developer wants to show an image even if it is not inside the viewport the lazySizes.loader.unveil(DOMNode) can be called:

lazySizes.loader.unveil(imgElem);

Note: As a more lazy alternative the lazypreload class can be set: $(imgElem).addClass('lazypreload');.

lazySizes.autoSizer.checkElems()

In case one or more image elements with the attribute data-sizes="auto" have changed in size lazySizes.autoSizer.updateElems can be called (For example to implement element queries):

lazySizes.autoSizer.checkElems();

lazySizes.loader.checkElems()

Tests whether new elements has came into view. Normally this method only needs to be called, if lazySizesConfig.loadMode was set to 0.

lazySizes.init()

LazySizes initializes itself automatically. In case you set lazySizesConfig.init to false you need to explicitly call lazySizes.init(). Note: You can speed up initial loading of in view images if you call lazySizesConfig.init() explicitly after lazysizes and all plugins are loaded.

<script>
window.lazySizesConfig = window.lazySizesConfig || {};
window.lazySizesConfig.init = false;
</script>

<script src="lazysizes.js"></script>
<script src="other-script.js"></script>
<script>
lazySizes.init();
</script>

Browser Support

lazysizes supports all browsers, that support document.getElementsByClassName (== all browsers but not IE8-). In case you need to support IE8, see also the noscript extension (or use a modified noscript pattern or the LQIP pattern).

Contributing

Fixes, PRs and issues are always welcome, make sure to create a new branch from the master (not the gh-pages branch), validate against JSHint and test in all browsers. In case of an API/documentation change make sure to also document it here in the readme.md.

Available plugins in this repo

It is recommended to concat all plugins together with lazySizes. In case you don't concat it is recommended to include the plugin scripts before the lazySizes main script.

respimg polyfill plugin

The respimg polyfill plugin is an extremely lightweight alternate polyfill for the most important subsets of responsive images (srcset and picture).

OPTIMUMX plugin

The srcset attribute with the w descriptor and sizes attribute automatically also includes high DPI images. But each image has a different optimal pixel density, which might be lower (for example 1.5x) than the pixel density of your device (2x or 3x). This information is unknown to the browser and therefore can't be optimized for. The lazySizes optimumx extension gives you more control to trade between perceived quality vs. perceived performance.

parent-fit extension

The parent fit plugin extends the data-sizes="auto" feature to also calculate the right sizes for object-fit: contain|cover image elements and other height ( and width) constrained image elements in general.

object-fit polyfill extension

The object fit polyfill plugin polyfills the object-fit and the object-position property in non supporting browsers.

blur up / effect plugin

The blur up / effect plugin allows you to create great over fade / blur up effects with low quality image placeholder, which improves the user experience and perceived performance in case you are using a low quality image approach.

attrchange / re-initialization extension (strongly recommended if you use React, Angular etc.)

In case you are changing the data-src/data-srcset attributes of already transformed lazyload elements, you must normally also re-add the lazyload class to the element.

This attrchange / re-initialization extension automatically detects changes to your data-* attributes and adds the class for you.

artdirect plugin

The artdirect plugin allows you to fully control art direction via CSS.

Other plugins/extensions

There are also other plugins/extension in the plugins folder. As always you are open to create new ones for your project.

Tip: Specifying image dimensions (minimizing reflows and avoiding page jumps)

To minimize reflows, content jumping or unpredictable behavior with some other JS widgets (isotope, masonry, some sliders/carousels...) the width and the height of an image should be calculable by the browser before the image source itself is loaded:

<img

    style="width: 350px; height: 150px;"
	data-srcset="http://placehold.it/350x150 1x,
    http://placehold.it/700x300 2x"
    data-src="http://placehold.it/350x150"
    class="lazyload" />

For flexible responsive images the CSS intrinsic ratio scaling technique should be used:

<style>
.ratio-container {
    position: relative;
}
.ratio-container:after {
    content: '';
    display: block;
    height: 0;
    width: 100%;
    /* 16:9 = 56.25% = calc(9 / 16 * 100%) */
    padding-bottom: 42.86%;
}
.ratio-container > * {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: block;
}
</style>

<div class="ratio-container">
    <img

        data-sizes="auto"
        data-srcset="http://placehold.it/175x75 175w,
        http://placehold.it/350x150 350w,
        http://placehold.it/700x300 700w,
        http://placehold.it/1400x600 1400w"
        data-src="http://placehold.it/700x300"
        class="lazyload" />
</div>

In case you want to dynamically calculate your intrinsic ratios for many different formats you can vary the pattern to something like this:

<style>
.ratio-box {
	position: relative;
	height: 0;
	display: block;
	width: 100%;
	/* padding-bottom is calculated and rendered in to HTML */
}

.ratio-box img,
.ratio-box iframe,
.ratio-box video {
	position: absolute;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	display: block;
}
</style>

<div class="ratio-box" style="padding-bottom: 42.85% /* calc(75 / 175 * 100%)*/;">
    <img

        data-sizes="auto"
        data-srcset="http://placehold.it/175x75 175w,
        http://placehold.it/350x150 350w,
        http://placehold.it/700x300 700w,
        http://placehold.it/1400x600 1400w"
        data-src="http://placehold.it/700x300"
        class="lazyload" />
</div>

In case the exact ratio of your image is unknown you can also vary the intrinsic ratio like this:

<style>
.ratio-container {
    position: relative;
}
.ratio-container:after {
    content: '';
    display: block;
    height: 0;
    width: 100%;
    /* 16:9 = 56.25% = calc(9 / 16 * 100%) */
    padding-bottom: 56.25%;
    content: "";
}
.ratio-container > * {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

/* unknown ration variation */
.unknown-ratio-container > * {
    max-width: 100%;
    max-height: 100%;
    width: auto;
    height: auto;
}
</style>

<div class="ratio-container unknown-ratio-container">
    <img

        data-src="http://placehold.it/350x150"
        class="lazyload" />
</div>

or at least add a min-height (and min-width) to minimize content jumps:

.lazyload,
.lazyloading {
	min-height: 200px;
}

Note:

• If you use the "unknown intrinsic ratio pattern" and the width of the loaded image will not (approximately) match the width of its container, the data-sizes="auto" feature will not be effective when used on its own. In this situation, the most appropriate size for the image to fit in the available space can be calculated automatically using the parent fit plugin.
• see also the aspectratio extension for an alternative way to add aspectratio.

Updating layout of JS widgets

In case you can't specify the image dimensions using CSS or one of the above suggested methods and your JS widgets have problems to calculate the right dimensions. You can use the following pattern to update your JS widgets (sliders/masonry):

$('.my-widget').each(function(){
    var $module = $(this);
    var update = function(){
        $module.myWidget('updateLayout');
    };

    // Note: Instead of waiting for all images until we initialize the widget
    // we use event capturing to update the widget's layout progressively.
    this.addEventListener('load', update, true);

    $module.myWidget();
});

For this update pattern you may want to combine this at least with the min-height pattern explained above.

Tip: Where/How to include lazySizes

While lazy loading is a great feature, it is important for users that crucial in view images are loaded as fast as possible. (Most users start to interact with a page after in view images are loaded.)

In case you normally combine all your scripts into one large script and add this to the bottom of your page, it can be better for perceived performance to generate two or sometimes more script packages: One small package, which includes all scripts which have heavy influence on the content or the UI and another larger one which includes the normal behavior of the page.

This smaller script, which should include lazySizes (and all its plugins), should than be placed before any other blocking elements (i.e.: script(s)) at the end of the body or after any blocking elements (i.e.: scripts, stylesheets) in the head to load the crucial content as fast possible. (Note: It might make also sense to call lazySizes.init(); explicitly right after lazySizes and all its plugins are added.)

Why lazysizes

In the past, I often struggled using lazy image loaders, because the "main check function" is called repeatedly and with a high frequency. Which makes it hard to fulfill two purposes runtime and memory efficiency. And looking into the source code of most so called lazy loaders often also unveils lazy developers...

But in a world of responsive retina optimized images on the one hand and JS widgets like carousels or tabs (a lot of initially hidden images) on the other hand lazy loading images becomes more and more important, so I created this project.

lazysizes is different:

Due to the fact, that it is designed to be invoked with a high frequency and therefore works highly efficient, it was possible to hook into all kinds of events as a mutationobserver meaning this lazyloader works as a simple drop in solution - you simply write/render your markup and no matter whether the .lazyload element was added by AJAX or revealed by a JS or CSS animation it will be picked up by lazysizes.

<!-- responsive example: -->
<img
	data-sizes="auto"

	data-srcset="image2.jpg 300w,
    image3.jpg 600w,
    image4.jpg 900w"
    data-src="image3.jpg"
    class="lazyload" />

<!-- or non-responsive: -->
<img
    data-src="image.jpg"
    class="lazyload" />

Changelog

5.1.2

• Fix visibility check (fixes #709)