lazySizes

lazySizes is the ultimate and lightweight lazyLoader which lazy loads images (including responsive images (picture/srcset)), iframes and scripts. It is written in VanillaJS and with high performance in mind.

Simply add the JS to your website and put the class lazyload to all elements, which should be lazy loaded. For a short API description go to the readme.md.

Boat

Image with LQIP technique

The low quality image placeholder pattern uses a low quality image for the first impression.

<img
	alt="100%x200"
	src="low-quality.jpg"
	data-src="normal-quality.jpg"
	class="lazyload" />
Desert Road

Normal lazy image

Of course the low quality image can be simply omitted to save more image data.

<img class="lazyload" data-src="image.jpg" alt="Desert Road" />

responsive image with srcset and sizes attribute

Simply use data-srcset and you can support responsive images.

<img
	alt=""
	sizes="(min-width: 1000px) 930px, 90vw"
	data-srcset="small.jpg 500w,
		medium.jpg 640w,
		big.jpg 1024w"
	data-src="medium.jpg"
	class="lazyload" />
image with artdirection

responsive image with the picture element

The picture element is also supported. Simply add the lazyload class to the img and use data-srcset on your source and the img element.

<picture>
	<!--[if IE 9]><video style="display: none"><![endif]-->
	<source
		data-srcset="500.jpg"
		media="(max-width: 500px)" />
	<source
			data-srcset="1024.jpg"
			media="(max-width: 1024px)" />
	<source
			data-srcset="1200.jpg" />
<!--[if IE 9]></video><![endif]-->
<img
		src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
		data-src="1024.jpg"
		class="lazyload"
		alt="image with artdirection" />
</picture>

automatic sizes feature: In case of lazy loading images the sizes attribute of the img/source elements can be calculated with JS.

This automatic sizes feature is directly included in lazySizes. Simply use the keyword auto inside of the data-sizes attributes (data-sizes="auto").

Important: How sizes is calculated: The automatic sizes calculation takes the width of the image if it is over 40 (see also minSize option). In case it's below the minSize threshold, it traverses up the DOM tree until it finds a parent which is over 40 and uses this number. Often the following general CSS rule might help: img[data-sizes="auto"] { display: block; }.

responsive image with srcset and automatic sizes attribute

The autosizes feature makes using responsive images with the right sizes value easy as hell.

<img
	alt=""
	data-sizes="auto"
	data-srcset="small.jpg 500w,
		medium.jpg 640w,
		big.jpg 1024w"
	data-src="medium"
	class="lazyload" />

For responsive images support you must use either use a full polyfill like picturefill or use the extreme lightweight partial respimg polyfill plugin or use the Responsive Images as a Service extension.

iframe

Iframes can be loaded too:

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

[data-expand]: More than lazyloading

LazySizes normally loads in view elements as fast as possible and near the view elements are lazily pre-loaded. This can be modified in general using the expand option. With the data-expand attribute this can be changed element specific to expand or shrink the viewport.

This can be used to create unveiling effects for elements with or even without loading anything.

<style>
.teaser.lazyload {
	opacity: 0;
	transform: scale(0.8);
}

.teaser.lazyloaded {
	opacity: 1;
	transform: scale(1);
	transition: all 700ms;
}
</style>

<script>

window.lazySizesConfig = {
	addClasses: true
};
</script>

<div class="teaser lazyload" data-expand="-110">
	<img data-src="image.jpg" class="lazyload" />
	<h1>Teaser Title</h1>
	<p>...</p>
</div>

Widgets/Javascript/Script

LazySizes can be extended to load nearly everything lazily. The ls.unveilhooks.js plugin can be used to lazy load scripts, background images, and videos. Simply add a data-script to your widget and you are done:

<div data-ride="carousel" data-script="assets/js/bootstrap.min.js" class="carousel lazyload">
	<!-- widget content -->
<div>
Desert Road
@ The Desert Tortoise Natural Area
Woman in water
Borobudur
A tree in the blue
Windows on Istanbul
Goldie Dawn
Avebury Stone Circle
el castil de tierra
sunset
Sky and earth
Missing Ulsoor lake (Explore)
Oxford Path 2

While other lazy loaders or responsive images solution need to be called, if new elements are added to the DOM or do become visible through a special user interaction or a JavaScript behavior.

LazySizes does automatically detect any changes to the DOM and the visibility of .lazyload elements.