LazyLoad is a fast, lightweight and flexible script to allow you enhance performance on your website by loading images only as they enter (or are about to enter) the viewport or a scrollable area, with best support to the img srcset attribute and to the progressive JPEG image format.
HTML JavaScript
Latest commit 2e318b1 Dec 10, 2016 @verlok Merge branch 'gh-pages'

README.md

LazyLoad (aka Vanilla LazyLoad)

LazyLoad is a fast, lightweight and flexible script for loading images only when they're about to enter the viewport of a scrollable area, which supports the srcset attribute and with and takes best advantage from the progressive JPEG image format (which is good for performance).

WEBSITE | DEMOS | GITHUB

Difference with jQuery_lazyload

This script was inspired by Mika Tuupola's jQuery_lazyLoad. The following are the main differences between the scripts.

  • LazyLoad is 6x faster than jQuery_lazyload. This is because LazyLoad uses only optimized, native javascript functions and methods, and not jQuery. Your users will see the difference, even in slow devices or computers. To measure the scripts performance yourself, see lazyLoad and jQuery_lazyLoad implementations in the perf folder.
  • Progressive JPEG support will improve perceived speed. Progressive JPEG is an image format good for perceived performance because it's rendered in progressive passes of detail. lazyLoad shows your images while they load, letting progressive JPEG do its magic. jQuery_lazyload will only show your images once fully loaded.
  • LazyLoad can be instanced on many scrolling containers on the same page, whereas jQuery_lazyload can't.
  • Throttled execution for optimized CPU usage. LazyLoad's listener to the container's scroll event is throttled by default, meaning that the main function of the script will not continuously be executed on devices with a smooth scroll such as mobile devices or Macs. jQuery_lazyload doesn't have this option, so its scroll handler function is executed more frequently then necessary, slowing down the user device.
  • Support for responsive images. LazyLoad also supports responsive images, both via the srcset attribute (x notation, i.g. 1x, 2x .. nx and w notation, i.g. 100w, 200w + sizes attribute), and via the picture tag.
  • jQuery independency. As LazyLoad doesn't rely on jQuery, you can use it in contexts where jQuery is not used (and it's much, much faster)!

How to use

HTML - img tags

Markup your images putting the image source inside the data-original attribute. Specify both width and height attributes if known upfront, so the browser can allocate the space on your page.

<img data-original="/your/image1.jpg" width="100" height="172">
<img data-original="/your/image2.jpg" width="100" height="172">
<img data-original="/your/image3.jpg" width="100" height="172">

Or if you want to use srcset:

<img data-original="/your/image1.jpg"
    data-original-set="/your/image1.jpg 1x, /your/image1@2x.jpg 2x"
    width="100" height="172">
<img data-original="/your/image2.jpg"
    data-original-set="/your/image2.jpg 1x, /your/image2@2x.jpg 2x"
    width="100" height="172">
<img data-original="/your/image3.jpg"
    data-original-set="/your/image3.jpg 1x, /your/image3@2x.jpg 2x"
    width="100" height="172">

Or if you want to use srcset and sizes:

<img data-original="/your/image1.jpg"
    data-original-set="/your/image1.jpg 200w, /your/image1@2x.jpg 400w"
    sizes="(min-width: 20em) 35vw, 100vw">
<img data-original="/your/image2.jpg"
    data-original-set="/your/image2.jpg 200w, /your/image2@2x.jpg 400w"
    sizes="(min-width: 20em) 35vw, 100vw">
<img data-original="/your/image3.jpg"
    data-original-set="/your/image3.jpg 200w, /your/image3@2x.jpg 400w"
    sizes="(min-width: 20em) 35vw, 100vw">

Or if you want to use the picture tag:

<picture>
    <source media="(min-width: 1024px)" data-original-set="/your/image1a.jpg" />
    <source media="(min-width: 500px)" data-original-set="/your/image1b.jpg" />
    <img alt="Stivaletti" data-original="/your/image1.jpg">
</picture>
<picture>
    <source media="(min-width: 1024px)" data-original-set="/your/image2a.jpg" />
    <source media="(min-width: 500px)" data-original-set="/your/image2b.jpg" />
    <img alt="Stivaletti" data-original="/your/image2.jpg">
</picture>
<picture>
    <source media="(min-width: 1024px)" data-original-set="/your/image3a.jpg" />
    <source media="(min-width: 500px)" data-original-set="/your/image3b.jpg" />
    <img alt="Stivaletti" data-original="/your/image3.jpg">
</picture>

Note that not all the images in the page needs to be lazy loaded. You can leave the first images (the amount that you're quite sure that fits in the majority of viewports) loaded normally, then start lazy loading the rest.

HTML - script tag

Include the script in the bottom of your HTML page, just before the closing </body> tag.

<body>
    <!-- Your content ... -->
    <script src="lazyload.min.js"></script>
</body>

Javascript

In your javascript code, create an instance of LazyLoad, doing so:

var myLazyLoad = new LazyLoad();

Or, if you want to customize the behaviour of LazyLoad passing some options in, you can do so:

var myLazyLoad = new LazyLoad({
    // example of options object -> see options section
    threshold: 500,
    container: document.getElementById('scrollPane'),
    elements_selector: ".showCase img",
    throttle: 30,
    data_src: "src",
    data_srcset: "srcset",
    callback_set: function() { /* ... */ }
});

See the demos and options sections for further defails.

CSS

Be sure that the images that are going to be lazy loaded occupy some vertical space (*), ideally the same space of the loaded images.

To do that you can size all your img elements using CSS, for example doing this:

/* Sets a min-height to all images
so that they occupy some space
before they are loaded */
img {
    display: block;
    width: 100%;
    height: auto;
    min-height: 300px;
}

In addition, something needs to be done to avoid the "broken image" icon to appear when the img element without the src attribute enters the viewport.

/* Prevents img without src to appear */
img:not([src]) {
    visibility: hidden;
}

Furthermore, we need to deal with a Firefox anomaly that still shows the "broken image" icon while the image is loading. The CSS code that does this trick is the following:

/* Fixes Firefox anomaly */
@-moz-document url-prefix() {
    img:-moz-loading {
        visibility: hidden;
    }
}

(*) if you won't do so, a lot of images would enter the viewport as the user scrolls down, so you would lose the advantages that LazyLoad would bring to your website.

Options

For every instance of LazyLoad you can pass in some options, to alter its default behaviour. Here's the list of the options.

Name Meaning Default value
container The container in which to start searching for elements, and from which to listen to the scroll event window
elements_selector The selector of the image elements inside the container "img"
threshold The distance out of the viewport, expressed in pixel, before which to start loading the images 300
throttle The time that has to pass between one element parsing and the following, when fast scroll events occur 40
data_src The name of the data attribute containing the original image source. The "data-" is automatically added. "original"
data_srcset The name of the data attribute containing the original image source set. The "data-" is automatically added. If you also need to add the sizes attribute, you can do it directly to you img tag, as sizes gets ignored when the srcset attribute is missing . "original-set"
class_loading The class applied to the img or iframe elements while the loading is in progress "loading"
class_loaded The class applied to the img or iframe elements when the loading is complete "loaded"
skip_invisible Specifies whether the script has to consider invisible images or not true
callback_load A function to be called when an image was loaded. null
callback_set A function to be called when the src of an image is set in the DOM. null
callback_processed A function to be called when an image was processed. null

Public methods

Method name Effect
update() Tells LazyLoad that new lazy images have arrived in the container, so it must start to manage them
destroy() Destroys the instance, unsetting instance variables and removing listeners.
handleScroll() A throttled scroll handler. This is called automatically from LazyLoad if the container element fires a scroll event, but it's exposed as a public method to allow you to use LazyLoad otherwise (i.g. when using iScroll)

Demos

Learn how to use LazyLoad in different contexts by looking at the code.

Simple demo

The images are in the page body, so LazyLoad is created with the default options.

See it in action | View source

With srcset attribute (1x 2x notation) demo

The images also rely on the srcset attribute to be loaded lazily. Just pass in the data_srcset option and the job will be done.

See it in action | View source

With srcset + sizes attributes demo

The images also rely on the srcset attribute to be loaded lazily, and on the sizes attribute to be sized. Just pass in the data_srcset option, set the sizes attribute normally and the job will be done.

See it in action | View source

With picture tag demo

The images can also be marked up using the picture tag and still be loaded lazily. Just pass in the data_srcset option and the job will be done.

See it in action | View source

Lazy loading iframe demo

To lazy load iframes, set the elements_selector to iframes and optionally change the data_src option (default data-original) to define which data attribute will contain the source of the iframe.

See it in action | View source

Using background images demo

To make lazy load use background images, set the elements_selector to whatever tag you want except img and iframe. The image will be loaded from the URL specified in the data attribute defined by the data_src option (default data-original).

See it in action | View source

Single scrolling container demo

The images are in scrolling container, a scrolling div inside the page body.

See it in action | View source

Multiple scrolling containers demo

The images are in multiple scrolling containers, two scrolling divs inside the page body.

See it in action | View source

Changing content demo

The images are added in different times, simulating content that gets added by AJAX requests or other actions.

See it in action | View source

Destroying LazyLoad demo

This demo shows how to destroy LazyLoad when you're finished using it, to free up some memory.

See it in action | View source

Browser support

Browser support is starting from Internet Explorer 8 up. Please mind that you can't use CSS 3 selectors in IE8, so beware of them if you change the elements_selector default option.

Install

It's time to give it a try and boost your website performance!

Download from GitHub

Download the package from GitHub and add dist/lazyload.min.js to your project files.

Zip file

Install using bower

Run the following command on your terminal or command prompt.

bower install vanilla-lazyload
Install using npm

Run the following command on your terminal or command prompt.

npm install vanilla-lazyload