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 - async script + auto init

Include the following scripts at the end of your HTML page, before the </body> tag.

<script>window.lazyLoadOptions = {/* your settings or array of settings */};</script>
<script async src="../src/lazyload.js"></script>

LazyLoad is then downloaded and automatically initialized right after with the options you passed in window.lazyLoadOptions.

Please note that if you put the script at the beginning of your HTML page, LazyLoad will probably initialized before the DOM ready event. In that case, you need to get the instance and use the update method on it. This will make it check the DOM again.

To get an auto-initialized instance of LazyLoad:

window.addEventListener('LazyLoad::Initialized', function (e) {
    lazyLoadInstance = e.detail.instance;
}, false);

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: 200,
    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	`150`
`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 elements (`img` or `iframe`) while the loading is in progress	`"loading"`
`class_loaded`	The class applied to the elements (`img` or `iframe`) when the loading is complete	`"loaded"`
`class_error`	The class applied to the elements (`img` or `iframe`) when the element causes an error	`"error"`
`class_initial`	The class applied to the first batch elements (`img` or `iframe`) to be loaded in the page	`"initial"`
`skip_invisible`	Specifies whether the script has to consider invisible images or not	`true`
`callback_load`	A function to be called when an element (`img` or `iframe`) was loaded.	`null`
`callback_error`	A function to be called when an element (`img` or `iframe`) triggers an error.	`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

Hundreds of images demo

This demo shows the performance of LazyLoad with 300+ images on the page.

See it in action | View source

Callbacks demo

This demo shows how to use LazyLoad's callbacks, including the error callback to provide a fallback image.

See it in action | View source

Fade-in + initial demo

This demo shows how to use LazyLoad to fade-in loaded images as they load, except the initial batch of images.

See it in action | View source

Async + auto init demo

This demo shows how to load LazyLoad with an async script, then auto-initialize it.

See it in action | View source

Browser support

Version 7.2.0+ of LazyLoad introduced support from Internet Explorer 11 up and it's OK, if you check the desktop browsers stats for the 1st quarter of 2017.

If you don't feel comfortable with leaving legacy IE versions behind, just use version 3.0.0 which supports back to IE 8.

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 npm

Run the following command on your terminal or command prompt.

npm install vanilla-lazyload

Install using bower

Run the following command on your terminal or command prompt.

bower install vanilla-lazyload

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README.v1.md

README.v1.md

LazyLoad (aka Vanilla LazyLoad)

Difference with jQuery_lazyload

How to use

HTML - `img` tags

HTML - async script + auto init

HTML - `script` tag

Javascript

CSS

Options

Public methods

Demos

Simple demo

With `srcset` attribute (1x 2x notation) demo

With `srcset` + `sizes` attributes demo

With `picture` tag demo

Lazy loading `iframe` demo

Using background images demo

Single scrolling container demo

Multiple scrolling containers demo

Changing content demo

Destroying LazyLoad demo

Hundreds of images demo

Callbacks demo

Fade-in + initial demo

Async + auto init demo

Browser support

Install

Download from GitHub

Install using npm

Install using bower

Files

README.v1.md

Latest commit

History

README.v1.md

File metadata and controls

LazyLoad (aka Vanilla LazyLoad)

Difference with jQuery_lazyload

How to use

HTML - img tags

HTML - async script + auto init

HTML - script tag

Javascript

CSS

Options

Public methods

Demos

Simple demo

With srcset attribute (1x 2x notation) demo

With srcset + sizes attributes demo

With picture tag demo

Lazy loading iframe demo

Using background images demo

Single scrolling container demo

Multiple scrolling containers demo

Changing content demo

Destroying LazyLoad demo

Hundreds of images demo

Callbacks demo

Fade-in + initial demo

Async + auto init demo

Browser support

Install

Download from GitHub

Install using npm

Install using bower

HTML - `img` tags

HTML - `script` tag

With `srcset` attribute (1x 2x notation) demo

With `srcset` + `sizes` attributes demo

With `picture` tag demo

Lazy loading `iframe` demo