LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading images as they enter the viewport. It's written in plain "vanilla" JavaScript, uses IntersectionObserver, and supports responsive images. It's also SEO-friendly and it has some other notable features.
JavaScript
Andrea Verlicchi
Andrea Verlicchi Removed emojis from links
Latest commit 7298d0c Aug 10, 2018

README.md

LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading images, video or iframes as they enter the viewport. It's written in plain "vanilla" JavaScript, uses Intersection Observers, and supports responsive images. It's also SEO-friendly and it has some other notable features.

➡️ Jump to: 👨‍💻 Include the script - 🥧 Recipes - 📺 Demos - 😋 Tips & tricks - 🔌 API - 😯 Notable features


👨‍💻 Include the script

Versions information

The universal, recommended version of LazyLoad is 8.x as it supports ALL browsers from IE9 up.

Version 10.x is best for performance since it leverages IntersectionObserver API, which is not supported by Internet Explorer and Safari, therefore all the images would be loaded at once in those browsers.

Version 8.x is recommended for local install, but you can be smart and conditionally load the best version from cdnjs instead.

Include as script from cdnjs

Version 8.x - versions info

<script src="https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/8.15.0/lazyload.min.js"></script>

Version 10.x - versions info

<script src="https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/10.15.0/lazyload.min.js"></script>

The file lazyload.min.js is provided as UMD (Universal Module Definition).
See bundles for more module types like AMD, IIFE and ES6 module.

Async script

It's possible to include it as an async script, see the recipes section for more information.

Conditional load

The best thing you can do for runtime performance is to conditionally load the appropriate version of LazyLoad depending on browser support of IntersectionObserver.

You can do it with the following script:

(function(w, d){
    var b = d.getElementsByTagName('body')[0];
    var s = d.createElement("script"); 
    var v = !("IntersectionObserver" in w) ? "8.15.0" : "10.15.0";
    s.async = true; // This includes the script as async. See the "recipes" section for more information about async loading of LazyLoad.
    s.src = "https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/" + v + "/lazyload.min.js";
    w.lazyLoadOptions = {/* Your options here */};
    b.appendChild(s);
}(window, document));

See demos/conditional_load.html to try and play around with it.

The file lazyload.min.js is provided as UMD (Universal Module Definition).
See bundles for more module types like AMD, IIFE and ES6 module.

Include via require.js

If you use require-js to dynamically load modules in your website, you can take advantage of it.

define("vanilla-lazyLoad", ["https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/8.15.0/lazyload.amd.min.js"], function (LazyLoad) {
    return LazyLoad;
});

You can also conditionally load the best version.

(function (w) {
    var v = !("IntersectionObserver" in w) ? "8.15.0" : "10.15.0";
    define("vanilla-lazyLoad", ["https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/" + v + "/lazyload.amd.min.js"], function (LazyLoad) {
        return LazyLoad;
    });
}(window));

Local install

If you prefer to install LazyLoad locally in your project, you can either:

Install with npm

Version 8.x, recommended - versions info

npm install vanilla-lazyload@8.15.0

Version 10.x - versions info

npm install vanilla-lazyload@10.15.0

Install with bower

Install with bower is also possible using bower install vanilla-lazyload#{version}

Manual download

Download one the latest releases. The files you need are inside the dist folder.

The file lazyload.min.js is provided as UMD (Universal Module Definition).
See bundles for more module types like AMD, IIFE and ES6 module.

Bundles

Inside dist folder you find different bundles.

Filename Module Type Advantages
lazyload.min.js UMD (Universal Module Definition) Works pretty much everywhere, even in common-js contexts
lazyload.iife.min.js IIFE (Immediately Invoked Function Expression) Works as in-page <script src="...">, ~0.5kb smaller than UMD version
lazyload.amd.min.js AMD (Asynchronous Module Definition) Works with require.js module loader, ~0.5kb smaller than UMD version
lazyload.es2015.js ES6 Module Exports LazyLoad so you can import it in your project both using <script type="module" src="..."> and a bundler like WebPack or Rollup

🥧 Recipes

This is the section where you can find copy & paste code for your convenience.

Simple

💡 Use case: your lazy images are (normally) located in the body of a scrolling page.

HTML

<img class="lazy" alt="..." 
     data-src="../img/44721746JJ_15_a.jpg"
     width="220" height="280">

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy"
});

DEMO - SOURCE - API

Scrolling panel

💡 Use case: when your scrolling container is not the main browser window, but a scrolling container.

HTML

<div class="scrollingPanel">
    <img alt="Image description" 
         data-src="../img/44721746JJ_15_a.jpg" 
         width="220" height="280">
    <!-- More images -->
</div>

CSS

.scrollingPanel {
    overflow-y: scroll;
    -webkit-overflow-scrolling: touch;
}

Javascript

var myLazyLoad = new LazyLoad({
    container: document.getElementById('scrollingPanel')
});

DEMO - SOURCE - API

Multiple scrolling panels

💡 Use case: when your scrolling container is not the main browser window, and you have multiple scrolling containers.

HTML

<div id="scrollingPanel1" class="scrollingPanel">
    <img alt="Image description" 
         data-src="../img/44721746JJ_15_a.jpg" 
         width="220" height="280">
    <!-- More images -->
</div>
<div id="scrollingPanel2" class="scrollingPanel">
    <img alt="Image description" 
         data-src="../img/44721746JJ_15_a.jpg" 
         width="220" height="280">
    <!-- More images -->
</div>

CSS

.scrollingPanel {
    overflow-y: scroll;
    -webkit-overflow-scrolling: touch;
}

Javascript

var myLazyLoad1 = new LazyLoad({
    container: document.getElementById('scrollingPanel1')
});
var myLazyLoad2 = new LazyLoad({
    container: document.getElementById('scrollingPanel2')
});

DEMO - SOURCE - API

Responsive images - img tag with srcset / sizes

💡 Use case: you want to lazily load responsive images using the srcset and the sizes attribute.

HTML

<img class="lazy" data-src="/your/image1.jpg"
    data-srcset="/your/image1.jpg 200w, /your/image1@2x.jpg 400w"
    data-sizes="(min-width: 20em) 35vw, 100vw">

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy"
});

DEMO - SOURCE - API

Responsive images - picture tag

💡 Use case: you want to lazily load responsive images using the picture tag.

HTML

<picture>
    <source media="(min-width: 1024px)" data-srcset="/your/image1a.jpg" />
    <source media="(min-width: 500px)" data-srcset="/your/image1b.jpg" />
    <img class="lazy" alt="Stivaletti" data-src="/your/image1.jpg">
</picture>

Please note that you just need to put the lazy class on the <img> tag but not in the <source> tags.

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy"
});

DEMO - SOURCE - API

Switch to WebP

💡 Use case: you want to dynamically switch your images' filename extension to .webp if the user's browser supports it.

HTML

<img class="lazy" data-src="/your/image1.jpg"
    data-srcset="/your/image1.jpg 200w, /your/image1@2x.jpg 400w"
    data-sizes="(min-width: 20em) 35vw, 100vw">

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy",
    to_webp: true
});

Hint: if you provide only some images in the WebP format, it's advisable to create 2 different instances of LazyLoad, as shown in the this demo and source code.

DEMO - SOURCE - API

Videos

💡 Use case: you want to lazily load videos using the video tag.

HTML

<video class="lazy" controls width="620"
    data-src="/your/video.mp4" poster="/your/poster.jpg">
    <source type="video/mp4" data-src="/your/video.mp4">
    <source type="video/ogg" data-src="/your/video.ogg">
    <source type="video/avi" data-src="/your/video.avi">
</video>

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy"
});

DEMO - SOURCE - API

Iframes

💡 Use case: you want to lazily load iframes.

HTML

<iframe class="lazy" data-src="https://some.page.com" frameborder="0"></iframe>

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy"
});

DEMO - SOURCE - API

Async script + auto initialization

💡 Use case: you want to use a non-blocking script (which is faster), and you don't need to have control on the exact moment when LazyLoad is created.

Include the following scripts at the end of your HTML page, right before closing the body tag.

HTML + Javascript

<script>
window.lazyLoadOptions = {
    /* your lazyload options */
};
</script>

<!-- Download the script and execute it after lazyLoadOptions is defined -->
<script async src="https://.../lazyload.min.js"></script>

If you need multiple async instances, just pass window.lazyLoadOptions an array of settings.

<script>
window.lazyLoadOptions = [{
    /* your instance 1 options */
}, {
    /* your instance 2 options */
}];
</script>

<!-- Download the script and execute it after lazyLoadOptions is defined -->
<script async src="https://.../lazyload.min.js"></script>

Please note that if you put the script at the beginning of your HTML page, LazyLoad will sometimes be executed before the browser has loaded all the DOM. In that case, you need to store the instance in a variable and use the update method on it. This will make it check the DOM again. See API.

DEMO - SOURCE - API

Auto init + store the instance in a variable

💡 Use case: you want to use a non-blocking script (which is faster), you don't need to have control on the exact moment when LazyLoad is created, but you need to assign the an auto-initialized instance to a variable, e.g. to use the API on it.

HTML + Javascript

<script>
// Listen to the Initialized event
window.addEventListener('LazyLoad::Initialized', function (e) {
    // Get the instance and puts it in the lazyLoadInstance variable
    lazyLoadInstance = e.detail.instance;
}, false);

// Set the lazyload options for async usage
lazyLoadOptions = {
    /* your lazyload options */
};
</script>

<!-- Download the script and execute it after lazyLoadOptions is defined -->
<script async src="https://.../lazyload.min.js"></script>

You will then have the auto-generated instance in the lazyLoadInstance variable.

DEMO - SOURCE - API

Note about Internet Explorer

LazyLoad uses CustomEvent (learn more to trigger the LazyLoad::Initialized, but this event type is not natively supported by Internet Explorer. If you want to use asynchronous loading and need to store the instance you can use the following polyfill to enable support for Internet Explorer.

(function () {
    if (typeof window.CustomEvent === "function") {
        return false;
    }

    function CustomEvent(event, params) {
        params = params || {bubbles: false, cancelable: false, detail: undefined};
        var evt = document.createEvent("CustomEvent");
        evt.initCustomEvent (event, params.bubbles, params.cancelable, params.detail);
        return evt;
    }

    CustomEvent.prototype = window.Event.prototype;
    window.CustomEvent = CustomEvent;
})();

Dynamic content

💡 Use case: when you want to lazily load images, but the number of images change in the scrolling area changes, maybe because they are added asynchronously.

HTML

The HTML to use depends on your case, see other recipes' HTML

Javascript

var myLazyLoad = new LazyLoad();
// After your content has changed...
myLazyLoad.update();

DEMO - SOURCE - API

Lazy iframes

💡 Use case: you want to lazily load iframes in your web page, maybe because you have many or just because you want to load only what your users actually want to see.

HTML

<iframe data-src="iframes/i01.html" frameborder="0"></iframe>

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: "iframe"
});

DEMO - SOURCE - API

Lazy background images

💡 Use case: your images are set as CSS background images instead of real img, but you still want to lazily load them.

HTML

<div class="lazy" data-src="../img/44721746JJ_15_a.jpg"></div>

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy"
});

That's it. Whenever the element selected by elements_selector is not an img or an iframe, LazyLoad puts the image found in the data-src attribute in the background-image of the element.

DEMO - SOURCE - API

Lazy LazyLoad

💡 Use case: when you have a lot of scrolling containers in the page and you want to instantiate a LazyLoad only on the ones that are in the viewport.

HTML

<div class="horzContainer">
    <img src="" alt="Row 01, col 01" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_01_col_01&amp;w=200&amp;h=200">
    <img src="" alt="Row 01, col 02" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_01_col_02&amp;w=200&amp;h=200">
    <!-- ... -->
</div>
<div class="horzContainer">
    <img src="" alt="Row 02, col 01" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_02_col_01&amp;w=200&amp;h=200">
    <img src="" alt="Row 02, col 02" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_02_col_02&amp;w=200&amp;h=200">
    <!-- ... -->
</div>

Javascript

var lazyLoadInstances = [];
// The "lazyLazy" instance of lazyload is used (kinda improperly) 
// to check when the .horzContainer divs enter the viewport
var lazyLazy = new LazyLoad({
    elements_selector: ".horzContainer",
    // When the .horzContainer div enters the viewport...
    callback_set: function(el) {
        // ...instantiate a new LazyLoad on it
        var oneLL = new LazyLoad({
            container: el
        });
        // Optionally push it in the lazyLoadInstances 
        // array to keep track of the instances
        lazyLoadInstances.push(oneLL);
    }
});

That's it. Whenever a .horzContainer element enters the viewport, LazyLoad calls the callback_set function, which creates a new instance of LazyLoad on the .horzContainer element.

DEMO - SOURCE - API


📺 Demos

Didn't find the recipe that exactly matches your case? We have demos!

The demos folder contains 15 use cases of LazyLoad. You might find there what you're looking for.


😋 Tips & tricks

Occupy vertical space and maintain ratio

You need to be sure that the images that are going to be lazy loaded occupy some vertical space (*), ideally the same space of the loaded images. Otherwise, all the images will be loaded at once.

In an elastic layout where images width change, you want to keep vertical space maintaining the images height, using a width/height ratio calculation.

.image-wrapper {
    width: 100%;
    height: 0;
    padding-bottom: 66.67%; /* You define this doing height / width * 100% */
    position: relative;
}
.image {
    width: 100%;
    /*height: auto;*/
    position: absolute;
}

More info in Sizing Fluid Image Containers with a Little CSS Padding Hack by Andy Shora.

There's also a useful SASS mixin to maintain aspect ratio on CSS tricks.

@mixin aspect-ratio($width, $height) {
  position: relative;
  &:before {
    display: block;
    content: "";
    width: 100%;
    padding-top: ($height / $width) * 100%;
  }
  > .content {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
  }
}

Show the images while they load

Images should be shown while they load, and not after, to give your users the best perceived performance. This is especially true if you use a progressive loading format like Progressive JPEG.

In order to make your images visible as soon as LazyLoad sets the src/srcset attribute to it, you can either:

Do it like that via CSS:

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

Or instead of the above :not() selector do it using the CSS classes of class_loading and class_loaded set by LazyLoad when loading starts or is completed - see API.

Do NOT use placeholder images

We do not recommend to use a placeholder image (like a transparent pixel GIF) in your HTML.

  • For best perceived preformance, leave the src and srcset attributes blank. Doing so, the image will be shown as soon as LazyLoad starts loading the image. See this video or this pen to test the difference (remember to disable the cache and to set a slower connection speed if you have a very fast one).
  • If you put anything in the src (like a transparent GIF), then LazyLoad starts loading the image but it won't be shown by browsers until the new image is loaded, leading to a worse perceived performance.

It's safe not to put any value in the src nor srcset attributes, even if your HTML won't validate by a static code analyzer. The reason is that once JavaScript is executed, those values will be set by LazyLoad. For SEO, if the client is a crawler like Googlebot, it will be detected by LazyLoad which will fix the HTML.

Dealing with Microsoft Edge problems

According to what reported in #152, for Microsoft Edge to fire the IntersectionObserver for an img element, it must have a size. Since imgs are displayed inline-block as standard, MS Edge (version not specified) doesn't read them correctly.

By setting the following, edge is able to see the images and they get loaded.

img[data-src],
img[data-srcset] {
  display: block;
  min-height: 1px;
}

🔌 API

Constructor arguments

The new LazyLoad() instruction you execute on your page can take 2 parameters

Required What to pass Type Default value
No The option object for this instance of LazyLoad Plain Object {}
No A NodeSet of elements to execute LazyLoad on NodeSet null

The most common usage of LazyLoad constructor is to pass only the options object (see "options" in the next section). For example:

var lazyLoadOptions = { /* options here */ };
var aLazyLoad = new LazyLoad(lazyLoadOptions);

In the rare cases where you can't or don't want to select the elements using elements_selector and you have a reference variable to your elements set (can be a NodeSet or an array of elements), you can pass the elements set as second parameter.

var lazyLoadOptions = { /* options here */ };
var elementsToLazyLoad = getElementSetFromSomewhere();
var aLazyLoad = new LazyLoad(lazyLoadOptions, elementsToLazyLoad);

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 scrolling container, and the container of the elements in the elements_selector option. document
elements_selector The string selector of the elements to load lazily, to be selected as descendants of the container object. For multiple elements, you can add the css selectors for the same followed by a comma. E.g.: 'iframe, img, .container_class'. This will lazy load images for iframe and img elements along with the images/background images under 'container_class' "img"
threshold The distance out of the viewport, expressed in pixel, before which to start loading the images 300
data_src The name of the data attribute containing the original image source, excluding the "data-" part. E.g. if your data attribute is named "data-src", just pass "src" "src"
data_srcset The name of the data attribute containing the original image source set in either img and source tags, excluding the "data-" part. E.g. if your data attribute is named "data-original-set", just pass "original-set" "srcset"
data_sizes The name of the data attribute containing the sizes attribute to use, excluding the "data-" part. E.g. if your data attribute is named "data-sizes", just pass "sizes" "sizes"
class_loading The class applied to the elements while the loading is in progress. "loading"
class_loaded The class applied to the elements when the loading is complete "loaded"
class_error The class applied to the elements when the element causes an error "error"
to_webp A boolean flag that activates the dynamic switch to WEBP feature. More info. false
callback_enter A function to be called when the DOM element enters the viewport. null
callback_set A function to be called after the src of an image is set in the DOM. null
callback_load A function to be called when an element was loaded. null
callback_error A function to be called when an element triggers an error. null

Methods

You can call the following public methods on any instance of LazyLoad.

Method name Effect
update() Make LazyLoad to check for new lazy images in the container, using the elements_selector option.
loadAll() Loads all the lazy images right away, no matter if they are inside or outside the viewport.
load(element, force) Immediately loads any lazy element, even if it isn't selectable by the elements_selector option. Note that this method works only once on a specific element, unless you force it passing true as second parameter.
destroy() Destroys the instance, unsetting instance variables and removing listeners.

😯 Notable features

SEO friendly

LazyLoad doesn't hide your images from search engines, even if you don't specify any initial src for your image.

It works with your favourite framework

As LazyLoad doesn't rely on jQuery, you can use it in web applications using Angular, React or Vue.js without the need to include jQuery.

Support for responsive images.

LazyLoad supports responsive images, both via the srcset & sizes attributes and via the picture tag.

Progressive JPEG support --> improve perceived performance

Progressive JPEG is an image format which is very good for perceived performance because it's rendered sooner, and refined in progressive passes. LazyLoad shows your images while they load, letting progressive JPEG do its magic.

Dynamic switch to WEBP

WebP is a modern image format that provides superior lossless and lossy compression for images on the web. If you are providing your images in the WebP format too, LazyLoad can switch the filenames extension to .webp before the image is loaded, given that the user's browser supports it. See WebP support table.

Intersection Observer API for optimized CPU usage

Instead of listening to the scroll and resize events, LazyLoad uses the Intersection Observer API which is a new, blazing fast method to detect if an element is inside the browser viewport. Your users will see the difference in slow and even in fast devices or computers.

Much faster than jQuery_lazyload

This script is comparable to the notorious jQuery_lazyload, but LazyLoad is 10x faster, because LazyLoad uses only optimized, native javascript functions and methods, instead of jQuery.