All about Lazy Load

LazyLoad (vanilla-lazyload)

LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading images only as they enter the viewport. LazyLoad supports responsive images.

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LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading your content images, videos and iframes only as they enter the viewport. It’s written in plain “vanilla” JavaScript, it leverages the IntersectionObserver API, it works with responsive images and it supports native lazy loading. See notable features for more.

➡️ Jump to: 👨‍💻 Getting started - HTML - 👩‍💻 Getting started - Script - 🥧 Recipes - 📺 Demos - 😋 Tips & tricks - 🔌 API - 😯 Notable features

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👨‍💻 Getting started - HTML

In order to make your content be loaded by LazyLoad, you must use some data- attributes instead of the actual attributes. Examples below.

Lazy image:

<img alt="A lazy image" data-src="lazy.jpg">

Lazy image with low quality placeholder:

<img alt="A lazy image" src="lazy-lowQuality.jpg" data-src="lazy.jpg">

Lazy responsive image with srcset and sizes:

<img alt="A lazy image" class="lazy" 
    data-src="lazy.jpg" 
    data-srcset="lazy_400.jpg 400w, lazy_800.jpg 800w" 
    data-sizes="100w">

To have a low quality placeholder, add the src attribute pointing to a very small version of the image. E.g. src="lazy_10.jpg".

Lazy responsive image with hi-dpi support using the picture tag:

<picture>
    <source 
        media="(min-width: 1200px)" 
        data-srcset="lazy_1200.jpg 1x, lazy_2400.jpg 2x">
    <source 
        media="(min-width: 800px)" 
        data-srcset="lazy_800.jpg 1x, lazy_1600.jpg 2x">
    <img alt="A lazy image" class="lazy" 
        data-src="lazy.jpg">
</picture>

To have a low quality placeholder, add the src attribute pointing to a very small version of the image to the img tag. E.g. src="lazy_10.jpg".

Lazy responsive image with automatic WebP format selection, using the picture tag:

<picture>
    <source type="image/webp" 
        data-srcset="lazy_400.webp 400w, lazy_800.webp 800w" 
        data-sizes="100w">
    <img alt="A lazy image" class="lazy" 
        data-src="lazy.jpg" 
        data-srcset="lazy_400.jpg 400w, lazy_800.jpg 800w"
        data-sizes="100w">
</picture>

To have a low quality placeholder, add the src attribute pointing to a very small version of the image to the img tag. E.g. src="lazy_10.jpg".

Lazy background image

Single background

<div class="lazy" data-bg="url(lazy.jpg)"></div>

Multiple backgrounds

<div class="lazy" 
    data-bg="url(lazy-head.jpg), url(lazy-body.jpg), linear-gradient(#fff, #ccc)">
    ...
</div>

Notes:

• you need to use url() in the value of your data-bg attribute, also for single background
• you shouldn’t use background images to load content images, they’re bad for SEO and for accessibility
• on background images, callback_loaded won’t be called and the class_loaded class won’t be added

Lazy video

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

Lazy iframe

<iframe class="lazy" data-src="lazyFrame.html" poster="lazy.jpg"></iframe>

👩‍💻 Getting started - Script

The latest, recommended version of LazyLoad is 12.1.0.

To polyfill or not to polyfill IntersectionObserver?

On browser NOT supporting IntersectionObserver such as Internet explorer and older versions of Safari you can choose whether or not to add a javascript polyfill for it.

If you don’t use a polyfill, LazyLoad will load all the images as soon as it’s downloaded and executed. The number of impacted users would be relatively small, so this is a completely acceptable choice.

If you prefer to load a polyfill, the regular LazyLoad behaviour is granted.

The simple, easiest way

The easiest way to use LazyLoad is to include the script from a CDN:

<script src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.1.0/dist/lazyload.min.js"></script>

Or, with the IntersectionObserver polyfill:

<script src="https://cdn.jsdelivr.net/npm/intersection-observer@0.7.0/intersection-observer.js"></script>
<script src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.1.0/dist/lazyload.min.js"></script>

Then, in your javascript code:

var lazyLoadInstance = new LazyLoad({
    elements_selector: ".lazy"
    // ... more custom settings?
});

To be sure that DOM for your lazy content is ready when you instantiate LazyLoad, place the script tag right before the closing </body> tag. If more DOM arrives later, e.g. via an AJAX call, you’ll need to call lazyLoadInstance.update(); to make LazyLoad check the DOM again.

if (lazyLoadInstance) {
    lazyLoadInstance.update();
}

Include via RequireJS

You can use RequireJS to dynamically and asynchronously load modules in your website.

You can also find the original W3C’S IntersectionObserver Polyfill packed in AMD so you can require it conditionally, along with LazyLoad.

Include RequireJS:

<script src="https://cdn.jsdelivr.net/npm/requirejs@2.3.6/bin/r.min.js"></script>

Then require the AMD version of LazyLoad, like this:

var lazyLoadAmdUrl = "https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.1.0/dist/lazyload.amd.min.js";
var polyfillAmdUrl = "https://cdn.jsdelivr.net/npm/intersection-observer-amd@2.1.0/intersection-observer-amd.js";

/// Dynamically define the dependencies
var dependencies = [
    "IntersectionObserver" in window
        ? null // <- Doesn't require the polyfill
        : polyfillAmdUrl,
    lazyLoadAmdUrl
];

// Initialize LazyLoad inside the callback
require(dependencies, function(_, LazyLoad) {
    var lazyLoadInstance = new LazyLoad({
        elements_selector: ".lazy"
        // ... more custom settings?
    });
}

DEMO - SOURCE

Using an async script

If you prefer, it’s possible to include LazyLoad’s script using async script and initialize it as soon as it’s loaded.

To do so, you must define the options before including the script. You can pass:

• {} an object to get a single instance of LazyLoad
• [{}, {}] an array of objects to get multiple instances of LazyLoad, each one with different options.

<script> 
    // Set the options to make LazyLoad self-initialize 
    window.lazyLoadOptions = { 
        elements_selector: ".lazy", 
        // ... more custom settings? 
    };
</script> 

Then include the script.

<script async src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.1.0/dist/lazyload.min.js"></script> 

Possibly place the script tag right before the closing </body> tag. If you can’t do that, LazyLoad could be executed before the browser has loaded all the DOM, and you’ll need to call its update() method to make it check the DOM again.

Using an async script + getting the instance reference

Same as above, but you must put the addEventListener code shown below before including the async script.

<script> 
    // Set the options to make LazyLoad self-initialize 
    window.lazyLoadOptions = { 
        elements_selector: ".lazy", 
        // ... more custom settings? 
    };
    // Listen to the initialization event and get the instance of LazyLoad 
    window.addEventListener('LazyLoad::Initialized', function (event) { 
        window.lazyLoadInstance = event.detail.instance; 
    }, false); 
</script> 

Then include the script.

<script async src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.1.0/dist/lazyload.min.js"></script> 

Now you’ll be able to call its methods, like:

if (lazyLoadInstance) {
    lazyLoadInstance.update();
}

Note about Internet Explorer: because this technique uses a CustomEvent (learn more) to trigger the LazyLoad::Initialized event, you might want to add this micro polyfill to make it work on Internet Explorer.

<script>
// CustomEvent micro-polyfill 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;
})();
</script>

Local install

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

Using npm

npm install vanilla-lazyload

Using bower

bower install vanilla-lazyload

Manual download

Download one the latest releases. The files you need are inside the dist folder. If you don’t know which one to pick, use lazyload.min.js, or read about bundles.

Local usage

Should you install LazyLoad locally, you can import it as ES module like the following:

import LazyLoad from "vanilla-lazyload";

It’s also possible (but unadvised) to use the require commonJS syntax.

More information about bundling LazyLoad with WebPack are available on this specific repo.

Usage with React

Take a look at this example of usage of React with LazyLoad on Sandbox.

This implementation takes the same props that you would normally pass to the img tag, but it renders a lazy image. Feel free to fork and improve it!

Bundles

Inside the dist folder you will 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 RequireJS module loader, ~0.5kb smaller than UMD version
lazyload.esm.js	ES 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 ready to copy & paste code for your convenience.

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

Scrolling panel(s)

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

HTML

<div class="scrollingPanel" id="scrollingPanel"> 
    <!-- Set of images -->
</div>

Javascript

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

DEMO - SOURCE - API

If you have multiple scrolling panels, you can use the following markup and code.
HTML

<div id="scrollingPanel1" class="scrollingPanel">
    <!-- Set of images -->
</div>
<div id="scrollingPanel2" class="scrollingPanel">
    <!-- Set of images -->
</div>

Javascript

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

DEMO - SOURCE - API

Delay loading

💡 Use case: if a your scrolls fast over your images, you might wait a short time before the images start loading. This is how.

HTML

<img class="lazy" alt="A lazy image" 
     data-src="lazy.jpg"
     width="220" height="280">

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy",
    load_delay: 300 //adjust according to use case
});

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_enter: 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_enter function, which creates a new instance of LazyLoad on the .horzContainer element.

Credit