lazyload: A JavaScript repository from emaklakov

LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading images only as they enter the viewport. LazyLoad is written in plain (vanilla) Javascript featuring the Intersection Observer API, it supports responsive images, it's SEO friendly and it has some other notable features.

Check out the LazyLoad website, in case you're reading this on GitHub.

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Include the script / browser support

Simple: direct include from cdnjs

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

<script src="https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/8.6.0/lazyload.min.js"></script>

Starting from version 9, LazyLoad uses the IntersectionObserver API, which is not supported by Internet Explorer and Safari (yet). As a result, if you included the latest version of LazyLoad, all the images would be loaded at once in those browsers.

To include the latest version of LazyLoad, use the following script:

<script src="https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/10.4.1/lazyload.min.js"></script>

Advanced and best option: conditionally load version 8 or 10

The best thing you can do is to conditionally load the best version of LazyLoad depending on the browser's support of the IntersectionObserver API. You can do it with the following script:

(function(w, d){
	var b = d.getElementsByTagName('body')[0];
	var s = d.createElement("script"); s.async = true;
	var v = !("IntersectionObserver" in w) ? "8.6.0" : "10.4.1";
	s.src = "https://cdnjs.cloudflare.com/ajax/libs/vanilla-lazyload/" + v + "/lazyload.min.js";
	w.lazyLoadOptions = {}; // Your options here. See "recipes" for more information about async.
	b.appendChild(s);
}(window, document));

See the conditional_load.html file in the demos folder to try it or play around with it.

Local install

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

download it from the dist folder The file you typically want to use is lazyload.min.js If you prefer the ES2015 version, use lazyload.es2015.js
install it with npm Recommended version npm install vanilla-lazyload@8.2.0
Latest version npm install vanilla-lazyload
install it with bower Recommended version bower install vanilla-lazyload#8.2.0 Latest version bower install vanilla-lazyload

Async script

It's possible to include it as an async script, see Recipes below.

Recipes

Simple

When to use: your lazy images are (normally) located in the body of a scrolling page.

HTML

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

Javascript

var myLazyLoad = new LazyLoad();

DEMO | SOURCE | API

Responsive images - srcset and sizes

When to use: you want to lazily load responsive images using the srcset and the sizes attribute.

HTML

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

Javascript

var myLazyLoad = new LazyLoad();

DEMO | SOURCE | API

Responsive images - picture

When to use: 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 alt="Stivaletti" data-src="/your/image1.jpg">
</picture>

Javascript

var myLazyLoad = new LazyLoad();

DEMO | SOURCE | API

Async script + auto initialization

When to use: 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

When to use: 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;
})();

Scrolling panel

When to use: when your scrolling container is not the main browser window, but a scrolling container.

HTML

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

Javascript

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

DEMO | SOURCE | API

Multiple scrolling panels

When to use: when your scrolling container is not the main browser window, and you have multiple scrolling containers.

HTML

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

Javascript

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

DEMO | SOURCE | API

Dynamic content

When to use: 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

When to use: 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

When to use: 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

When to use: 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 do it using the CSS classes set by LazyLoad when loading starts - see API.

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

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.	`"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"`
`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"`
`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()`	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.

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.

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.

emaklakov/lazyload

Include the script / browser support

Simple: direct include from cdnjs

Advanced and best option: conditionally load version 8 or 10

Local install

Async script

Recipes

Simple

Responsive images - srcset and sizes

Responsive images - picture

Async script + auto initialization

Auto init + store the instance in a variable

Scrolling panel

Multiple scrolling panels

Dynamic content

Lazy iframes

Lazy background images

Lazy LazyLoad

Demos

Tips & tricks

Occupy vertical space and maintain ratio

Show the images while they load

Dealing with Microsoft Edge problems

API

Options

Methods

Notable features

SEO friendly

It works with your favourite framework

Support for responsive images.

Progressive JPEG support --> improve perceived performance

Intersection Observer API for optimized CPU usage

Much faster than jQuery_lazyload