jQuery & Zepto Lazy - Delayed Content, Image and Background Loader

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Table of Contents

About Lazy

Lazy is a fast, feature-rich and lightweight delayed content loading plugin for jQuery and Zepto. It’s designed to speed up page loading times and decrease traffic to your users by only loading the content in view. You can use Lazy in all vertical and horizontal scroll ways. It supports images in <img /> tags and backgrounds, supplied with css like background-image, by default. On those elements Lazy can set an default image or a placeholder while loading and supports retina displays as well. But Lazy is even able to load any other content you want by plugins and custom loaders.


Lazy will work with a wide range of browsers and support jQuery versions for years backwards and Zepto as alternative. You can pick any version since jQuery 1.7.2 or Zepto 1.1.6 or greater. There is no way to guarantee, that Lazy will work with all browsers, but all I’ve tested worked great so far. If you find any problems in specific browsers, please let me know.

Tested in: IE, Chrome (+ mobile), Firefox (+ mobile), Safari (+ mobile) and Android Browser.

Documentation / Examples

For documentation, examples and other information take a look on the project page.


First of all, you will need a copy of jQuery or Zepto to use Lazy successfully on your project. If you get this you can install Lazy by different ways. Some examples below:


Lazy and all plugins are available over cdnjs and jsDelivr CDN and can directly included to every page.

<!-- jsDeliver -->
<script type="text/javascript" src="//cdn.jsdelivr.net/gh/eisbehr-/[email protected]/jquery.lazy.min.js"></script>
<script type="text/javascript" src="//cdn.jsdelivr.net/gh/eisbehr-/[email protected]/jquery.lazy.plugins.min.js"></script>

<!-- cdnjs -->
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/jquery.lazy/1.7.10/jquery.lazy.min.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/jquery.lazy/1.7.10/jquery.lazy.plugins.min.js"></script>


Download and save one of two available files to include Lazy to your page, either the development or the minified version.

<script type="text/javascript" src="jquery.lazy.min.js"></script>

Package Managers

Lazy is even available through NPM and Bower. Just use one of the following commands below:


$ npm install jquery-lazy
$ bower install jquery-lazy

Basic Usage

1.) The basic usage of Lazy ist pretty easy. First of all you need to prepare all elements you want to lazy load. By default add a data-src attribute to images containing the loadable image and/or a data-loader attribute to elements witch shall use a plugin or custom loaders.

<img class="lazy" data-src="path/to/image_to_load.jpg" src="" />

2.) Start using Lazy by calling it after page load. You don’t have to specify your elements exactly, but for better performance, or different options, load your elements over unique classes or any other selector.

$(function($) {

Take a look at the documentation to get an idea what Lazy is capable of.

Callbacks / Events

Lazy comes with a bunch of callbacks and events you can assign to. Just add them by initialization settings:

Instances and public Functions

Lazy supports multiple parallel instances. Just initialize them with different selectors. To access an instances public functions you can initialize them in an object oriented manner or grab the instance bind to every element by default:

// object oriented way
var instance = $("img.lazy").Lazy({chainable: false});

// grab from elements (only works well if you use same selectors)
var instance = $("img.lazy").data("plugin_lazy");

Every instance has some public available functions to control it’s behavior. There are currently six available:

instance.config(entryName[, newValue]); // get or set an configuration entry
instance.addItems(items); // add new items to current instance
instance.getItems(); // get all unhandled items left of current instance
instance.update([useThrottle]); // loads all elements in current viewport
instance.force(items); // force loading specific items, ignoring the viewport
instance.loadAll(); // loads all remaining available elements from this instance
instance.destroy(); // unbinds all events and stop execution directly

Custom Content Loaders

With the custom loaders option there is a powerful solution to load every contents the Lazy way. Lazy will handle everything, you just create a loading method witch got triggered whenever the element hits the visibility threshold. It is still possible to load images and custom loaders in the same Lazy instance.

To use this just define a loader function inside the Lazy initialisation and pass the loader name to the data-loader attribute of the elements witch should be lazy loaded.

<div class="lazy" data-loader="customLoaderName"></div>
<img class="lazy" data-src="path/to/image_to_load.jpg" src="" />
<div class="lazy" data-loader="customLoaderName"></div>
<div class="lazy" data-loader="asyncLoader"></div>
    // callback
    beforeLoad: function(element) {
        console.log("start loading " + element.prop("tagName"));

    // custom loaders
    customLoaderName: function(element) {
        element.html("element handled by custom loader");
    asyncLoader: function(element, response) {
        setTimeout(function() {
            element.html("element handled by async loader");
        }, 1000);

Loader Plugins

The loader plugins can extend the functionality of Lazy, like loading other elements and data. It is basically the same as the custom content loaders, with the difference, that plugins can extend all further instances globally at once permanently and let them handle specific elements like <video> by default, without data-loader attribute set. With custom content loaders you have to initialize each instance on setup with the loader. With plugins you only load the plugin file and you’re done for all instances from now on.

For more information and examples, take a look at the existing plugins or the readme.md in there.

Configuration Parameters

The following configurations is available by default:

Name Type Default Description
name string ‘lazy’ Internal name, used for namespaces and bindings.
chainable boolean true By default Lazy is chainable and will return all elements. If set to false Lazy will return the created plugin instance itself for further use.
autoDestroy boolean true Will automatically destroy the instance when no further elements are available to handle.
bind string ‘load’ If set to load’ Lazy starts working directly after page load. If you want to use Lazy on own events set it to event’.
threshold integer 500 Amount of pixels below the viewport, in which all images gets loaded before the user sees them.
visibleOnly boolean false Determine if only visible elements should be load.
appendScroll integer window An element to listen on for scroll events, useful when images are stored in a container.
scrollDirection string ‘both’ Determines the handles scroll direction. Possible values are both, vertical and horizontal.
imageBase string null If defined this will be used as base path for all images loaded by this instance.
defaultImage string blank image Base64 image string, set as default image source for every image without a predefined source attribute.
placeholder string null Base64 image string, set a background on every element as loading placeholder.
delay integer -1 If you want to load all elements at once after page load, then you can specify a delay time in milliseconds.
combined boolean false With this parameter, Lazy will combine the event driven and delayed element loading.
attribute string ‘data-src’ Name of the image tag src attribute, where the image path is stored.
srcsetAttribute string ‘data-srcset’ Name of the image tag srcset attribute, where the source set is stored.
sizesAttribute string ‘data-sizes’ Name of the image tag sizes attribute, where the size definition for source set is stored.
retinaAttribute string ‘data-retina’ Name of the image tag attribute, where the path for optional retina image is stored.
loaderAttribute string ‘data-loader’ Name or the element attribute, where the identifier of the plugin or customer loader is sored.
imageBaseAttribute string ‘data-imagebase’ Name ot the image tag element, where the specific image base is stored. This will overwrite the global imageBase config.
removeAttribute boolean true Determine if the attribute should be removed from the element after loading.
handledName string ‘handled’ Name of the element tag data attribute, to determine if element is already handled.
loadedName string ‘loaded’ Name of the element tag data attribute, to determine if element is already loaded.
effect string ‘show’ Function name of the effect you want to use to show the loaded images, like show or fadeIn.
effectTime integer 0 Time in milliseconds the effect should use to view the image.
enableThrottle boolean true Throttle down the loading calls on scrolling event.
throttle integer 250 Time in milliseconds the throttle will use to limit the loading calls.
beforeLoad function undefined Callback function, which will be called before the element gets loaded. Has current element and response function as parameters. this is the current Lazy instance.
afterLoad function undefined Callback function, which will be called after the element was loaded. Has current element and response function as parameters. this is the current Lazy instance.
onError function undefined Callback function, which will be called if the element could not be loaded. Has current element and response function as parameters. this is the current Lazy instance.
onFinishedAll function undefined Callback function, which will be called after all elements was loaded or returned an error. The callback has no parameters. this is the current Lazy instance.

Build and Validation

This project includes an automated build script using gulp. To build your own versions of Lazy you need to install it via npm first. Afterwards you can use the following command in your console to automatically generate all productive files. While building these files everything will be checked with jshint for validity too.

$ gulp build

Available gulp Tasks:

Name Description
build check & build everything
build-main check & build main project file
build-plugins check & build single plugin files
concat-plugins build concatenated plugins file
validate check all files with jshint
watch watches all files to check & build everything on change

Bugs / Feature request

Please report bugs and feel free to ask for new features directly on GitHub.


Lazy is dual-licensed under MIT and GPL-2.0 license.


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