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).
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.
lazyLoadhas an option calledshow_while_loadingthat, when set totrue, shows your images while they load, letting progressive JPEG do its magic. As jQuery_lazyload doesn't have this option, it 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
scrollevent 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
srcsetattribute. LazyLoad also supports the srcset attribute, today supported in modern browsers to provide HD images for high-density displays (aka Retina display). - jQuery independency. As LazyLoad doesn't rely on jQuery, you can use it in contexts where jQuery is not available.
Markup your images putting the image source inside the data-original attribute.
Specify both width and height attributes 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 200x, /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">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.
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>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: 30,
data_src: "src",
data_srcset: "srcset",
show_while_loading: true,
callback_set: function() { /* ... */ }
});See the demos and options sections for further defails.
Be sure that the images that are going to be lazy loaded occupy the same space of loaded images (*).
To do that, you can either set a placeholder image in your HTML (which nullifies the effect of the show_while_loading option), or 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, if we are using the show_while_loading option, 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.
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 | 40 |
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 while the loading is in progress | "loading" |
class_loaded |
The class applied to the elements when the loading is complete | "loaded" |
skip_invisible |
Specifies whether the script has to consider invisible images or not | true |
show_while_loading |
Specifies whether the script must show the images while they are loading. Set it to true when you use progressive JPEG format for your images. Note: to make the image visible while loading, you will have to avoid using the src attribute in the img tag |
false |
callback_load |
A function to be called when an image was loaded. | 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 |
placeholder |
The URL of a placeholder image to be shown while the original image is loading. This option is ignored when the option show_while_loading is set to true |
(A base64 encoded 1x1 transp. gif) |
| 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) |
Learn how to use LazyLoad in different contexts by looking at the code.
The images are in the page body, so LazyLoad is created with the default options.
See it in action | View source
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
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
The images are in the page body, but as they are stored in the Progressive JPEG format we want them to be shown while loading and not only when they are fully loaded.
See it in action | View source
The images are in scrolling container, a scrolling div inside the page body.
See it in action | View source
The images are in multiple scrolling containers, two scrolling divs inside the page body.
See it in action | View source
The images are added in different times, simulating content that gets added by AJAX requests or other actions.
See it in action | View source
This demo shows how to destroy LazyLoad when you're finished using it, to free up some memory.
See it in action | View source
Browser support is starting from Internet Explorer 8 up.
Please mind that you can't use CSS 3 selectors in IE8, so beware of them if you change the elements_selector default option.
It's time to give it a try and boost your website performance!
Download the package from GitHub and add dist/lazyload.min.js to your project files.
Run the following command on your terminal or command prompt.
bower install vanilla-lazyload