Hey, be lazy! bLazy.JS is a lightweight pure JavaScript script for lazy loading and multi-serving images. It's working in all modern browsers including IE7+.
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README.md

hey, be lazy

Downloads Latest Stable Version License

bLazy is a lightweight script for lazy loading and multi-serving images, iframes, videos and more (less than 1.4KB minified and gzipped). It’s written in pure JavaScript why it doesn’t depend on 3rd-party libraries such as jQuery. It lets you lazy load and multi-serve your images so you can save bandwidth and server requests. The user will have faster load times and save data usage if he/she doesn't browse the whole page.

Table of Contents
1. Demo
2. Usage & API
3. Why be lazy?
4. Changelog
5. License

DEMO

http://dinbror.dk/blazy/

More examples: http://dinbror.dk/blazy/examples/

Codepen playground: http://codepen.io/dinbror/pen/HzCAJ

USAGE & API

http://dinbror.dk/blog/blazy/

INSTALL

You can install blazy.js with npm:

npm install blazy --save

or bower:

bower install blazy --save

CDN

If you don't want to host the script yourself you can link to the latest minified file:
//cdn.jsdelivr.net/blazy/latest/blazy.min.js on jsDelivr. Exchange latest with the specific version number if you want to lock it in.

WHY BE LAZY?

  • bLazy is used on big sites with millions of monthly visitors so it has been tested out in the real world.
  • bLazy is written in pure JavaScript why it doesn’t depend on 3rd-party libraries such as jQuery.
  • bLazy is lightweight, less than 1.4KB and less than 1.25KB if you don't need IE7- support.
  • bLazy is very fast. It has focus on performance why it also auto-destroys when it's done lazy loading.
  • bLazy can lazy load all types of images including background images.
  • bLazy is future-proof. It supports srcset and the picture element.
  • bLazy can serve retina images on retina devices.
  • bLazy can lazy load everything with a src like iframes, HTML5 videos, scripts, unity games etc.
  • bLazy supports all browsers used today including legacy browsers like IE7 and 8.
  • bLazy supports all main module formats like AMD, CommonJS and globals.

WISHLIST/NEW FEATURES REQUESTED BY YOU

  • Only preload "first frame" of progressive jpegs.
  • Add support for CSS background property; image-set (caniuse).
  • Add a class when the lazyloading begins.
  • Option to keen load once on screen images have loaded.
  • Add option to disable success/error classes
  • Animate the container that contains the image you lazy load. You can do that today by adding/removing a class in the success callback.

CHANGELOG

v 1.8.2 (2016/10/25)

  • Added null check in public load function.
  • Bugfix: Fixed this issue defaulting to window when passing revalidate or destroy in setTimeout as reference #73 and #112. Thanks PeteDuncanson.
  • Bugfix: If parts of container is outside window use the viewport boundaries #113 and #114. Thanks dbirkbeck.

v 1.8.1 (2016/10/22)

  • Bugfix: Created polyfill and check for support of Element.closest which was introduced in the container fix in v. 1.8.0.

v 1.8.0 (2016/10/16)

  • Bugfix: Non-visible images being loaded inside container #23 and #96.

v 1.7.1 (2016/10/14)

  • Bugfix: In safari the picture element always loaded the default/fallback image #92.

v 1.7.0 (2016/10/10)

  • Bugfix: When lazyloading picture elements it also loaded the fallback/regular image #92 and 108. Thanks @idoshamun
  • Refactored loadElement function to avoid redundancy.

v 1.6.4 (2016/10/08)

  • Bugfix: When lazyloading srcset images it also loaded the fallback/regular image #99. Thanks @m0uH

v 1.6.3 (2016/09/30)

  • Changed event listener to passive listener #106. Thanks @idoshamun
  • Added support for web components (shadow dom) #107. Thanks again @idoshamun

v 1.6.2 (2016/05/09)

  • Fixed bug introduced in v.1.6.0, not using retina/breakpoint src #90.

v 1.6.1 (2016/05/02)

  • Implemented a workaround for onload/onerror bug introduced in chrome v50, LINK. Fixed #85.

v 1.6.0 (2016/04/30)

  • Added support for srcset and the picture element. Fixed #69, #75, #77 and #82.
  • Added support for lazy load of videos with sources. Fixed #81.
  • Bugfix. Ensuring that error and success classes won't be added multiple times. Fixed #84.
  • Marked breakpoints as obsolete. Will be removed in upcoming version. Use srcset and/or the picture element instead.

v 1.5.4 (2016/03/06)

  • Fixed two Safari bugs: #66 and #78. Ensuring "DOM ready".

v 1.5.3 (2016/03/01)

  • Implemented #30. Keeping data source until success.
  • Fixed #47. After implementing #30 you can now get the image src and more information in the error/success callbacks.
  • Added example page to repo /example/index.html.

v 1.5.2 (2015/12/01)

  • Fixed minor bug where the error class was added when calling revalidate().
  • Minor refactoring

v 1.5.1 (2015/11/14)

  • Fixed toArray function so it now works in IE7 + 8 again. Bug introduced in 1.4.0. Thanks for reporting @imcotton.
  • Fixed #41. Added options for validate and saveViewportOffset delay.

v 1.5.0 (2015/10/30)

  • Added new feature. Now you can lazy load everything with a src attribute like iframes, unity games etc.
  • Fixed #45. Now you can pass an option if you always want to load invisible images/elements.
  • Fixed #49. Expanded the load function so it's now possible to pass a list of elements instead of only one element. Tested with getElementById, getElementsByClassName, querySelectorAll, querySelector and jQuery selector.
  • Fixed #63.

v 1.4.1 (2015/10/12)

  • Fixed #60. An "Uncaught TypeError" when options is null introduced in the refactoring in version 1.4.0.

v 1.4.0 (2015/09/28)

  • Fixed #56. Now it's possible to create multiple versions of blazy without overriding options.

v 1.3.1 (2015/02/01)

  • Added support for CommonJS-like environments that support module.exports like node.

v 1.3.0 (2015/01/23)

  • Fixed #34. Expanded public load function with force attribute, so you can force hidden images to be loaded.
  • Fixed #24, #32 and #35. Updated "elementInView" function with intersection check. Thanks @teohhanhui.

v 1.2.2 (2014/05/04)

  • Fixed #15, when you resize the browser window in another tab bLazy didn't trigger new images in view. Thanks joshribakoff.

v 1.2.1 (2014/03/23)

  • When lazy loading background images it now only updates the background-image css attribute. Thanks Saku.

v 1.2.0 (2014/02/15)

  • Important note: renamed option multi to breakpoints because it's much more descriptive.
  • Added AMD support.
  • Minor refactoring.

v 1.1.3 (2014/01/21)

  • Fixed hardcoded retina check (isRetina = true).
  • Fixed "Uncaught TypeError" when data-src is null. Instead it'll trigger the error callback.

v 1.1.2 (2014/01/03)

  • New feature: After many requests I added the possibility to handle retina images (if you’re not doing retina-first).
  • New feature: Now you can also lazy load background images.
  • Added new option, separator. Used if you want to pass retina images, default separator is ‘|’. (data-src=“image.jpg|image@2x.jpg”).

v 1.1.1 (2013/12/27)

  • Fixed #1, resize/scroll events may be detached when adding elements by ajax.
  • Added new option, errorClass. Classname an image will get if something goes wrong, default is ‘b-error’.
  • Renamed option loadedClass to successClass so naming is aligned. Default is still ‘b-loaded’.

v 1.1.0 (2013/11/22)

  • Renamed success callback from onLoaded to success.
  • Added onerror callback; error.
  • Added the possibility to pass multiple containers instead of one.

v 1.0.5 (2013/10/7)

  • Fixed "Uncaught TypeError" when container isn't default (window).

v 1.0.4 (2013/8/29)

  • Added null check so we won't try to load an image if it's missing a data source.

v 1.0.3 (2013/8/27)

  • Added new option, loadedClass. Classname an image will get when loaded.
  • Added support for horizontal lazy loading.
  • Reduced throttle time for validate.

v 1.0.2 (2013/8/7)

  • Fixed typo in unbindEvent function.
  • Added support for IE7 as promised (fallback for querySelectorAll).

v 1.0.1 (2013/8/6)

  • Performance improvements.
  • Added throttle function to ensure that we don't call resize/scroll functions too often.
  • Cleaning image markup when image has loaded.

LICENSE:

Copyright (c) 2013-16 Bjørn Klinggaard. Licensed under the The MIT License (MIT).