lzy.js - The littlest lazy loader

A teeny lazy loader for images to make your site more performant, by only loading them as they approach the viewport.

30 lines of code, vanilla JavaScript only, and under 300 bytes when minified & gzipped.

Written by Adam and Neef

Features

• Very small! 30 lines and under 300 bytes when minified and gzipped
• Written in vanilla JS (ES2015+), with no other dependencies
• Will take a data-src attribute and load in an src for images
• If not on an img element, lzy.js will fallback to applying background-image to the element's style
• Will remove the data-src attribute once loading is done
• Can pass in an offset value, which controls how far the images are from the viewport before being loaded. By default this is 200px
• Uses the IntersectionObserver API, with a super easy polyfill method for unsupported browsers near the bottom of this Readme

Usage

To use lzy.js download the file and add it to your project folder, and include the script in your html file with it's correct path, for example (using the minified version):

<script src="javascript/lzy.min.js> </script>

Setup

Ensure all the elements you want lazily-loaded have a data-src attribute, with the path to the image you want to use.

<img data-src="path_to_image.jpg">

This will resolve to the following as the element enters the offset region

<img src="path_to_image.jpg">

Or for any non img element it would resove to

<div style="background-image: url("path_to_image.jpg);"> </div>

Calling

To call lzy, add the following function at the bottom of the lzy.js file, or in a script that's loaded afterwards if you want more control.

lzy();

Options

The only option in lzy.js is the offset value, which controls how far the images are from the viewport before being loaded, which by default is 200px. To customise this value, include it as an argument when calling lzy. The following example will load images as they get 500px from the viewport.

lzy(500);

Polyfill for Intersection Observer

lzy.js uses the IntersectionObserver API, which at the time of writing doesn't have an amazing amount of browser support. To ensure support for all browsers, add the following polyfill.io script to your html file, before including the script for lzy.js.

<script src="https://polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"> </script>

License

Licensed under the The MIT License (MIT).