responsive-lazyload

1.2.0 • Public • Published

Lazyload Images Responsively

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This package was inspired by https://github.com/ivopetkov/responsively-lazy/. It uses very similar markup, but significantly simplifies the way image replacement is handled under the hood. It also adds an optional fallback for when JavaScript is disabled.

Quick Start

Check out the examples for copy-pasteable code and more information about usage.

Option 1: Using a Build Tool

This example assumes webpack.

1. Install the module using npm.

npm install --save responsive-lazyload

2. Include the module and initialize lazyloading.

Load the module and initialize lazyloading in your app's script:

import responsiveLazyload from 'responsive-lazyload';
 
responsiveLazyload();

3. Include the stylesheet.

Include the following in the <head> of your document.

<link rel="stylesheet" 
      href="node_modules/responsive-lazyload/dist/responsive-lazyload.min.css">

4. Add a lazyloaded image to your markup.

Place the lazyload markup anywhere in your app's markup:

<div class="js--lazyload">
  <img alt="a lazyloaded image"
       src="http://placekitten.com/400/300"
       srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
       data-lazyload="http://placekitten.com/400/300 1x,
                      http://placekitten.com/800/600 2x">
</div>

For more information and configuration options, see the examples.

Option 2: Using unpkg

NOTE: While unpkg is a fantastic tool, adding additional HTTP requests to your project will slow it down. For that reason, this approach is not recommended.

1. Include the script in your markup.

Just before the closing </body> tag, add the lazyloading script:

<script src="https://unpkg.com/responsive-lazyload/dist/responsive-lazyload.umd.js" defer></script>

2. Include the styles in your markup.

Include the following in the <head> of your document:

<link rel="stylesheet" 
      href="https://unpkg.com/responsive-lazyload/dist/responsive-lazyload.min.css">

3. Initialize lazyloading.

The initialization function is stored inside a global object called responsiveLazyload. Initialize lazyloading by adding the following just below the script include:

<script>
  responsiveLazyload();
</script> 

4. Add a lazyloaded image to your markup.

Place the lazyload markup anywhere in your app's markup:

<div class="js--lazyload">
  <img alt="a lazyloaded image"
       src="http://placekitten.com/400/300"
       srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
       data-lazyload="http://placekitten.com/400/300 1x,
                      http://placekitten.com/800/600 2x">
</div>

For more information and configuration options, see the examples.

Markup

The markup to implement this is:

<div class="js--lazyload js--lazyload--loading">
  <img alt="image description"
       src="/images/image@2x.jpg"
       srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
       data-lazyload="/images/image-300x150.jpg 300w,
                    /images/image-600x300.jpg 600w,
                    /images/image.jpg 690w,
                    /images/image@2x.jpg 1380w">
</div>

Markup Details

  • The classes can be changed, but must be updated in the call to responsiveLazyload().
  • The initial srcset is a blank GIF, which avoids an unnecessary HTTP request.
  • The actual srcset is added as data-lazyload.

The way responsiveLazyload() works is to check if the image is inside the viewport, and — if so — swap out the srcset for the data-lazyload. This is much simpler than duplicating browser behavior to choose the optimal image size; instead, we just give the browser a srcset and let it do its thing.

On browsers that don’t support srcset, the regular src attribute is used, so this should degrade gracefully.

Markup With Fallback for Browsers Without JavaScript Enabled

To ensure that an image is still visible, even if JavaScript is disabled, add a <noscript> block with the properly marked up image using srcset without the lazyloading solution:

<div class="js--lazyload js--lazyload--loading">
  <img alt="image description"
       src="/images/image@2x.jpg"
       srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
       data-lazyload="/images/image-300x150.jpg 300w,
                    /images/image-600x300.jpg 600w,
                    /images/image.jpg 690w,
                    /images/image@2x.jpg 1380w">
  <noscript>
    <img alt="image description"
         src="/images/image@2x.jpg"
         srcset="/images/image-300x150.jpg 300w,
                 /images/image-600x300.jpg 600w,
                 /images/image.jpg 690w,
                 /images/image@2x.jpg 1380w">
  </noscript>
</div>

JavaScript Options

To enable lazyloading, add the following to your initialization script:

import responsiveLazyload from 'responsive-lazyload';
 
responsiveLazyload({
    containerClass: 'js--lazyload',
    loadingClass: 'js--lazyload--loading',
    callback: () => {},
});
option default description
containerClass js--lazyload Determines which elements are targeted for lazyloading.
loadingClass js--lazyload--loading Applied to containers before loading. This is useful for adding loading animations.
callback () => {} Fired on each image load. Useful for adding custom functionality after an image has loaded.

Development

To run this module locally for development, follow these steps:

# Clone the repo. 
git clone git@github.com:jlengstorf/responsive-lazyload.js.git
 
# Move into the repo. 
cd responsive-lazyload.js/
 
# Install dependencies. 
npm install
 
# Run the build script. 
npm run build

Testing

Tests are built using Jest. Run them with:

npm test
 
# Or, to remove all the extra crap npm spits out and only show test output: 
npm test --silent

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npm i responsive-lazyload

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Version

1.2.0

License

ISC

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Collaborators

  • jlengstorf