LazyImages plugin for 11ty

What this plugin does:

• 🔍 Finds IMG elements in your markup
• ➕ Adds width and height attributes to the element
• ✋ Defers loading the image until it is in/near the viewport (lazy loading)
• 🖼️ Displays a blurry low-res placeholder until the image has loaded (LQIP)

This plugin supports:

• Any 11ty template format that outputs to a .html file
• Absolute image paths
• Relative image paths (improved in v2.1!)
• Custom image selectors; target all images or only images in a certain part of the page
• Placeholder generation for all image formats supported by Sharp; including JPEG, PNG, WebP, TIFF, GIF, & SVG
• Responsive images using srcset; the image in the src attribute will be used for determining the placeholder image and width/height attributes

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v2.1 just released! View the release/upgrade notes

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Getting started

Install the plugin

In your project directory run:

# Using npm
npm install eleventy-plugin-lazyimages --save-dev

# Or using yarn
yarn add eleventy-plugin-lazyimages --dev

Then update your project's .eleventy.js to include the plugin:

const lazyImagesPlugin = require('eleventy-plugin-lazyimages');

module.exports = function (eleventyConfig) {
  eleventyConfig.addPlugin(lazyImagesPlugin);
};

Tweak your CSS (optional)

This plugin will automatically set the width and height attributes for each image based on the source image dimensions. You might want to overwrite this with the following CSS:

img {
  display: block;
  width: 100%;
  max-width: 100%;
  height: auto;
}

The above CSS will ensure the image is never wider than its container and the aspect ratio is maintained.

Configure the plugin (optional)

You can pass an object with configuration options as the second parameter:

eleventyConfig.addPlugin(lazyImagesPlugin, {
  imgSelector: '.post-content img', // custom image selector
  cacheFile: '', // don't cache results to a file
});

A full list of available configuration options are listed below, and some common questions are covered at the end of this file.

Configuration options

Key	Type	Description
maxPlaceholderWidth	Integer	The maximum width in pixels of the generated placeholder image. Recommended values are between 15 and 40. Default: 25
maxPlaceholderHeight	Integer	The maximum height in pixels of the generated placeholder image. Recommended values are between 15 and 40. Default: 25
imgSelector	String	The DOM selector used to find IMG elements in the markup. Default: img
transformImgPath	Function	A function that takes the IMG src attribute and returns a string representing the actual file path to your image.
cacheFile	String	Cache image metadata and placeholder images to this filename. Greatly speeds up subsequent builds. Pass an empty string to turn off the cache. Default: .lazyimages.json
appendInitScript	Boolean	Appends code to initialise lazy loading of images to the generated markup. Set this to false if you include your own lazy load script. Default: true
scriptSrc	String	The URI for the lazy load script that is injected into the markup via appendInitScript. Default: https://cdn.jsdelivr.net/npm/lazysizes@5/lazysizes.min.js
preferNativeLazyLoad	Boolean	Use the native browser loading="lazy" instead of the lazy load script (if available). Default: false
setWidthAndHeightAttrs	Boolean	Set the width and height attributes on img elements to the actual size of the image file. Default: true
className	String[]	The class names added to found IMG elements. You usually don't need to change this unless you're using a different scriptSrc. Default: ['lazyload']

Example projects

Example projects using the plugin can be found in the /example directory.

• Basic - using default configuration
• Custom selector - using a custom image selector to only target image in certain DIVs
• Usage with eleventy-plugin-local-images - using this plugin with eleventy-plugin-local-images
• Usage with vanilla-lazyload - using this plugin with vanilla-lazyload

Built with

• JSDOM - To find and modify image elements in 11ty's generated markup
• Sharp - To read image metadata and generate low-res placeholders
• LazySizes - Handles lazy loading

Contributing

This project welcomes suggestions and Pull Requests!

Authors

• Liam Fiddler - Initial work / maintainer - @liamfiddler

See also the list of contributors who participated in this project.

License

This project is licensed under the MIT License - see the LICENSE file for details

Acknowledgments

• The wonderfully supportive team at Mentally Friendly
• Everyone who has contributed to the 11ty project, without whom this plugin wouldn't run
• José M. Pérez's blog post about progressive image loading which served as the inspiration for this plugin
• Addy Osmani's blog post about lazy loading which served as the inspiration for the init script

Common questions

Can I host the lazy load script locally?

Yes! This plugin defaults to LazySizes from JSDelivr but you can specify a relative path via the scriptSrc configuration option.

Does my local image path have to match the output path?

(a.k.a Why do I have "Input file is missing" messages in my terminal?)

By default this plugin will look for the file referenced in a src attribute like <img src="/images/dog.jpg" /> at <project root>/images/dog.jpg or <project root>/src/images/dog.jpg.

Whereas a file referenced like <img src="./images/dog.jpg" /> or <img src="images/dog.jpg" /> is expected to be found at <input file directory>/images/dog.jpg.

If you prefer to store your images elsewhere the transformImgPath config option allows you to specify a function that points the plugin to your internal image path.

For example, if your file structure stores <img src="/images/dog.jpg" /> at <project root>/assets/dog.jpg you could set transformImgPath like:

// .eleventy.js
eleventyConfig.addPlugin(lazyImagesPlugin, {
  transformImgPath: (imgPath) => imgPath.replace('/images/', './assets/'),
});

The transformImgPath configuration option takes a function that receives two parameters; src, and options.

src is a string containing the value of the img elements src attribute.

options is an object containing the outputPath of the file being processed, as well as the outputDir, inputPath, inputDir, and extraOutputSubdirectory values from eleventy config.

Can I use a different lazy load script?

Yes! By default this plugin uses LazySizes to handle lazy loading, but any lazy load script that reads from the data-src attribute is supported via the scriptSrc configuration option.

We've included an example project in this repo demonstrating this plugin using vanilla-lazyload.

Note: if you need to modify the custom script's parameters the recommended approach is to set appendInitScript: false in this plugin's config. This tells the plugin to skip adding the script loader code to the page. It ignores any value set for scriptSrc and allows you to use your own method for including the custom script. The plugin will still set the data-src + width + height attributes on IMG tags and generate the low quality image placeholders, it just doesn't manage the actual lazy loading.

Can I use this plugin with a plugin that moves/renames image files?

Yes! The key to solving this problem is the order in which the plugins are defined in .eleventy.js. It is important this plugin runs after the plugin that moves/renames files otherwise this plugin may still be referencing the original filepath in the markup, not the one generated by the other plugin.

We've included an example project in this repo demonstrating this plugin with eleventy-plugin-local-images.

Upgrade notes

v2.1.0

This release improves support for relative file paths in src attributes.

transformImgPath now receives an optional second parameter containing the outputPath of the file being processed, as well as the outputDir, inputPath, inputDir, and extraOutputSubdirectory values from eleventy config.

This release also adds the setWidthAndHeightAttrs config option which allows you to turn off the setting of width and height attributes being added to img elements.

v2.0.0

The underlying tool used to generate placeholders has switched from JIMP to Sharp. This allows the plugin to handle a greater variety of image formats, while also increasing in speed.

The API remains largely the same so most sites should not need to adjust their config.

• The default values for maxPlaceholderWidth and maxPlaceholderHeight have been increased from 12 to 25 - this increases the quality of the LQIP without a significant change in filesize
• placeholderQuality has been removed - at the size of the LQIP it didn't make much of a difference to filesize or image quality
• The default value for preferNativeLazyLoad is now false - most users install this plugin to generate LQIP and the previous default meant the LQIP weren't visible in modern browsers

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