lazySizes is a high-performance, lightweight JavaScript library for lazy loading images. It intelligently loads images only when they’re near the viewport, significantly improving page load times and reducing bandwidth consumption. Unlike many other lazy loading solutions, lazySizes handles various image types (including <picture> and <img srcset>), responsive images, and offers advanced features like placeholder support and smooth transitions. It’s designed to be easily integrated into existing projects and requires minimal configuration.
Using lazySizes offers several compelling advantages:
<img srcset>: Handles responsive images seamlessly, selecting the appropriate image based on screen resolution.<picture> element: Allows for advanced image selection based on various factors (e.g., device pixel ratio, media queries).There are several ways to install lazySizes:
1. Download and Include:
Download the lazysizes.min.js file from the official lazySizes GitHub repository and include it in your HTML file. Place the <script> tag before the closing </body> tag for optimal performance:
<script src="lazysizes.min.js" async></script>2. Using a CDN:
Use a CDN like jsDelivr for easy integration:
<script src="https://cdn.jsdelivr.net/npm/lazysizes@5/lazysizes.min.js" async></script>3. Using a Package Manager (npm):
If you’re using a package manager like npm, install lazySizes:
npm install lazysizesThen import it into your JavaScript code. Note that you’ll still need to include the necessary HTML attributes on your images (see below).
Adding lazySizes Attributes to Images:
Once lazySizes is included, you need to add the data-src attribute to your <img> tags to specify the actual image URL:
<img data-src="image.jpg" alt="My Image">For responsive images using <img srcset> or the <picture> element, data-src and data-srcset attributes are used respectively. lazySizes automatically handles these. For example:
<img data-srcset="image-small.jpg 300w, image-large.jpg 800w"
src="placeholder.jpg" alt="My Image">
<picture>
<source data-srcset="image-small.webp 300w, image-large.webp 800w" type="image/webp">
<source data-srcset="image-small.jpg 300w, image-large.jpg 800w" type="image/jpeg">
<img data-src="image-default.jpg" alt="My Image">
</picture>Remember to replace "placeholder.jpg", "image-small.jpg", "image-large.jpg", and "image-default.jpg" with your actual image paths. Using a low-resolution placeholder image as the src attribute improves initial load times and user experience.
That completes the basic setup. Further configuration options and advanced features are discussed in the subsequent sections of this manual.
Lazy loading is a performance optimization technique that delays the loading of resources (in this case, images) until they are needed. Instead of loading all images on page load, lazySizes only loads images that are visible or about to become visible within the browser’s viewport. This significantly reduces the initial page load time and bandwidth consumption, leading to a better user experience and improved SEO. lazySizes achieves this by monitoring the user’s scrolling and loading images as they enter the viewport. It intelligently handles various factors like image size, screen resolution, and network conditions to optimize the loading process. The images’ actual URLs are stored in data attributes, and lazySizes replaces these placeholders with the real images when appropriate.
lazySizes provides a minimal but powerful API for interacting with its functionality. While generally not required for basic usage, the API allows advanced customization and integration. The primary method available is lazySizes.cfg, allowing runtime configuration changes.
lazySizes.cfg: This object allows modification of lazySizes’ configuration after initialization. Modifying this object after lazySizes has already processed the page will usually have an effect but it is best practice to modify this object before lazySizes loads. See the Configuration Options section for a detailed list of configurable properties.
Example: To change the lazyClass (the class applied to an image before loading), you can do:
lazySizes.cfg.lazyClass = 'my-lazy-class';Note: Direct manipulation of the internal lazySizes objects is generally discouraged unless you’re developing a plugin or extending the library’s core functionality. Always prefer using the provided API whenever possible.
lazySizes offers a wide range of configuration options, accessible through lazySizes.cfg. These options allow fine-tuning the library’s behavior to suit your project’s specific needs. Most options have default values that work well in most cases, but customizing them can lead to further performance improvements or tailored behavior. Here are some key configuration options:
lazyClass (String, default: lazyload): The class name added to elements before they’re loaded. You can use CSS to style elements with this class as placeholders.loadedClass (String, default: lazyloaded): The class name added to elements after they’ve been successfully loaded.errorClass (String, default: lazyerror): The class name added to elements if loading fails.expand (Number, default: 200): The number of pixels before an element enters the viewport that lazySizes starts loading the image. A higher value starts loading earlier.throttle (Number, default: 250): The time in milliseconds to wait before checking for new images in the viewport. A smaller value checks more frequently.preload (Number, default: 1): The number of elements to preload beyond the viewport.hFac (Number, default: 0.5): The horizontal factor used for calculating viewport boundaries. Values closer to 0 prioritize elements closer to the left side of the screen.vFac (Number, default: 0.5): The vertical factor used for calculating viewport boundaries. Values closer to 0 prioritize elements closer to the top of the screen.thresholds (Array, default: [250]): This is a more advanced option. You can provide an array of values (in pixels) to dynamically change the lazy load threshold based on a hierarchy.useNative (Boolean, default: false): If true, uses browser native lazy loading if available. This should generally be left as false to ensure consistent cross-browser behavior with lazySizes.nativeLoading (Boolean, default: true): If useNative is true, this controls whether to use browser native loading.For a complete list and explanation of all configuration options, refer to the lazySizes documentation on GitHub. It’s best to experiment with different values to find what works best for your specific use case.
lazySizes triggers several custom events throughout its lifecycle, allowing you to hook into specific stages of the lazy loading process. These events are dispatched on the image element itself. You can listen for these events using JavaScript’s addEventListener method. Here are some key events:
lazybeforeunveil: Dispatched just before an image is about to be unveiled (loaded). Useful for performing actions before the image load begins.lazyunveil: Dispatched when an image is unveiled (its loading process starts).lazyloaded: Dispatched after an image has successfully loaded.lazyerror: Dispatched if an image fails to load.Example: To add a console log message when an image is successfully loaded:
document.addEventListener('lazyloaded', (event) => {
console.log('Image loaded:', event.target.src);
});By listening to these events, you can implement custom behaviors, such as displaying loading indicators, handling errors gracefully, or triggering animations after image load. The complete list of events and their details can be found in the official lazySizes documentation.
While lazySizes doesn’t enforce the use of placeholder images, using them significantly enhances the user experience by providing immediate visual feedback. You can customize placeholder images in several ways:
src attribute: The simplest method is to set a low-resolution or placeholder image as the src attribute of your <img> tag. This image will be displayed immediately, then replaced by the high-resolution image loaded by lazySizes.<img src="placeholder-small.jpg" data-src="image-large.jpg" alt="My Image">display to block on the img tag, and the container to relative so the image sits on top of the background.<div style="background-image: url('placeholder.jpg'); background-size: cover; width: 300px; height: 200px;">
<img style="display:block;" data-src="image.jpg" alt="My Image">
</div>lazybeforeunveil event.Remember to choose placeholder images that are small in file size to minimize initial page load time.
lazySizes seamlessly handles various image formats, including JPEG, PNG, WebP, GIF, and more. It automatically detects the image format based on the file extension. No special configuration is typically required. However, for optimal performance, consider using optimized image formats like WebP, which often offer better compression than JPEG or PNG.
Using <picture> elements allows you to serve different image formats depending on browser capabilities:
<picture>
<source data-srcset="image.webp" type="image/webp">
<source data-srcset="image.jpg" type="image/jpeg">
<img data-src="image.jpg" alt="My Image">
</picture>srcsetlazySizes fully supports responsive images using the <img srcset> attribute. It automatically selects the appropriate image based on the screen’s resolution and pixel density. Simply provide different image sizes and their corresponding resolutions in the data-srcset attribute.
<img data-srcset="image-small.jpg 300w, image-medium.jpg 600w, image-large.jpg 1200w"
src="placeholder.jpg" alt="My Image">This tells lazySizes to choose the most appropriate image based on the screen’s width. The w descriptor indicates the image’s width. You can also use x for pixel density (e.g., image-high.jpg 2x).
Beyond the basic configuration options, several strategies can further optimize lazySizes’ performance:
expand and throttle: Experiment with these configuration options to find the optimal balance between responsiveness and performance. Higher expand values load images earlier, but increase resource consumption. Lower throttle values check for new images more frequently, increasing CPU load.lazySizes is designed to be compatible with most other JavaScript libraries. However, potential conflicts can arise if other libraries manipulate the DOM in ways that affect the images lazySizes manages. If you encounter issues, ensure that the libraries’ initialization order is appropriate, or consider using techniques like event listeners to coordinate actions between lazySizes and other libraries.
If you encounter problems with lazySizes, here are some debugging steps:
lazySizes.cfg settings to ensure that they are correctly configured.Remember to always check the official lazySizes documentation for the most up-to-date information and best practices.
The lazySizes API provides methods for interacting with and controlling the library’s behavior beyond the basic configuration. While most use cases can be handled with configuration alone, the API provides advanced control and integration capabilities. Remember that directly manipulating lazySizes’ internal objects is generally discouraged; use the provided API methods whenever possible.
lazySizes.cfgThis is the primary method for configuring lazySizes. It’s an object containing various settings. Modifying this object before lazySizes initializes is recommended for predictable behavior. Changes made after initialization might have an effect but are not guaranteed to be fully applied.
Access: lazySizes.cfg
Type: Object
Example: To change the expand setting to 300 pixels:
lazySizes.cfg.expand = 300;See the “Configuration Options” section for a complete list of configurable properties within lazySizes.cfg.
lazySizes.init()Manually initializes lazySizes. Normally, lazySizes initializes automatically when the script is loaded. This method is useful if you need to initialize lazySizes at a specific time or after modifying the DOM.
Access: lazySizes.init()
Type: Function
Returns: undefined
Example: To initialize lazySizes after dynamically adding images to the page:
// ... add images to the DOM ...
lazySizes.init();lazySizes.autoInitA boolean property that controls whether lazySizes initializes automatically. Setting this to false prevents automatic initialization, requiring manual initialization with lazySizes.init().
Access: lazySizes.autoInit
Type: Boolean
Example: To disable auto-initialization:
lazySizes.autoInit = false;lazySizes.listenTo(target, eventName, callback)Adds an event listener to a specific element or elements. This provides a way to listen for custom lazySizes events on elements other than the images themselves.
Access: lazySizes.listenTo(target, eventName, callback)
Type: Function
Parameters:
target: The element or elements to listen to (can be a CSS selector string or a DOM element).eventName: The name of the event to listen for (e.g., lazyloaded).callback: The function to execute when the event is triggered.Example: Listening for the lazyloaded event on all elements with the class my-element:
lazySizes.listenTo('.my-element', 'lazyloaded', (e) => {
console.log('Image loaded:', e.target);
});lazySizes.unlistenTo(target, eventName, callback)Removes an event listener previously added with lazySizes.listenTo.
Access: lazySizes.unlistenTo(target, eventName, callback)
Type: Function
Parameters:
target: The element or elements to remove the listener from.eventName: The name of the event.callback: The callback function (optional; if omitted, all callbacks for that event are removed).Example: Removing the previously added listener:
lazySizes.unlistenTo('.my-element', 'lazyloaded');lazySizes.on(eventName, callback)Adds an event listener for a specific lazySizes event. This is similar to addEventListener but specifically for lazySizes events.
Access: lazySizes.on(eventName, callback)
Type: Function
Parameters:
eventName: The name of the lazySizes event.callback: The function to execute when the event occurs.lazySizes.once(eventName, callback)Adds an event listener that is executed only once for the specified event.
Access: lazySizes.once(eventName, callback)
Type: Function
Parameters:
eventName: The name of the lazySizes event.callback: The function to execute when the event occurs.lazySizes.off(eventName, callback)Removes an event listener added with lazySizes.on or lazySizes.once.
Access: lazySizes.off(eventName, callback)
Type: Function
Parameters:
eventName: The name of the lazySizes event.callback: The callback function (optional; if omitted, all callbacks for that event are removed).lazySizes.trigger(element, eventName, details)Manually triggers a lazySizes event on a specified element. Useful for testing or creating custom interactions.
Access: lazySizes.trigger(element, eventName, details)
Type: Function
Parameters:
element: The DOM element on which to trigger the event.eventName: The name of the event to trigger.details: An optional object containing additional event details.lazySizes.update(isForced)Forces lazySizes to recalculate the viewport and check for images to load. Useful after significant changes to the DOM, such as adding or removing images dynamically.
Access: lazySizes.update(isForced)
Type: Function
Parameters:
isForced: (Optional, Boolean) If true, forces an immediate update even if the throttle setting is in effect. Defaults to false.This API provides powerful tools for controlling and extending lazySizes functionality beyond its default behavior. Remember to consult the official lazySizes documentation for the most up-to-date information and examples.
This section demonstrates various ways to integrate lazySizes into your projects, showcasing its versatility and power.
The most basic implementation involves adding the data-src attribute to your <img> tags. This attribute holds the actual image URL, while a low-resolution placeholder (or no src attribute at all) is used for immediate display. LazySizes handles the rest.
<!DOCTYPE html>
<html>
<head>
<title>lazySizes Example</title>
<script src="https://cdn.jsdelivr.net/npm/lazysizes@5/lazysizes.min.js" async></script>
</head>
<body>
<img data-src="image1.jpg" alt="Image 1">
<img data-src="image2.jpg" alt="Image 2">
<img data-src="image3.jpg" alt="Image 3">
</body>
</html>Remember to replace "image1.jpg", "image2.jpg", and "image3.jpg" with the actual paths to your images. Ensure that lazysizes.min.js is included before the closing </body> tag.
LazySizes works seamlessly with image galleries. Simply apply the data-src attribute to each image within your gallery structure. This is particularly beneficial for large galleries, as it drastically reduces the initial load time.
<div class="gallery">
<img data-src="gallery-image1.jpg" alt="Gallery Image 1">
<img data-src="gallery-image2.jpg" alt="Gallery Image 2">
<img data-src="gallery-image3.jpg" alt="Gallery Image 3">
<!-- ... more gallery images ... -->
</div>LazySizes also supports lazy loading for background images. Use the data-bg attribute instead of data-src. This attribute should contain the URL of the background image.
<div style="width: 300px; height: 200px;" data-bg="background-image.jpg"></div>This will load the background image only when the <div> element is within the viewport.
Note: Ensure the container has defined width and height for the background image to display correctly.
LazySizes integrates seamlessly with responsive design techniques like srcset and the <picture> element. It automatically selects the appropriate image based on the user’s device and screen size.
<img data-srcset="small.jpg 300w, medium.jpg 600w, large.jpg 1200w" alt="Responsive Image">or using <picture>:
<picture>
<source data-srcset="image-small.webp 300w, image-large.webp 800w" type="image/webp">
<source data-srcset="image-small.jpg 300w, image-large.jpg 800w" type="image/jpeg">
<img data-src="image-default.jpg" alt="My Image">
</picture>For infinite scrolling implementations, lazySizes automatically handles loading images as new content is added to the page. You don’t need any special configuration. Just ensure that new images with data-src attributes are added to the DOM, and lazySizes will detect and load them as they enter the viewport.
LazySizes’ versatility allows for custom implementations tailored to specific needs. By utilizing the API and event listeners, you can incorporate lazy loading into almost any image loading scenario. For example, you can:
lazybeforeunveil and lazyloaded events.lazyerror event.By combining the core features of lazySizes with your custom code, you can create highly optimized and efficient image loading solutions for your projects. Remember to consult the API reference for available methods and events.
This section covers common issues encountered when using lazySizes and provides solutions and debugging techniques.
data-src is correctly set.data-src are correct.lazysizes.min.js script is included correctly before the closing </body> tag.data-src attribute is present and not being overwritten by other JavaScript code.src attribute for a placeholder image, or that you have CSS placeholders in place. If you’re using a src attribute placeholder and the image isn’t appearing, verify that the image exists at that URL.<body>.lazySizes.init() to control when lazySizes initializes.expand and throttle configuration options in lazySizes.cfg. Experiment with different values to find the best settings for your use case. Larger expand values load images sooner. Smaller throttle values check more frequently, but consume more CPU cycles.console.log statements to your code to track the execution flow and identify potential problems. For example, you can log events triggered by lazySizes to verify that images are being processed as expected.expand and throttle: Fine-tune these configuration options for optimal balance between responsiveness and performance.lazySizes generally supports most modern browsers. However, for older browsers, you might need polyfills for certain features. While useNative might seem attractive, generally keeping it false ensures cross-browser consistency with lazySizes’ own, more robust handling. Refer to the lazySizes documentation for the most current compatibility information. Generally, if a browser supports modern JavaScript, it will support lazySizes. Older browsers might require polyfills for some specific features but the core functionality will remain unaffected.
We welcome contributions to lazySizes! Whether you’re fixing bugs, adding features, or improving documentation, your help is valuable.
Fork the Repository: Fork the official lazySizes repository on GitHub.
Create a Branch: Create a new branch for your contribution. Use descriptive branch names (e.g., feature/add-new-feature, fix/bug-fix-123).
Make Your Changes: Make your code changes, following the code style guidelines (see below).
Test Your Changes: Thoroughly test your changes to ensure they work correctly and don’t introduce new bugs. See the “Testing and Development” section below.
Commit Your Changes: Commit your changes with clear and concise commit messages.
Push Your Branch: Push your branch to your forked repository on GitHub.
Create a Pull Request: Create a pull request on the official lazySizes repository, explaining your changes and their purpose.
Address Feedback: Respond to any feedback from the maintainers and make necessary revisions.
lazySizes follows a consistent coding style to maintain readability and maintainability. Please adhere to these guidelines when contributing:
Before contributing code, ensure that it works correctly and doesn’t break existing functionality. You can use the following steps to test your changes:
Run the Tests: lazySizes uses a test suite. The instructions for running the tests can be found in the project’s README.md file or the repository’s contributing guide.
Manual Testing: Manually test your changes in a browser environment. Try different scenarios and edge cases to ensure comprehensive coverage.
Browser Compatibility Testing: Test your changes in different browsers to ensure compatibility.
If you encounter a bug or have a feature request, please report it on the lazySizes GitHub issue tracker. Provide as much information as possible, including:
By following these guidelines, you can contribute effectively to lazySizes and help improve this valuable library for everyone. Your contributions are greatly appreciated!