Lazy load offscreen images with lazysizes

Katie Hempenius
Katie Hempenius

Lazy loading is the approach of waiting to load resources until they are needed, rather than loading them in advance. This can improve performance by reducing the amount of resources that need to be loaded and parsed on initial page load.

Images that are offscreen during the initial pageload are ideal candidates for this technique. Best of all, lazysizes makes this a very simple strategy to implement.

Add the lazysizes script to the page

  • Click the Remix to Edit button to make the project editable.

lazysizes.min.js has already been downloaded and added to this Glitch. To include it in the page:

  • Add the following <script> tag to index.html:
  <script src="lazysizes.min.js" async></script>
<!-- Images End -->

The lazysizes.min.js file has already been added to this project, so there is no need to add it separately. The script you just added can use this script.

lazysizes will intelligently load images as the user scrolls through the page and prioritize the images that the user is going to encounter soon.

Indicate the images to lazy load

  • Add the class lazyload to images that should be lazy loaded. In addition, change the src attribute to data-src.

For example, the changes for flower3.png would look like this:

<img src="images/flower3.png" alt="">
<img data-src="images/flower3.png" class="lazyload" alt="">

For this example, try lazy loading flower3.png, flower4.jpg, and flower5.jpg.

You may be wondering why it is necessary to change the src attribute to data-src. If this attribute is not changed, all the images will load immediately instead of being lazy-loaded. data-src is not an attribute that the browser recognizes, so when it encounters an image tag with this attribute, it doesn't load the image. In this case, that is a good thing, because it then allows the lazysizes script to decide when the image should be loaded, rather than the browser.

See it in action

That's it! To see these changes in action, follow these steps:

  • To preview the site, mouse over the editor, press the App button, then the Show button.

  • Open the console and find the images that were just added. Their classes should change from lazyload to lazyloaded as you scroll down the page.

Images being lazy loaded

  • Watch the network panel to see the image files load individually as you scroll down the page.

Images being lazy loaded

Verify using Lighthouse

Lastly, it's a good idea to use Lighthouse to verify these changes. Lighthouse's "Defer offscreen images" performance audit will indicate if you've forgotten to add lazy loading to any offscreen images.

  • Preview the site with the Show button.

  • Run the Lighthouse performance audit (Lighthouse > Options > Performance) on the live version of your Glitch and verify that the "Defer offscreen images" audit was passed.

Passing 'Efficiently encode images' audit in Lighthouse

Success! You have used lazysizes to lazy load the images on your page.