cssAudio - Activefile-genericCSS - ActiveGeneric - ActiveHTML - ActiveImage - ActiveJS - ActiveSVG - ActiveText - Activefile-genericVideo - ActiveLovehtmlicon-new-collectionicon-personicon-teamlog-outoctocatpop-outspinnerstartv

Performant lazy-load technique for 2018

A Pen By mike foskett

Pen Settings

About HTML Preprocessors

HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

Learn more · Versions

Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

!
Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

Learn more · Versions

CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

     

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

     

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

Powered by Algolia
About External Resources

You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

If the stylesheet you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the CSS from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

Learn more

+ add another resource

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient. For instance, CoffeeScript can help prevent easy-to-make mistakes and offer a cleaner syntax and Babel can bring ECMAScript 6 features to browsers that only support ECMAScript 5.

Learn more · Versions

You're using npm packages, so we've auto-selected Babel for you here, which we require to process imports and make it all work. If you need to use a different JavaScript preprocessor, remove the packages in the npm tab.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

Powered by Algolia
About External Resources

You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

Learn more

+ add another resource

Use npm Packages

We can make npm packages available for you to use in your JavaScript. We use webpack to prepare them and make them available to import. We'll also process your JavaScript with Babel.

⚠️ This feature can only be used by logged in users.

Code Indentation

     

Save Automatically?

If active, Pens will autosave every 30 seconds after being saved once.

Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

HTML

            
              <header>
  <div class=hero_bg></div>
  <h1>Performant lazy-load method using the Intersection Observer API</h1>
</header>


<section>
  
  <h2 class=centered-1>No visual jank, runs at the full 60fps</h2>

<!--   <img data-LL-src="https://source.unsplash.com/pFqrYbhIAXs/?w=1920x1080"> -->
  <img data-LL-src="https://websemantics.uk/unsplash/needle-on-the-record.jpg">
  
  <noscript>
    <img src="https://websemantics.uk/unsplash/needle-on-the-record.jpg">
  </noscript>
  
  <a href="https://unsplash.com/@lukechesser" target=_blank rel="noopener noreferrer">
    <svg><use xlink:href="#icon_camera"></use></svg>
    Luke Chesser
  </a>
  
</section>


<section>
  
  <h2 class=centered-2>Ultra lightweight vanilla JavaScript</h2>
  
<!--   <img data-LL-src="https://source.unsplash.com/bq-1seg1cPI/?w=1920x1080"> -->
  <img data-LL-src="https://websemantics.uk/unsplash/busy-bike.jpg">
  
  <noscript>
    <img src="https://websemantics.uk/unsplash/busy-bike.jpg">
  </noscript>
  
  <a href="https://unsplash.com/@clemono2" target=_blank rel="noopener noreferrer">
    <svg><use xlink:href="#icon_camera"></use></svg>
    Clem Onojeghuo
  </a>
  
</section>


<section>
  
  <h2 class=centered-3>Images loaded when JavaScript is unavailable</h2>

<!--   <img data-LL-src="https://source.unsplash.com/X2gM-SIufpU/?w=1920x2390"> -->
  <img data-LL-src="https://websemantics.uk/unsplash/sunday-delight.jpg">
  
  <noscript>
    <img src="https://websemantics.uk/unsplash/sunday-delight.jpg">
  </noscript>
  
  <a href="https://unsplash.com/@foodess" target=_blank rel="noopener noreferrer">
    <svg><use xlink:href="#icon_camera"></use></svg>
    Jennifer Pallian
  </a>
  
</section>


<section>
  
  <h2 class=centered-4>Images loaded when IntersectionObserver is unsupported.</h2>

<!--   <img data-LL-src="https://source.unsplash.com/AYSorKGDm10/?w=1920x1080"> -->
  <img data-LL-src="https://websemantics.uk/unsplash/red-door.jpg">
  
  <noscript>
    <img src="https://websemantics.uk/unsplash/red-door.jpg">
  </noscript>
  
  <a href="https://unsplash.com/@shttefan" target=_blank rel="noopener noreferrer">
    <svg><use xlink:href="#icon_camera"></use></svg>
    SHTTEFAN
  </a>
  
</section>


<section>
  
  <h2 class=centered>Neat huh?</h2>

<!--   <img data-LL-src="https://source.unsplash.com/PC_lbSSxCZE/?w=1920x1080"> -->
  <img data-LL-src="https://websemantics.uk/unsplash/excalibur-sparkles.jpg">
  
  <noscript>
    <img src="https://websemantics.uk/unsplash/excalibur-sparkles.jpg">
  </noscript>
  
  <a href="https://unsplash.com/@krisroller" target=_blank rel="noopener noreferrer">
    <svg><use xlink:href="#icon_camera"></use></svg>
    Kristopher Roller
  </a>
  
</section>




<footer>

  <h2>Details</h2>

  <p>Method utilises the new Intersection Observer API. Which has zero jank and runs at the full 60fps using ultra lightweight vanilla JavaScript.</p>

  <p>Under fallback conditions (method unsupported, no JavaScript available) images are  loaded as normal.</p>

  <h2>Credits</h2>

  <p>Based heavily on Dean Humes article: <a class=details_lnk title="[new window]" target=_blank href="http://www.deanhume.com/Home/BlogPost/lazy-loading-images-using-intersection-observer/10163">Lazy-loading images using Intersection Observer</a></p>

  <p>Images from <a class=details_lnk target=_blank title="[new window]" href="https://unsplash.com/">Unsplash</a></p>
  
  <h2>More info</h2>

  <p>MDN: <a class=details_lnk target=_blank title="[new window]" href="https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API">Intersection Observer API</a></p>
  
  <p>Google Developers: <a class=details_lnk target=_blank title="[new window]" href="https://developers.google.com/web/updates/2016/04/intersectionobserver">IntersectionObserver’s coming into view</a></p>
  
  <p>Can I Use: <a class=details_lnk target=_blank title="[new window]" href="http://caniuse.com/#feat=intersectionobserver">browser support</a></p>


  <p class=myStuff><a class=details_lnk target=_blank title="[new window]" href="https://codepen.io/2kool2/pens/popular/?grid_type=list#">Pens by Mike Foskett</a> &mdash; <a class=details_lnk target=_blank title="[new window]" href="https://websemantics.uk/">webSemantics</a></p>

  
</footer>





<svg class=visually-hidden>
  <defs>
    <symbol id="icon_camera" viewBox="0 0 32 32">
      <path d="M20.8 18.1c0 2.7-2.2 4.8-4.8 4.8s-4.8-2.1-4.8-4.8c0-2.7 2.2-4.8 4.8-4.8 2.7.1 4.8 2.2 4.8 4.8zm11.2-7.4v14.9c0 2.3-1.9 4.3-4.3 4.3h-23.4c-2.4 0-4.3-1.9-4.3-4.3v-15c0-2.3 1.9-4.3 4.3-4.3h3.7l.8-2.3c.4-1.1 1.7-2 2.9-2h8.6c1.2 0 2.5.9 2.9 2l.8 2.4h3.7c2.4 0 4.3 1.9 4.3 4.3zm-8.6 7.5c0-4.1-3.3-7.5-7.5-7.5-4.1 0-7.5 3.4-7.5 7.5s3.3 7.5 7.5 7.5c4.2-.1 7.5-3.4 7.5-7.5z"></path>
    </symbol>
  </defs>
</svg>
!

CSS

            
              @media (min-width: 20em) {
  :root {
    font-size: calc(1rem + ((1vw - 0.2em) * 1));
  }
}
@media (min-width: 120em) {
  :root {
    font-size: 2em;
  }
}
body {
  font-family: sans-serif;
  line-height: 1.5;
  margin: 0;
  text-align: center;
}

h1 {
  color: #fff;
  
  font-size: 1.5em;
  font-weight: 100;
  margin: 1rem;
  padding: 1rem .5rem;
  position: absolute; /* Collapses box */
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  text-shadow: 0 0 2px #000, 0 1px 1px #000;
  background-color: rgba(0,0,0, .5);
/*   backdrop-filter: grayscale(1) contrast(3) blur(5px); */
}


h2  {
  margin: 2em 0 1em;
  font-size: 1.25em;
  font-weight: 100;
}

[class^="centered"] {
  color: #fff;
  background-color: rgba(0,0,0, .5);
  position: absolute; /* Collapses box */
  top: 50px;
  left: 50%;
  transform: translate(-50%, 0);
  text-shadow: 0 0 2px #000, 0 1px 1px #000;
  letter-spacing: 0.05em;
/*   white-space: nowrap; */
  z-index: 1;
  margin:0;
  padding: .25em .5em;
}


header,
footer,
section {
  display: block;
  position: relative;
  min-height: calc(100vw / 16 * 10);
}
footer {
  min-height: 0;
  max-width: 36em;
  margin: 0 auto;
  padding: 0 1rem;
}
.myStuff, .myStuff .details_lnk {
  background-color: #f7f7f7;
}


.hero_bg {
  position: absolute;
  width: 100%;
  top: 0;
  right: 0;
  bottom: 0;
  left;
  background-image: url(https://websemantics.uk/unsplash/duvet-days.jpg);
  background-size:cover;
/*   filter: blur(1px); */
}
 
img {
  display:block;
  max-width: 100%;
  margin:0 auto;
}

a {
  position: absolute;
  right: 0;
  bottom: 0;
  background-color: rgba(0,0,0,.5);
  color: #fff;
  text-decoration: none;
  padding: .25em 1em;
}
.details_lnk {
  position: relative;
  padding: 0;
  text-decoration: underline;
  background-color: #fff;
  color: #000;
}

svg {
  width: .9em;
  height: .9em;
  vertical-align: text-top;
  fill: #ccc;
  margin-right:.35em;
}

.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip: rect(1px, 1px, 1px, 1px);
}


.-js-fadeIn {
  animation-name: fadeIn;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0, 0, 0.4, 1);
  animation-fill-mode: forwards;
}

@keyframes fadeIn {
  from {opacity: 0;}
  to {opacity: 1;}
}
!

JS

            
              // Dean Hume:  https://deanhume.github.io/lazy-observer-load/

// TODO: background-images

var lazy_loader = (() => {

  "use strict";
  
  const dataAttr = "data-LL-src";
  const imageLoaded = "-js-lazyImageLoaded";
  const imageFadeClass = "-js-fadeIn";

  // Get all of the images that are marked up to lazy load
  const images = document.querySelectorAll("[" + dataAttr + "]");

  const config = {
    // If the image gets within 50px in the Y axis, start the download.
    //rootMargin: '50px 0px',
    rootMargin: '-50px 0px', // 50px into viewport before loading so you can see it kick in
    threshold: 0.01
  };

  let imageCount = images.length;
  let observer;

  // If we don't have support for intersection observer, loads the images immediately
  if (!('IntersectionObserver' in window)) {

    console.log("No IntersectionObserver support");
    Array.from(images).forEach(image => preloadImage(image));

  } else {
    // It is supported, load the images
    console.log("IntersectionObserver supported");
    observer = new IntersectionObserver(onIntersection, config);
    images.forEach(image => {
      if (image.classList.contains(imageLoaded)) {
        return;
      }

      observer.observe(image);
    });
  }

  // Fetch image for given URL
  function fetchImage(url) {
    return new Promise((resolve, reject) => {
      const image = new Image();
      image.src = url;
      image.onload = resolve;
      image.onerror = reject;
    });
  }

  // Preloads the image
  function preloadImage(image) {
    const src = image.getAttribute(dataAttr);
    if (!src) {
      return;
    }

    return fetchImage(src).then(() => {
      applyImage(image, src);
    });
  }

  // Load all of the images immediately
  function loadImagesImmediately(images) {
    Array.from(images).forEach(image => preloadImage(image));
  }

  // Disconnect the observer
  function disconnect() {
    if (!observer) {
      return;
    }

    observer.disconnect();
  }

  // On intersection
  function onIntersection(entries) {
    // Disconnect if we've already loaded all of the images
    if (imageCount === 0) {
      observer.disconnect();
    }

    // Loop through the entries
    entries.forEach(entry => {
      // Are we in viewport?
      if (entry.intersectionRatio > 0) {
        imageCount--;

        // Stop watching and load the image
        observer.unobserve(entry.target);
        preloadImage(entry.target);
      }
    });
  }

  // Apply the image
  function applyImage(img, src) {
    // Prevent this from being lazy loaded a second time.
    img.classList.add(imageLoaded);
    img.src = src;
    img.classList.add(imageFadeClass);
  }

})();
!
999px
🕑 One or more of the npm packages you are using needs to be built. You're the first person to ever need it! We're building it right now and your preview will start updating again when it's ready.
Loading ..................

Console